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postgresql/src/backend/access/nbtree/nbtsearch.c

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/*-------------------------------------------------------------------------
*
* nbtsearch.c
* Search code for postgres btrees.
*
*
* Portions Copyright (c) 1996-2019, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* IDENTIFICATION
* src/backend/access/nbtree/nbtsearch.c
*
*-------------------------------------------------------------------------
*/
#include "postgres.h"
#include "access/nbtree.h"
#include "access/relscan.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "storage/predicate.h"
#include "utils/lsyscache.h"
#include "utils/rel.h"
static void _bt_drop_lock_and_maybe_pin(IndexScanDesc scan, BTScanPos sp);
static OffsetNumber _bt_binsrch(Relation rel, BTScanInsert key, Buffer buf);
static bool _bt_readpage(IndexScanDesc scan, ScanDirection dir,
OffsetNumber offnum);
static void _bt_saveitem(BTScanOpaque so, int itemIndex,
OffsetNumber offnum, IndexTuple itup);
static bool _bt_steppage(IndexScanDesc scan, ScanDirection dir);
static bool _bt_readnextpage(IndexScanDesc scan, BlockNumber blkno, ScanDirection dir);
static bool _bt_parallel_readpage(IndexScanDesc scan, BlockNumber blkno,
ScanDirection dir);
static Buffer _bt_walk_left(Relation rel, Buffer buf, Snapshot snapshot);
static bool _bt_endpoint(IndexScanDesc scan, ScanDirection dir);
static inline void _bt_initialize_more_data(BTScanOpaque so, ScanDirection dir);
/*
* _bt_drop_lock_and_maybe_pin()
*
* Unlock the buffer; and if it is safe to release the pin, do that, too. It
* is safe if the scan is using an MVCC snapshot and the index is WAL-logged.
* This will prevent vacuum from stalling in a blocked state trying to read a
* page when a cursor is sitting on it -- at least in many important cases.
*
* Set the buffer to invalid if the pin is released, since the buffer may be
* re-used. If we need to go back to this block (for example, to apply
* LP_DEAD hints) we must get a fresh reference to the buffer. Hopefully it
* will remain in shared memory for as long as it takes to scan the index
* buffer page.
*/
static void
_bt_drop_lock_and_maybe_pin(IndexScanDesc scan, BTScanPos sp)
{
LockBuffer(sp->buf, BUFFER_LOCK_UNLOCK);
if (IsMVCCSnapshot(scan->xs_snapshot) &&
RelationNeedsWAL(scan->indexRelation) &&
!scan->xs_want_itup)
{
ReleaseBuffer(sp->buf);
sp->buf = InvalidBuffer;
}
}
/*
* _bt_search() -- Search the tree for a particular scankey,
* or more precisely for the first leaf page it could be on.
*
* The passed scankey is an insertion-type scankey (see nbtree/README),
* but it can omit the rightmost column(s) of the index.
*
* Return value is a stack of parent-page pointers. *bufP is set to the
* address of the leaf-page buffer, which is read-locked and pinned.
* No locks are held on the parent pages, however!
*
* If the snapshot parameter is not NULL, "old snapshot" checking will take
* place during the descent through the tree. This is not needed when
* positioning for an insert or delete, so NULL is used for those cases.
*
* The returned buffer is locked according to access parameter. Additionally,
* access = BT_WRITE will allow an empty root page to be created and returned.
* When access = BT_READ, an empty index will result in *bufP being set to
* InvalidBuffer. Also, in BT_WRITE mode, any incomplete splits encountered
* during the search will be finished.
*/
BTStack
_bt_search(Relation rel, BTScanInsert key, Buffer *bufP, int access,
Snapshot snapshot)
{
BTStack stack_in = NULL;
int page_access = BT_READ;
/* Get the root page to start with */
*bufP = _bt_getroot(rel, access);
/* If index is empty and access = BT_READ, no root page is created. */
if (!BufferIsValid(*bufP))
return (BTStack) NULL;
/* Loop iterates once per level descended in the tree */
for (;;)
{
Page page;
BTPageOpaque opaque;
OffsetNumber offnum;
ItemId itemid;
IndexTuple itup;
BlockNumber blkno;
BlockNumber par_blkno;
BTStack new_stack;
/*
* Race -- the page we just grabbed may have split since we read its
* pointer in the parent (or metapage). If it has, we may need to
* move right to its new sibling. Do that.
*
* In write-mode, allow _bt_moveright to finish any incomplete splits
* along the way. Strictly speaking, we'd only need to finish an
* incomplete split on the leaf page we're about to insert to, not on
* any of the upper levels (they are taken care of in _bt_getstackbuf,
* if the leaf page is split and we insert to the parent page). But
* this is a good opportunity to finish splits of internal pages too.
*/
*bufP = _bt_moveright(rel, key, *bufP, (access == BT_WRITE), stack_in,
page_access, snapshot);
/* if this is a leaf page, we're done */
page = BufferGetPage(*bufP);
opaque = (BTPageOpaque) PageGetSpecialPointer(page);
if (P_ISLEAF(opaque))
break;
/*
* Find the appropriate item on the internal page, and get the child
* page that it points to.
*/
offnum = _bt_binsrch(rel, key, *bufP);
itemid = PageGetItemId(page, offnum);
itup = (IndexTuple) PageGetItem(page, itemid);
blkno = BTreeInnerTupleGetDownLink(itup);
par_blkno = BufferGetBlockNumber(*bufP);
/*
* We need to save the location of the index entry we chose in the
* parent page on a stack. In case we split the tree, we'll use the
* stack to work back up to the parent page. We also save the actual
* downlink (block) to uniquely identify the index entry, in case it
* moves right while we're working lower in the tree. See the paper
* by Lehman and Yao for how this is detected and handled. (We use the
* child link during the second half of a page split -- if caller ends
* up splitting the child it usually ends up inserting a new pivot
* tuple for child's new right sibling immediately after the original
* bts_offset offset recorded here. The downlink block will be needed
* to check if bts_offset remains the position of this same pivot
* tuple.)
*/
new_stack = (BTStack) palloc(sizeof(BTStackData));
new_stack->bts_blkno = par_blkno;
new_stack->bts_offset = offnum;
new_stack->bts_btentry = blkno;
new_stack->bts_parent = stack_in;
/*
* Page level 1 is lowest non-leaf page level prior to leaves. So, if
* we're on the level 1 and asked to lock leaf page in write mode,
* then lock next page in write mode, because it must be a leaf.
*/
if (opaque->btpo.level == 1 && access == BT_WRITE)
page_access = BT_WRITE;
/* drop the read lock on the parent page, acquire one on the child */
*bufP = _bt_relandgetbuf(rel, *bufP, blkno, page_access);
/* okay, all set to move down a level */
stack_in = new_stack;
}
/*
* If we're asked to lock leaf in write mode, but didn't manage to, then
* relock. This should only happen when the root page is a leaf page (and
* the only page in the index other than the metapage).
*/
if (access == BT_WRITE && page_access == BT_READ)
{
/* trade in our read lock for a write lock */
LockBuffer(*bufP, BUFFER_LOCK_UNLOCK);
LockBuffer(*bufP, BT_WRITE);
/*
* If the page was split between the time that we surrendered our read
* lock and acquired our write lock, then this page may no longer be
* the right place for the key we want to insert. In this case, we
* need to move right in the tree. See Lehman and Yao for an
* excruciatingly precise description.
*/
*bufP = _bt_moveright(rel, key, *bufP, true, stack_in, BT_WRITE,
snapshot);
}
return stack_in;
}
/*
* _bt_moveright() -- move right in the btree if necessary.
*
* When we follow a pointer to reach a page, it is possible that
* the page has changed in the meanwhile. If this happens, we're
* guaranteed that the page has "split right" -- that is, that any
* data that appeared on the page originally is either on the page
* or strictly to the right of it.
*
* This routine decides whether or not we need to move right in the
* tree by examining the high key entry on the page. If that entry is
* strictly less than the scankey, or <= the scankey in the
* key.nextkey=true case, then we followed the wrong link and we need
* to move right.
*
* The passed insertion-type scankey can omit the rightmost column(s) of the
* index. (see nbtree/README)
*
* When key.nextkey is false (the usual case), we are looking for the first
* item >= key. When key.nextkey is true, we are looking for the first item
* strictly greater than key.
*
* If forupdate is true, we will attempt to finish any incomplete splits
* that we encounter. This is required when locking a target page for an
* insertion, because we don't allow inserting on a page before the split
* is completed. 'stack' is only used if forupdate is true.
*
* On entry, we have the buffer pinned and a lock of the type specified by
* 'access'. If we move right, we release the buffer and lock and acquire
* the same on the right sibling. Return value is the buffer we stop at.
*
* If the snapshot parameter is not NULL, "old snapshot" checking will take
* place during the descent through the tree. This is not needed when
* positioning for an insert or delete, so NULL is used for those cases.
*/
Buffer
_bt_moveright(Relation rel,
BTScanInsert key,
Buffer buf,
bool forupdate,
BTStack stack,
int access,
Snapshot snapshot)
{
Page page;
BTPageOpaque opaque;
int32 cmpval;
/*
* When nextkey = false (normal case): if the scan key that brought us to
* this page is > the high key stored on the page, then the page has split
* and we need to move right. (pg_upgrade'd !heapkeyspace indexes could
* have some duplicates to the right as well as the left, but that's
* something that's only ever dealt with on the leaf level, after
* _bt_search has found an initial leaf page.)
*
* When nextkey = true: move right if the scan key is >= page's high key.
* (Note that key.scantid cannot be set in this case.)
*
* The page could even have split more than once, so scan as far as
* needed.
*
* We also have to move right if we followed a link that brought us to a
* dead page.
*/
cmpval = key->nextkey ? 0 : 1;
for (;;)
{
page = BufferGetPage(buf);
TestForOldSnapshot(snapshot, rel, page);
opaque = (BTPageOpaque) PageGetSpecialPointer(page);
if (P_RIGHTMOST(opaque))
break;
/*
* Finish any incomplete splits we encounter along the way.
*/
if (forupdate && P_INCOMPLETE_SPLIT(opaque))
{
BlockNumber blkno = BufferGetBlockNumber(buf);
/* upgrade our lock if necessary */
if (access == BT_READ)
{
LockBuffer(buf, BUFFER_LOCK_UNLOCK);
LockBuffer(buf, BT_WRITE);
}
if (P_INCOMPLETE_SPLIT(opaque))
_bt_finish_split(rel, buf, stack);
else
_bt_relbuf(rel, buf);
/* re-acquire the lock in the right mode, and re-check */
buf = _bt_getbuf(rel, blkno, access);
continue;
}
if (P_IGNORE(opaque) || _bt_compare(rel, key, page, P_HIKEY) >= cmpval)
{
/* step right one page */
buf = _bt_relandgetbuf(rel, buf, opaque->btpo_next, access);
continue;
}
else
break;
}
if (P_IGNORE(opaque))
elog(ERROR, "fell off the end of index \"%s\"",
RelationGetRelationName(rel));
return buf;
}
/*
* _bt_binsrch() -- Do a binary search for a key on a particular page.
*
* On a leaf page, _bt_binsrch() returns the OffsetNumber of the first
* key >= given scankey, or > scankey if nextkey is true. (NOTE: in
* particular, this means it is possible to return a value 1 greater than the
* number of keys on the page, if the scankey is > all keys on the page.)
*
* On an internal (non-leaf) page, _bt_binsrch() returns the OffsetNumber
* of the last key < given scankey, or last key <= given scankey if nextkey
* is true. (Since _bt_compare treats the first data key of such a page as
* minus infinity, there will be at least one key < scankey, so the result
* always points at one of the keys on the page.) This key indicates the
* right place to descend to be sure we find all leaf keys >= given scankey
* (or leaf keys > given scankey when nextkey is true).
*
* This procedure is not responsible for walking right, it just examines
* the given page. _bt_binsrch() has no lock or refcount side effects
* on the buffer.
*/
static OffsetNumber
_bt_binsrch(Relation rel,
BTScanInsert key,
Buffer buf)
{
Page page;
BTPageOpaque opaque;
OffsetNumber low,
high;
int32 result,
cmpval;
/* Requesting nextkey semantics while using scantid seems nonsensical */
Assert(!key->nextkey || key->scantid == NULL);
page = BufferGetPage(buf);
opaque = (BTPageOpaque) PageGetSpecialPointer(page);
low = P_FIRSTDATAKEY(opaque);
high = PageGetMaxOffsetNumber(page);
/*
* If there are no keys on the page, return the first available slot. Note
* this covers two cases: the page is really empty (no keys), or it
* contains only a high key. The latter case is possible after vacuuming.
* This can never happen on an internal page, however, since they are
* never empty (an internal page must have children).
*/
if (unlikely(high < low))
return low;
/*
* Binary search to find the first key on the page >= scan key, or first
* key > scankey when nextkey is true.
*
* For nextkey=false (cmpval=1), the loop invariant is: all slots before
* 'low' are < scan key, all slots at or after 'high' are >= scan key.
*
* For nextkey=true (cmpval=0), the loop invariant is: all slots before
* 'low' are <= scan key, all slots at or after 'high' are > scan key.
*
* We can fall out when high == low.
*/
high++; /* establish the loop invariant for high */
cmpval = key->nextkey ? 0 : 1; /* select comparison value */
while (high > low)
{
OffsetNumber mid = low + ((high - low) / 2);
/* We have low <= mid < high, so mid points at a real slot */
result = _bt_compare(rel, key, page, mid);
if (result >= cmpval)
low = mid + 1;
else
high = mid;
}
/*
* At this point we have high == low, but be careful: they could point
* past the last slot on the page.
*
* On a leaf page, we always return the first key >= scan key (resp. >
* scan key), which could be the last slot + 1.
*/
if (P_ISLEAF(opaque))
return low;
/*
* On a non-leaf page, return the last key < scan key (resp. <= scan key).
* There must be one if _bt_compare() is playing by the rules.
*/
Assert(low > P_FIRSTDATAKEY(opaque));
return OffsetNumberPrev(low);
}
/*
*
* bt_binsrch_insert() -- Cacheable, incremental leaf page binary search.
*
* Like _bt_binsrch(), but with support for caching the binary search
* bounds. Only used during insertion, and only on the leaf page that it
* looks like caller will insert tuple on. Exclusive-locked and pinned
* leaf page is contained within insertstate.
*
* Caches the bounds fields in insertstate so that a subsequent call can
* reuse the low and strict high bounds of original binary search. Callers
* that use these fields directly must be prepared for the case where low
* and/or stricthigh are not on the same page (one or both exceed maxoff
* for the page). The case where there are no items on the page (high <
* low) makes bounds invalid.
*
* Caller is responsible for invalidating bounds when it modifies the page
* before calling here a second time.
*/
OffsetNumber
_bt_binsrch_insert(Relation rel, BTInsertState insertstate)
{
BTScanInsert key = insertstate->itup_key;
Page page;
BTPageOpaque opaque;
OffsetNumber low,
high,
stricthigh;
int32 result,
cmpval;
page = BufferGetPage(insertstate->buf);
opaque = (BTPageOpaque) PageGetSpecialPointer(page);
Assert(P_ISLEAF(opaque));
Assert(!key->nextkey);
if (!insertstate->bounds_valid)
{
/* Start new binary search */
low = P_FIRSTDATAKEY(opaque);
high = PageGetMaxOffsetNumber(page);
}
else
{
/* Restore result of previous binary search against same page */
low = insertstate->low;
high = insertstate->stricthigh;
}
/* If there are no keys on the page, return the first available slot */
if (unlikely(high < low))
{
/* Caller can't reuse bounds */
insertstate->low = InvalidOffsetNumber;
insertstate->stricthigh = InvalidOffsetNumber;
insertstate->bounds_valid = false;
return low;
}
/*
* Binary search to find the first key on the page >= scan key. (nextkey
* is always false when inserting).
*
* The loop invariant is: all slots before 'low' are < scan key, all slots
* at or after 'high' are >= scan key. 'stricthigh' is > scan key, and is
* maintained to save additional search effort for caller.
*
* We can fall out when high == low.
*/
if (!insertstate->bounds_valid)
high++; /* establish the loop invariant for high */
stricthigh = high; /* high initially strictly higher */
cmpval = 1; /* !nextkey comparison value */
while (high > low)
{
OffsetNumber mid = low + ((high - low) / 2);
/* We have low <= mid < high, so mid points at a real slot */
result = _bt_compare(rel, key, page, mid);
if (result >= cmpval)
low = mid + 1;
else
{
high = mid;
if (result != 0)
stricthigh = high;
}
}
/*
* On a leaf page, a binary search always returns the first key >= scan
* key (at least in !nextkey case), which could be the last slot + 1. This
* is also the lower bound of cached search.
*
* stricthigh may also be the last slot + 1, which prevents caller from
* using bounds directly, but is still useful to us if we're called a
* second time with cached bounds (cached low will be < stricthigh when
* that happens).
*/
insertstate->low = low;
insertstate->stricthigh = stricthigh;
insertstate->bounds_valid = true;
return low;
}
/*----------
* _bt_compare() -- Compare insertion-type scankey to tuple on a page.
*
* page/offnum: location of btree item to be compared to.
*
* This routine returns:
* <0 if scankey < tuple at offnum;
* 0 if scankey == tuple at offnum;
* >0 if scankey > tuple at offnum.
* NULLs in the keys are treated as sortable values. Therefore
* "equality" does not necessarily mean that the item should be
* returned to the caller as a matching key!
*
* CRUCIAL NOTE: on a non-leaf page, the first data key is assumed to be
* "minus infinity": this routine will always claim it is less than the
* scankey. The actual key value stored is explicitly truncated to 0
* attributes (explicitly minus infinity) with version 3+ indexes, but
* that isn't relied upon. This allows us to implement the Lehman and
* Yao convention that the first down-link pointer is before the first
* key. See backend/access/nbtree/README for details.
*----------
*/
int32
_bt_compare(Relation rel,
BTScanInsert key,
Page page,
OffsetNumber offnum)
{
TupleDesc itupdesc = RelationGetDescr(rel);
BTPageOpaque opaque = (BTPageOpaque) PageGetSpecialPointer(page);
IndexTuple itup;
ItemPointer heapTid;
ScanKey scankey;
int ncmpkey;
int ntupatts;
Assert(_bt_check_natts(rel, key->heapkeyspace, page, offnum));
Assert(key->keysz <= IndexRelationGetNumberOfKeyAttributes(rel));
Assert(key->heapkeyspace || key->scantid == NULL);
/*
* Force result ">" if target item is first data item on an internal page
* --- see NOTE above.
*/
if (!P_ISLEAF(opaque) && offnum == P_FIRSTDATAKEY(opaque))
return 1;
itup = (IndexTuple) PageGetItem(page, PageGetItemId(page, offnum));
ntupatts = BTreeTupleGetNAtts(itup, rel);
/*
* The scan key is set up with the attribute number associated with each
* term in the key. It is important that, if the index is multi-key, the
* scan contain the first k key attributes, and that they be in order. If
* you think about how multi-key ordering works, you'll understand why
* this is.
*
* We don't test for violation of this condition here, however. The
* initial setup for the index scan had better have gotten it right (see
* _bt_first).
*/
ncmpkey = Min(ntupatts, key->keysz);
Assert(key->heapkeyspace || ncmpkey == key->keysz);
scankey = key->scankeys;
for (int i = 1; i <= ncmpkey; i++)
{
Datum datum;
bool isNull;
int32 result;
datum = index_getattr(itup, scankey->sk_attno, itupdesc, &isNull);
/* see comments about NULLs handling in btbuild */
if (scankey->sk_flags & SK_ISNULL) /* key is NULL */
{
if (isNull)
result = 0; /* NULL "=" NULL */
else if (scankey->sk_flags & SK_BT_NULLS_FIRST)
result = -1; /* NULL "<" NOT_NULL */
else
result = 1; /* NULL ">" NOT_NULL */
}
else if (isNull) /* key is NOT_NULL and item is NULL */
{
if (scankey->sk_flags & SK_BT_NULLS_FIRST)
result = 1; /* NOT_NULL ">" NULL */
else
result = -1; /* NOT_NULL "<" NULL */
}
else
{
/*
* The sk_func needs to be passed the index value as left arg and
* the sk_argument as right arg (they might be of different
* types). Since it is convenient for callers to think of
* _bt_compare as comparing the scankey to the index item, we have
* to flip the sign of the comparison result. (Unless it's a DESC
* column, in which case we *don't* flip the sign.)
*/
result = DatumGetInt32(FunctionCall2Coll(&scankey->sk_func,
scankey->sk_collation,
datum,
scankey->sk_argument));
if (!(scankey->sk_flags & SK_BT_DESC))
INVERT_COMPARE_RESULT(result);
}
/* if the keys are unequal, return the difference */
if (result != 0)
return result;
scankey++;
}
/*
* All non-truncated attributes (other than heap TID) were found to be
* equal. Treat truncated attributes as minus infinity when scankey has a
* key attribute value that would otherwise be compared directly.
*
* Note: it doesn't matter if ntupatts includes non-key attributes;
* scankey won't, so explicitly excluding non-key attributes isn't
* necessary.
*/
if (key->keysz > ntupatts)
return 1;
/*
* Use the heap TID attribute and scantid to try to break the tie. The
* rules are the same as any other key attribute -- only the
* representation differs.
*/
heapTid = BTreeTupleGetHeapTID(itup);
if (key->scantid == NULL)
{
/*
* Most searches have a scankey that is considered greater than a
* truncated pivot tuple if and when the scankey has equal values for
* attributes up to and including the least significant untruncated
* attribute in tuple.
*
* For example, if an index has the minimum two attributes (single
* user key attribute, plus heap TID attribute), and a page's high key
* is ('foo', -inf), and scankey is ('foo', <omitted>), the search
* will not descend to the page to the left. The search will descend
* right instead. The truncated attribute in pivot tuple means that
* all non-pivot tuples on the page to the left are strictly < 'foo',
* so it isn't necessary to descend left. In other words, search
* doesn't have to descend left because it isn't interested in a match
* that has a heap TID value of -inf.
*
* However, some searches (pivotsearch searches) actually require that
* we descend left when this happens. -inf is treated as a possible
* match for omitted scankey attribute(s). This is needed by page
* deletion, which must re-find leaf pages that are targets for
* deletion using their high keys.
*
* Note: the heap TID part of the test ensures that scankey is being
* compared to a pivot tuple with one or more truncated key
* attributes.
*
* Note: pg_upgrade'd !heapkeyspace indexes must always descend to the
* left here, since they have no heap TID attribute (and cannot have
* any -inf key values in any case, since truncation can only remove
* non-key attributes). !heapkeyspace searches must always be
* prepared to deal with matches on both sides of the pivot once the
* leaf level is reached.
*/
if (key->heapkeyspace && !key->pivotsearch &&
key->keysz == ntupatts && heapTid == NULL)
return 1;
/* All provided scankey arguments found to be equal */
return 0;
}
/*
* Treat truncated heap TID as minus infinity, since scankey has a key
* attribute value (scantid) that would otherwise be compared directly
*/
Assert(key->keysz == IndexRelationGetNumberOfKeyAttributes(rel));
if (heapTid == NULL)
return 1;
Assert(ntupatts >= IndexRelationGetNumberOfKeyAttributes(rel));
return ItemPointerCompare(key->scantid, heapTid);
}
/*
* _bt_first() -- Find the first item in a scan.
*
* We need to be clever about the direction of scan, the search
* conditions, and the tree ordering. We find the first item (or,
* if backwards scan, the last item) in the tree that satisfies the
* qualifications in the scan key. On success exit, the page containing
* the current index tuple is pinned but not locked, and data about
* the matching tuple(s) on the page has been loaded into so->currPos.
* scan->xs_ctup.t_self is set to the heap TID of the current tuple,
* and if requested, scan->xs_itup points to a copy of the index tuple.
*
* If there are no matching items in the index, we return false, with no
* pins or locks held.
*
* Note that scan->keyData[], and the so->keyData[] scankey built from it,
* are both search-type scankeys (see nbtree/README for more about this).
* Within this routine, we build a temporary insertion-type scankey to use
* in locating the scan start position.
*/
bool
_bt_first(IndexScanDesc scan, ScanDirection dir)
{
Relation rel = scan->indexRelation;
BTScanOpaque so = (BTScanOpaque) scan->opaque;
Buffer buf;
BTStack stack;
OffsetNumber offnum;
StrategyNumber strat;
bool nextkey;
bool goback;
BTScanInsertData inskey;
ScanKey startKeys[INDEX_MAX_KEYS];
ScanKeyData notnullkeys[INDEX_MAX_KEYS];
int keysCount = 0;
int i;
bool status = true;
StrategyNumber strat_total;
BTScanPosItem *currItem;
BlockNumber blkno;
Assert(!BTScanPosIsValid(so->currPos));
pgstat_count_index_scan(rel);
/*
* Examine the scan keys and eliminate any redundant keys; also mark the
* keys that must be matched to continue the scan.
*/
_bt_preprocess_keys(scan);
/*
* Quit now if _bt_preprocess_keys() discovered that the scan keys can
* never be satisfied (eg, x == 1 AND x > 2).
*/
if (!so->qual_ok)
return false;
/*
* For parallel scans, get the starting page from shared state. If the
* scan has not started, proceed to find out first leaf page in the usual
* way while keeping other participating processes waiting. If the scan
* has already begun, use the page number from the shared structure.
*/
if (scan->parallel_scan != NULL)
{
status = _bt_parallel_seize(scan, &blkno);
if (!status)
return false;
else if (blkno == P_NONE)
{
_bt_parallel_done(scan);
return false;
}
else if (blkno != InvalidBlockNumber)
{
if (!_bt_parallel_readpage(scan, blkno, dir))
return false;
goto readcomplete;
}
}
/*----------
* Examine the scan keys to discover where we need to start the scan.
*
* We want to identify the keys that can be used as starting boundaries;
* these are =, >, or >= keys for a forward scan or =, <, <= keys for
* a backwards scan. We can use keys for multiple attributes so long as
* the prior attributes had only =, >= (resp. =, <=) keys. Once we accept
* a > or < boundary or find an attribute with no boundary (which can be
* thought of as the same as "> -infinity"), we can't use keys for any
* attributes to its right, because it would break our simplistic notion
* of what initial positioning strategy to use.
*
* When the scan keys include cross-type operators, _bt_preprocess_keys
* may not be able to eliminate redundant keys; in such cases we will
* arbitrarily pick a usable one for each attribute. This is correct
* but possibly not optimal behavior. (For example, with keys like
* "x >= 4 AND x >= 5" we would elect to scan starting at x=4 when
* x=5 would be more efficient.) Since the situation only arises given
* a poorly-worded query plus an incomplete opfamily, live with it.
*
* When both equality and inequality keys appear for a single attribute
* (again, only possible when cross-type operators appear), we *must*
* select one of the equality keys for the starting point, because
* _bt_checkkeys() will stop the scan as soon as an equality qual fails.
* For example, if we have keys like "x >= 4 AND x = 10" and we elect to
* start at x=4, we will fail and stop before reaching x=10. If multiple
* equality quals survive preprocessing, however, it doesn't matter which
* one we use --- by definition, they are either redundant or
* contradictory.
*
* Any regular (not SK_SEARCHNULL) key implies a NOT NULL qualifier.
* If the index stores nulls at the end of the index we'll be starting
* from, and we have no boundary key for the column (which means the key
* we deduced NOT NULL from is an inequality key that constrains the other
* end of the index), then we cons up an explicit SK_SEARCHNOTNULL key to
* use as a boundary key. If we didn't do this, we might find ourselves
* traversing a lot of null entries at the start of the scan.
*
* In this loop, row-comparison keys are treated the same as keys on their
* first (leftmost) columns. We'll add on lower-order columns of the row
* comparison below, if possible.
*
* The selected scan keys (at most one per index column) are remembered by
* storing their addresses into the local startKeys[] array.
*----------
*/
strat_total = BTEqualStrategyNumber;
if (so->numberOfKeys > 0)
{
AttrNumber curattr;
ScanKey chosen;
ScanKey impliesNN;
ScanKey cur;
/*
* chosen is the so-far-chosen key for the current attribute, if any.
* We don't cast the decision in stone until we reach keys for the
* next attribute.
*/
curattr = 1;
chosen = NULL;
/* Also remember any scankey that implies a NOT NULL constraint */
impliesNN = NULL;
/*
* Loop iterates from 0 to numberOfKeys inclusive; we use the last
* pass to handle after-last-key processing. Actual exit from the
* loop is at one of the "break" statements below.
*/
for (cur = so->keyData, i = 0;; cur++, i++)
{
if (i >= so->numberOfKeys || cur->sk_attno != curattr)
{
/*
* Done looking at keys for curattr. If we didn't find a
* usable boundary key, see if we can deduce a NOT NULL key.
*/
if (chosen == NULL && impliesNN != NULL &&
((impliesNN->sk_flags & SK_BT_NULLS_FIRST) ?
ScanDirectionIsForward(dir) :
ScanDirectionIsBackward(dir)))
{
/* Yes, so build the key in notnullkeys[keysCount] */
chosen = &notnullkeys[keysCount];
ScanKeyEntryInitialize(chosen,
(SK_SEARCHNOTNULL | SK_ISNULL |
(impliesNN->sk_flags &
(SK_BT_DESC | SK_BT_NULLS_FIRST))),
curattr,
((impliesNN->sk_flags & SK_BT_NULLS_FIRST) ?
BTGreaterStrategyNumber :
BTLessStrategyNumber),
InvalidOid,
InvalidOid,
InvalidOid,
(Datum) 0);
}
/*
* If we still didn't find a usable boundary key, quit; else
* save the boundary key pointer in startKeys.
*/
if (chosen == NULL)
break;
startKeys[keysCount++] = chosen;
/*
* Adjust strat_total, and quit if we have stored a > or <
* key.
*/
strat = chosen->sk_strategy;
if (strat != BTEqualStrategyNumber)
{
strat_total = strat;
if (strat == BTGreaterStrategyNumber ||
strat == BTLessStrategyNumber)
break;
}
/*
* Done if that was the last attribute, or if next key is not
* in sequence (implying no boundary key is available for the
* next attribute).
*/
if (i >= so->numberOfKeys ||
cur->sk_attno != curattr + 1)
break;
/*
* Reset for next attr.
*/
curattr = cur->sk_attno;
chosen = NULL;
impliesNN = NULL;
}
/*
* Can we use this key as a starting boundary for this attr?
*
* If not, does it imply a NOT NULL constraint? (Because
* SK_SEARCHNULL keys are always assigned BTEqualStrategyNumber,
* *any* inequality key works for that; we need not test.)
*/
switch (cur->sk_strategy)
{
case BTLessStrategyNumber:
case BTLessEqualStrategyNumber:
if (chosen == NULL)
{
if (ScanDirectionIsBackward(dir))
chosen = cur;
else
impliesNN = cur;
}
break;
case BTEqualStrategyNumber:
/* override any non-equality choice */
chosen = cur;
break;
case BTGreaterEqualStrategyNumber:
case BTGreaterStrategyNumber:
if (chosen == NULL)
{
if (ScanDirectionIsForward(dir))
chosen = cur;
else
impliesNN = cur;
}
break;
}
}
}
/*
* If we found no usable boundary keys, we have to start from one end of
* the tree. Walk down that edge to the first or last key, and scan from
* there.
*/
if (keysCount == 0)
{
bool match;
match = _bt_endpoint(scan, dir);
if (!match)
{
/* No match, so mark (parallel) scan finished */
_bt_parallel_done(scan);
}
return match;
}
/*
* We want to start the scan somewhere within the index. Set up an
* insertion scankey we can use to search for the boundary point we
* identified above. The insertion scankey is built using the keys
* identified by startKeys[]. (Remaining insertion scankey fields are
* initialized after initial-positioning strategy is finalized.)
*/
Assert(keysCount <= INDEX_MAX_KEYS);
for (i = 0; i < keysCount; i++)
{
ScanKey cur = startKeys[i];
Assert(cur->sk_attno == i + 1);
if (cur->sk_flags & SK_ROW_HEADER)
{
/*
* Row comparison header: look to the first row member instead.
*
* The member scankeys are already in insertion format (ie, they
* have sk_func = 3-way-comparison function), but we have to watch
* out for nulls, which _bt_preprocess_keys didn't check. A null
* in the first row member makes the condition unmatchable, just
* like qual_ok = false.
*/
ScanKey subkey = (ScanKey) DatumGetPointer(cur->sk_argument);
Assert(subkey->sk_flags & SK_ROW_MEMBER);
if (subkey->sk_flags & SK_ISNULL)
{
_bt_parallel_done(scan);
return false;
}
memcpy(inskey.scankeys + i, subkey, sizeof(ScanKeyData));
/*
* If the row comparison is the last positioning key we accepted,
* try to add additional keys from the lower-order row members.
* (If we accepted independent conditions on additional index
* columns, we use those instead --- doesn't seem worth trying to
* determine which is more restrictive.) Note that this is OK
* even if the row comparison is of ">" or "<" type, because the
* condition applied to all but the last row member is effectively
* ">=" or "<=", and so the extra keys don't break the positioning
* scheme. But, by the same token, if we aren't able to use all
* the row members, then the part of the row comparison that we
* did use has to be treated as just a ">=" or "<=" condition, and
* so we'd better adjust strat_total accordingly.
*/
if (i == keysCount - 1)
{
bool used_all_subkeys = false;
Assert(!(subkey->sk_flags & SK_ROW_END));
for (;;)
{
subkey++;
Assert(subkey->sk_flags & SK_ROW_MEMBER);
if (subkey->sk_attno != keysCount + 1)
break; /* out-of-sequence, can't use it */
if (subkey->sk_strategy != cur->sk_strategy)
break; /* wrong direction, can't use it */
if (subkey->sk_flags & SK_ISNULL)
break; /* can't use null keys */
Assert(keysCount < INDEX_MAX_KEYS);
memcpy(inskey.scankeys + keysCount, subkey,
sizeof(ScanKeyData));
keysCount++;
if (subkey->sk_flags & SK_ROW_END)
{
used_all_subkeys = true;
break;
}
}
if (!used_all_subkeys)
{
switch (strat_total)
{
case BTLessStrategyNumber:
strat_total = BTLessEqualStrategyNumber;
break;
case BTGreaterStrategyNumber:
strat_total = BTGreaterEqualStrategyNumber;
break;
}
}
break; /* done with outer loop */
}
}
else
{
/*
* Ordinary comparison key. Transform the search-style scan key
* to an insertion scan key by replacing the sk_func with the
* appropriate btree comparison function.
*
* If scankey operator is not a cross-type comparison, we can use
* the cached comparison function; otherwise gotta look it up in
* the catalogs. (That can't lead to infinite recursion, since no
* indexscan initiated by syscache lookup will use cross-data-type
* operators.)
*
* We support the convention that sk_subtype == InvalidOid means
* the opclass input type; this is a hack to simplify life for
* ScanKeyInit().
*/
if (cur->sk_subtype == rel->rd_opcintype[i] ||
cur->sk_subtype == InvalidOid)
{
FmgrInfo *procinfo;
procinfo = index_getprocinfo(rel, cur->sk_attno, BTORDER_PROC);
ScanKeyEntryInitializeWithInfo(inskey.scankeys + i,
cur->sk_flags,
cur->sk_attno,
InvalidStrategy,
cur->sk_subtype,
cur->sk_collation,
procinfo,
cur->sk_argument);
}
else
{
RegProcedure cmp_proc;
cmp_proc = get_opfamily_proc(rel->rd_opfamily[i],
rel->rd_opcintype[i],
cur->sk_subtype,
BTORDER_PROC);
if (!RegProcedureIsValid(cmp_proc))
elog(ERROR, "missing support function %d(%u,%u) for attribute %d of index \"%s\"",
BTORDER_PROC, rel->rd_opcintype[i], cur->sk_subtype,
cur->sk_attno, RelationGetRelationName(rel));
ScanKeyEntryInitialize(inskey.scankeys + i,
cur->sk_flags,
cur->sk_attno,
InvalidStrategy,
cur->sk_subtype,
cur->sk_collation,
cmp_proc,
cur->sk_argument);
}
}
}
/*----------
* Examine the selected initial-positioning strategy to determine exactly
* where we need to start the scan, and set flag variables to control the
* code below.
*
* If nextkey = false, _bt_search and _bt_binsrch will locate the first
* item >= scan key. If nextkey = true, they will locate the first
* item > scan key.
*
* If goback = true, we will then step back one item, while if
* goback = false, we will start the scan on the located item.
*----------
*/
switch (strat_total)
{
case BTLessStrategyNumber:
/*
* Find first item >= scankey, then back up one to arrive at last
* item < scankey. (Note: this positioning strategy is only used
* for a backward scan, so that is always the correct starting
* position.)
*/
nextkey = false;
goback = true;
break;
case BTLessEqualStrategyNumber:
/*
* Find first item > scankey, then back up one to arrive at last
* item <= scankey. (Note: this positioning strategy is only used
* for a backward scan, so that is always the correct starting
* position.)
*/
nextkey = true;
goback = true;
break;
case BTEqualStrategyNumber:
/*
* If a backward scan was specified, need to start with last equal
* item not first one.
*/
if (ScanDirectionIsBackward(dir))
{
/*
* This is the same as the <= strategy. We will check at the
* end whether the found item is actually =.
*/
nextkey = true;
goback = true;
}
else
{
/*
* This is the same as the >= strategy. We will check at the
* end whether the found item is actually =.
*/
nextkey = false;
goback = false;
}
break;
case BTGreaterEqualStrategyNumber:
/*
* Find first item >= scankey. (This is only used for forward
* scans.)
*/
nextkey = false;
goback = false;
break;
case BTGreaterStrategyNumber:
/*
* Find first item > scankey. (This is only used for forward
* scans.)
*/
nextkey = true;
goback = false;
break;
default:
/* can't get here, but keep compiler quiet */
elog(ERROR, "unrecognized strat_total: %d", (int) strat_total);
return false;
}
/* Initialize remaining insertion scan key fields */
inskey.heapkeyspace = _bt_heapkeyspace(rel);
inskey.anynullkeys = false; /* unused */
inskey.nextkey = nextkey;
inskey.pivotsearch = false;
inskey.scantid = NULL;
inskey.keysz = keysCount;
/*
* Use the manufactured insertion scan key to descend the tree and
* position ourselves on the target leaf page.
*/
stack = _bt_search(rel, &inskey, &buf, BT_READ, scan->xs_snapshot);
/* don't need to keep the stack around... */
_bt_freestack(stack);
if (!BufferIsValid(buf))
{
/*
* We only get here if the index is completely empty. Lock relation
* because nothing finer to lock exists.
*/
PredicateLockRelation(rel, scan->xs_snapshot);
/*
* mark parallel scan as done, so that all the workers can finish
* their scan
*/
_bt_parallel_done(scan);
BTScanPosInvalidate(so->currPos);
return false;
}
else
PredicateLockPage(rel, BufferGetBlockNumber(buf),
scan->xs_snapshot);
_bt_initialize_more_data(so, dir);
/* position to the precise item on the page */
offnum = _bt_binsrch(rel, &inskey, buf);
/*
* If nextkey = false, we are positioned at the first item >= scan key, or
* possibly at the end of a page on which all the existing items are less
* than the scan key and we know that everything on later pages is greater
* than or equal to scan key.
*
* If nextkey = true, we are positioned at the first item > scan key, or
* possibly at the end of a page on which all the existing items are less
* than or equal to the scan key and we know that everything on later
* pages is greater than scan key.
*
* The actually desired starting point is either this item or the prior
* one, or in the end-of-page case it's the first item on the next page or
* the last item on this page. Adjust the starting offset if needed. (If
* this results in an offset before the first item or after the last one,
* _bt_readpage will report no items found, and then we'll step to the
* next page as needed.)
*/
if (goback)
offnum = OffsetNumberPrev(offnum);
/* remember which buffer we have pinned, if any */
Assert(!BTScanPosIsValid(so->currPos));
so->currPos.buf = buf;
/*
* Now load data from the first page of the scan.
*/
if (!_bt_readpage(scan, dir, offnum))
{
/*
* There's no actually-matching data on this page. Try to advance to
* the next page. Return false if there's no matching data at all.
*/
LockBuffer(so->currPos.buf, BUFFER_LOCK_UNLOCK);
if (!_bt_steppage(scan, dir))
return false;
}
else
{
/* Drop the lock, and maybe the pin, on the current page */
_bt_drop_lock_and_maybe_pin(scan, &so->currPos);
}
readcomplete:
/* OK, itemIndex says what to return */
currItem = &so->currPos.items[so->currPos.itemIndex];
scan->xs_heaptid = currItem->heapTid;
if (scan->xs_want_itup)
scan->xs_itup = (IndexTuple) (so->currTuples + currItem->tupleOffset);
return true;
}
/*
* _bt_next() -- Get the next item in a scan.
*
* On entry, so->currPos describes the current page, which may be pinned
* but is not locked, and so->currPos.itemIndex identifies which item was
* previously returned.
*
* On successful exit, scan->xs_ctup.t_self is set to the TID of the
* next heap tuple, and if requested, scan->xs_itup points to a copy of
* the index tuple. so->currPos is updated as needed.
*
* On failure exit (no more tuples), we release pin and set
* so->currPos.buf to InvalidBuffer.
*/
bool
_bt_next(IndexScanDesc scan, ScanDirection dir)
{
BTScanOpaque so = (BTScanOpaque) scan->opaque;
BTScanPosItem *currItem;
/*
* Advance to next tuple on current page; or if there's no more, try to
* step to the next page with data.
*/
if (ScanDirectionIsForward(dir))
{
if (++so->currPos.itemIndex > so->currPos.lastItem)
{
if (!_bt_steppage(scan, dir))
return false;
}
}
else
{
if (--so->currPos.itemIndex < so->currPos.firstItem)
{
if (!_bt_steppage(scan, dir))
return false;
}
}
/* OK, itemIndex says what to return */
currItem = &so->currPos.items[so->currPos.itemIndex];
scan->xs_heaptid = currItem->heapTid;
if (scan->xs_want_itup)
scan->xs_itup = (IndexTuple) (so->currTuples + currItem->tupleOffset);
return true;
}
/*
* _bt_readpage() -- Load data from current index page into so->currPos
*
* Caller must have pinned and read-locked so->currPos.buf; the buffer's state
* is not changed here. Also, currPos.moreLeft and moreRight must be valid;
* they are updated as appropriate. All other fields of so->currPos are
* initialized from scratch here.
*
* We scan the current page starting at offnum and moving in the indicated
* direction. All items matching the scan keys are loaded into currPos.items.
* moreLeft or moreRight (as appropriate) is cleared if _bt_checkkeys reports
* that there can be no more matching tuples in the current scan direction.
*
* In the case of a parallel scan, caller must have called _bt_parallel_seize
* prior to calling this function; this function will invoke
* _bt_parallel_release before returning.
*
* Returns true if any matching items found on the page, false if none.
*/
static bool
_bt_readpage(IndexScanDesc scan, ScanDirection dir, OffsetNumber offnum)
{
BTScanOpaque so = (BTScanOpaque) scan->opaque;
Page page;
BTPageOpaque opaque;
OffsetNumber minoff;
OffsetNumber maxoff;
int itemIndex;
bool continuescan;
int indnatts;
/*
* We must have the buffer pinned and locked, but the usual macro can't be
* used here; this function is what makes it good for currPos.
*/
Assert(BufferIsValid(so->currPos.buf));
page = BufferGetPage(so->currPos.buf);
opaque = (BTPageOpaque) PageGetSpecialPointer(page);
/* allow next page be processed by parallel worker */
if (scan->parallel_scan)
{
if (ScanDirectionIsForward(dir))
_bt_parallel_release(scan, opaque->btpo_next);
else
_bt_parallel_release(scan, BufferGetBlockNumber(so->currPos.buf));
}
continuescan = true; /* default assumption */
indnatts = IndexRelationGetNumberOfAttributes(scan->indexRelation);
minoff = P_FIRSTDATAKEY(opaque);
maxoff = PageGetMaxOffsetNumber(page);
/*
* We note the buffer's block number so that we can release the pin later.
* This allows us to re-read the buffer if it is needed again for hinting.
*/
so->currPos.currPage = BufferGetBlockNumber(so->currPos.buf);
/*
* We save the LSN of the page as we read it, so that we know whether it
* safe to apply LP_DEAD hints to the page later. This allows us to drop
* the pin for MVCC scans, which allows vacuum to avoid blocking.
*/
so->currPos.lsn = BufferGetLSNAtomic(so->currPos.buf);
/*
* we must save the page's right-link while scanning it; this tells us
* where to step right to after we're done with these items. There is no
* corresponding need for the left-link, since splits always go right.
*/
so->currPos.nextPage = opaque->btpo_next;
/* initialize tuple workspace to empty */
so->currPos.nextTupleOffset = 0;
/*
* Now that the current page has been made consistent, the macro should be
* good.
*/
Assert(BTScanPosIsPinned(so->currPos));
if (ScanDirectionIsForward(dir))
{
/* load items[] in ascending order */
itemIndex = 0;
offnum = Max(offnum, minoff);
while (offnum <= maxoff)
{
ItemId iid = PageGetItemId(page, offnum);
IndexTuple itup;
/*
* If the scan specifies not to return killed tuples, then we
* treat a killed tuple as not passing the qual
*/
if (scan->ignore_killed_tuples && ItemIdIsDead(iid))
{
offnum = OffsetNumberNext(offnum);
continue;
}
itup = (IndexTuple) PageGetItem(page, iid);
if (_bt_checkkeys(scan, itup, indnatts, dir, &continuescan))
{
/* tuple passes all scan key conditions, so remember it */
_bt_saveitem(so, itemIndex, offnum, itup);
itemIndex++;
}
/* When !continuescan, there can't be any more matches, so stop */
if (!continuescan)
break;
offnum = OffsetNumberNext(offnum);
}
/*
* We don't need to visit page to the right when the high key
* indicates that no more matches will be found there.
*
* Checking the high key like this works out more often than you might
* think. Leaf page splits pick a split point between the two most
* dissimilar tuples (this is weighed against the need to evenly share
* free space). Leaf pages with high key attribute values that can
* only appear on non-pivot tuples on the right sibling page are
* common.
*/
if (continuescan && !P_RIGHTMOST(opaque))
{
ItemId iid = PageGetItemId(page, P_HIKEY);
IndexTuple itup = (IndexTuple) PageGetItem(page, iid);
int truncatt;
truncatt = BTreeTupleGetNAtts(itup, scan->indexRelation);
_bt_checkkeys(scan, itup, truncatt, dir, &continuescan);
}
if (!continuescan)
so->currPos.moreRight = false;
Assert(itemIndex <= MaxIndexTuplesPerPage);
so->currPos.firstItem = 0;
so->currPos.lastItem = itemIndex - 1;
so->currPos.itemIndex = 0;
}
else
{
/* load items[] in descending order */
itemIndex = MaxIndexTuplesPerPage;
offnum = Min(offnum, maxoff);
while (offnum >= minoff)
{
ItemId iid = PageGetItemId(page, offnum);
IndexTuple itup;
bool tuple_alive;
bool passes_quals;
/*
* If the scan specifies not to return killed tuples, then we
* treat a killed tuple as not passing the qual. Most of the
* time, it's a win to not bother examining the tuple's index
* keys, but just skip to the next tuple (previous, actually,
* since we're scanning backwards). However, if this is the first
* tuple on the page, we do check the index keys, to prevent
* uselessly advancing to the page to the left. This is similar
* to the high key optimization used by forward scans.
*/
if (scan->ignore_killed_tuples && ItemIdIsDead(iid))
{
Assert(offnum >= P_FIRSTDATAKEY(opaque));
if (offnum > P_FIRSTDATAKEY(opaque))
{
offnum = OffsetNumberPrev(offnum);
continue;
}
tuple_alive = false;
}
else
tuple_alive = true;
itup = (IndexTuple) PageGetItem(page, iid);
passes_quals = _bt_checkkeys(scan, itup, indnatts, dir,
&continuescan);
if (passes_quals && tuple_alive)
{
/* tuple passes all scan key conditions, so remember it */
itemIndex--;
_bt_saveitem(so, itemIndex, offnum, itup);
}
if (!continuescan)
{
/* there can't be any more matches, so stop */
so->currPos.moreLeft = false;
break;
}
offnum = OffsetNumberPrev(offnum);
}
Assert(itemIndex >= 0);
so->currPos.firstItem = itemIndex;
so->currPos.lastItem = MaxIndexTuplesPerPage - 1;
so->currPos.itemIndex = MaxIndexTuplesPerPage - 1;
}
return (so->currPos.firstItem <= so->currPos.lastItem);
}
/* Save an index item into so->currPos.items[itemIndex] */
static void
_bt_saveitem(BTScanOpaque so, int itemIndex,
OffsetNumber offnum, IndexTuple itup)
{
BTScanPosItem *currItem = &so->currPos.items[itemIndex];
currItem->heapTid = itup->t_tid;
currItem->indexOffset = offnum;
if (so->currTuples)
{
Size itupsz = IndexTupleSize(itup);
currItem->tupleOffset = so->currPos.nextTupleOffset;
memcpy(so->currTuples + so->currPos.nextTupleOffset, itup, itupsz);
so->currPos.nextTupleOffset += MAXALIGN(itupsz);
}
}
/*
* _bt_steppage() -- Step to next page containing valid data for scan
*
* On entry, if so->currPos.buf is valid the buffer is pinned but not locked;
* if pinned, we'll drop the pin before moving to next page. The buffer is
* not locked on entry.
*
* For success on a scan using a non-MVCC snapshot we hold a pin, but not a
* read lock, on that page. If we do not hold the pin, we set so->currPos.buf
* to InvalidBuffer. We return true to indicate success.
*/
static bool
_bt_steppage(IndexScanDesc scan, ScanDirection dir)
{
BTScanOpaque so = (BTScanOpaque) scan->opaque;
BlockNumber blkno = InvalidBlockNumber;
bool status = true;
Assert(BTScanPosIsValid(so->currPos));
/* Before leaving current page, deal with any killed items */
if (so->numKilled > 0)
_bt_killitems(scan);
/*
* Before we modify currPos, make a copy of the page data if there was a
* mark position that needs it.
*/
if (so->markItemIndex >= 0)
{
/* bump pin on current buffer for assignment to mark buffer */
if (BTScanPosIsPinned(so->currPos))
IncrBufferRefCount(so->currPos.buf);
memcpy(&so->markPos, &so->currPos,
offsetof(BTScanPosData, items[1]) +
so->currPos.lastItem * sizeof(BTScanPosItem));
if (so->markTuples)
memcpy(so->markTuples, so->currTuples,
so->currPos.nextTupleOffset);
so->markPos.itemIndex = so->markItemIndex;
so->markItemIndex = -1;
}
if (ScanDirectionIsForward(dir))
{
/* Walk right to the next page with data */
if (scan->parallel_scan != NULL)
{
/*
* Seize the scan to get the next block number; if the scan has
* ended already, bail out.
*/
status = _bt_parallel_seize(scan, &blkno);
if (!status)
{
/* release the previous buffer, if pinned */
BTScanPosUnpinIfPinned(so->currPos);
BTScanPosInvalidate(so->currPos);
return false;
}
}
else
{
/* Not parallel, so use the previously-saved nextPage link. */
blkno = so->currPos.nextPage;
}
/* Remember we left a page with data */
so->currPos.moreLeft = true;
/* release the previous buffer, if pinned */
BTScanPosUnpinIfPinned(so->currPos);
}
else
{
/* Remember we left a page with data */
so->currPos.moreRight = true;
if (scan->parallel_scan != NULL)
{
/*
* Seize the scan to get the current block number; if the scan has
* ended already, bail out.
*/
status = _bt_parallel_seize(scan, &blkno);
BTScanPosUnpinIfPinned(so->currPos);
if (!status)
{
BTScanPosInvalidate(so->currPos);
return false;
}
}
else
{
/* Not parallel, so just use our own notion of the current page */
blkno = so->currPos.currPage;
}
}
if (!_bt_readnextpage(scan, blkno, dir))
return false;
/* Drop the lock, and maybe the pin, on the current page */
_bt_drop_lock_and_maybe_pin(scan, &so->currPos);
return true;
}
/*
* _bt_readnextpage() -- Read next page containing valid data for scan
*
* On success exit, so->currPos is updated to contain data from the next
* interesting page. Caller is responsible to release lock and pin on
* buffer on success. We return true to indicate success.
*
* If there are no more matching records in the given direction, we drop all
* locks and pins, set so->currPos.buf to InvalidBuffer, and return false.
*/
static bool
_bt_readnextpage(IndexScanDesc scan, BlockNumber blkno, ScanDirection dir)
{
BTScanOpaque so = (BTScanOpaque) scan->opaque;
Relation rel;
Page page;
BTPageOpaque opaque;
bool status = true;
rel = scan->indexRelation;
if (ScanDirectionIsForward(dir))
{
for (;;)
{
/*
* if we're at end of scan, give up and mark parallel scan as
* done, so that all the workers can finish their scan
*/
if (blkno == P_NONE || !so->currPos.moreRight)
{
_bt_parallel_done(scan);
BTScanPosInvalidate(so->currPos);
return false;
}
/* check for interrupts while we're not holding any buffer lock */
CHECK_FOR_INTERRUPTS();
/* step right one page */
so->currPos.buf = _bt_getbuf(rel, blkno, BT_READ);
page = BufferGetPage(so->currPos.buf);
TestForOldSnapshot(scan->xs_snapshot, rel, page);
opaque = (BTPageOpaque) PageGetSpecialPointer(page);
/* check for deleted page */
if (!P_IGNORE(opaque))
{
PredicateLockPage(rel, blkno, scan->xs_snapshot);
/* see if there are any matches on this page */
/* note that this will clear moreRight if we can stop */
if (_bt_readpage(scan, dir, P_FIRSTDATAKEY(opaque)))
break;
}
else if (scan->parallel_scan != NULL)
{
/* allow next page be processed by parallel worker */
_bt_parallel_release(scan, opaque->btpo_next);
}
/* nope, keep going */
if (scan->parallel_scan != NULL)
{
_bt_relbuf(rel, so->currPos.buf);
status = _bt_parallel_seize(scan, &blkno);
if (!status)
{
BTScanPosInvalidate(so->currPos);
return false;
}
}
else
{
blkno = opaque->btpo_next;
_bt_relbuf(rel, so->currPos.buf);
}
}
}
else
{
/*
* Should only happen in parallel cases, when some other backend
* advanced the scan.
*/
if (so->currPos.currPage != blkno)
{
BTScanPosUnpinIfPinned(so->currPos);
so->currPos.currPage = blkno;
}
/*
* Walk left to the next page with data. This is much more complex
* than the walk-right case because of the possibility that the page
* to our left splits while we are in flight to it, plus the
* possibility that the page we were on gets deleted after we leave
* it. See nbtree/README for details.
*
* It might be possible to rearrange this code to have less overhead
* in pinning and locking, but that would require capturing the left
* pointer when the page is initially read, and using it here, along
* with big changes to _bt_walk_left() and the code below. It is not
* clear whether this would be a win, since if the page immediately to
* the left splits after we read this page and before we step left, we
* would need to visit more pages than with the current code.
*
* Note that if we change the code so that we drop the pin for a scan
* which uses a non-MVCC snapshot, we will need to modify the code for
* walking left, to allow for the possibility that a referenced page
* has been deleted. As long as the buffer is pinned or the snapshot
* is MVCC the page cannot move past the half-dead state to fully
* deleted.
*/
if (BTScanPosIsPinned(so->currPos))
LockBuffer(so->currPos.buf, BT_READ);
else
so->currPos.buf = _bt_getbuf(rel, so->currPos.currPage, BT_READ);
for (;;)
{
/* Done if we know there are no matching keys to the left */
if (!so->currPos.moreLeft)
{
_bt_relbuf(rel, so->currPos.buf);
_bt_parallel_done(scan);
BTScanPosInvalidate(so->currPos);
return false;
}
/* Step to next physical page */
so->currPos.buf = _bt_walk_left(rel, so->currPos.buf,
scan->xs_snapshot);
/* if we're physically at end of index, return failure */
if (so->currPos.buf == InvalidBuffer)
{
_bt_parallel_done(scan);
BTScanPosInvalidate(so->currPos);
return false;
}
/*
* Okay, we managed to move left to a non-deleted page. Done if
* it's not half-dead and contains matching tuples. Else loop back
* and do it all again.
*/
page = BufferGetPage(so->currPos.buf);
TestForOldSnapshot(scan->xs_snapshot, rel, page);
opaque = (BTPageOpaque) PageGetSpecialPointer(page);
if (!P_IGNORE(opaque))
{
PredicateLockPage(rel, BufferGetBlockNumber(so->currPos.buf), scan->xs_snapshot);
/* see if there are any matches on this page */
/* note that this will clear moreLeft if we can stop */
if (_bt_readpage(scan, dir, PageGetMaxOffsetNumber(page)))
break;
}
else if (scan->parallel_scan != NULL)
{
/* allow next page be processed by parallel worker */
_bt_parallel_release(scan, BufferGetBlockNumber(so->currPos.buf));
}
/*
* For parallel scans, get the last page scanned as it is quite
* possible that by the time we try to seize the scan, some other
* worker has already advanced the scan to a different page. We
* must continue based on the latest page scanned by any worker.
*/
if (scan->parallel_scan != NULL)
{
_bt_relbuf(rel, so->currPos.buf);
status = _bt_parallel_seize(scan, &blkno);
if (!status)
{
BTScanPosInvalidate(so->currPos);
return false;
}
so->currPos.buf = _bt_getbuf(rel, blkno, BT_READ);
}
}
}
return true;
}
/*
* _bt_parallel_readpage() -- Read current page containing valid data for scan
*
* On success, release lock and maybe pin on buffer. We return true to
* indicate success.
*/
static bool
_bt_parallel_readpage(IndexScanDesc scan, BlockNumber blkno, ScanDirection dir)
{
BTScanOpaque so = (BTScanOpaque) scan->opaque;
_bt_initialize_more_data(so, dir);
if (!_bt_readnextpage(scan, blkno, dir))
return false;
/* Drop the lock, and maybe the pin, on the current page */
_bt_drop_lock_and_maybe_pin(scan, &so->currPos);
return true;
}
/*
* _bt_walk_left() -- step left one page, if possible
*
* The given buffer must be pinned and read-locked. This will be dropped
* before stepping left. On return, we have pin and read lock on the
* returned page, instead.
*
* Returns InvalidBuffer if there is no page to the left (no lock is held
* in that case).
*
* When working on a non-leaf level, it is possible for the returned page
* to be half-dead; the caller should check that condition and step left
* again if it's important.
*/
static Buffer
_bt_walk_left(Relation rel, Buffer buf, Snapshot snapshot)
{
Page page;
BTPageOpaque opaque;
page = BufferGetPage(buf);
opaque = (BTPageOpaque) PageGetSpecialPointer(page);
for (;;)
{
BlockNumber obknum;
BlockNumber lblkno;
BlockNumber blkno;
int tries;
/* if we're at end of tree, release buf and return failure */
if (P_LEFTMOST(opaque))
{
_bt_relbuf(rel, buf);
break;
}
/* remember original page we are stepping left from */
obknum = BufferGetBlockNumber(buf);
/* step left */
blkno = lblkno = opaque->btpo_prev;
_bt_relbuf(rel, buf);
/* check for interrupts while we're not holding any buffer lock */
CHECK_FOR_INTERRUPTS();
buf = _bt_getbuf(rel, blkno, BT_READ);
page = BufferGetPage(buf);
TestForOldSnapshot(snapshot, rel, page);
opaque = (BTPageOpaque) PageGetSpecialPointer(page);
/*
* If this isn't the page we want, walk right till we find what we
* want --- but go no more than four hops (an arbitrary limit). If we
* don't find the correct page by then, the most likely bet is that
* the original page got deleted and isn't in the sibling chain at all
* anymore, not that its left sibling got split more than four times.
*
* Note that it is correct to test P_ISDELETED not P_IGNORE here,
* because half-dead pages are still in the sibling chain. Caller
* must reject half-dead pages if wanted.
*/
tries = 0;
for (;;)
{
if (!P_ISDELETED(opaque) && opaque->btpo_next == obknum)
{
/* Found desired page, return it */
return buf;
}
if (P_RIGHTMOST(opaque) || ++tries > 4)
break;
blkno = opaque->btpo_next;
buf = _bt_relandgetbuf(rel, buf, blkno, BT_READ);
page = BufferGetPage(buf);
TestForOldSnapshot(snapshot, rel, page);
opaque = (BTPageOpaque) PageGetSpecialPointer(page);
}
/* Return to the original page to see what's up */
buf = _bt_relandgetbuf(rel, buf, obknum, BT_READ);
page = BufferGetPage(buf);
TestForOldSnapshot(snapshot, rel, page);
opaque = (BTPageOpaque) PageGetSpecialPointer(page);
if (P_ISDELETED(opaque))
{
/*
* It was deleted. Move right to first nondeleted page (there
* must be one); that is the page that has acquired the deleted
* one's keyspace, so stepping left from it will take us where we
* want to be.
*/
for (;;)
{
if (P_RIGHTMOST(opaque))
elog(ERROR, "fell off the end of index \"%s\"",
RelationGetRelationName(rel));
blkno = opaque->btpo_next;
buf = _bt_relandgetbuf(rel, buf, blkno, BT_READ);
page = BufferGetPage(buf);
TestForOldSnapshot(snapshot, rel, page);
opaque = (BTPageOpaque) PageGetSpecialPointer(page);
if (!P_ISDELETED(opaque))
break;
}
/*
* Now return to top of loop, resetting obknum to point to this
* nondeleted page, and try again.
*/
}
else
{
/*
* It wasn't deleted; the explanation had better be that the page
* to the left got split or deleted. Without this check, we'd go
* into an infinite loop if there's anything wrong.
*/
if (opaque->btpo_prev == lblkno)
elog(ERROR, "could not find left sibling of block %u in index \"%s\"",
obknum, RelationGetRelationName(rel));
/* Okay to try again with new lblkno value */
}
}
return InvalidBuffer;
}
/*
* _bt_get_endpoint() -- Find the first or last page on a given tree level
*
* If the index is empty, we will return InvalidBuffer; any other failure
* condition causes ereport(). We will not return a dead page.
*
* The returned buffer is pinned and read-locked.
*/
Buffer
_bt_get_endpoint(Relation rel, uint32 level, bool rightmost,
Snapshot snapshot)
{
Buffer buf;
Page page;
BTPageOpaque opaque;
OffsetNumber offnum;
BlockNumber blkno;
IndexTuple itup;
/*
* If we are looking for a leaf page, okay to descend from fast root;
* otherwise better descend from true root. (There is no point in being
* smarter about intermediate levels.)
*/
if (level == 0)
buf = _bt_getroot(rel, BT_READ);
else
buf = _bt_gettrueroot(rel);
if (!BufferIsValid(buf))
return InvalidBuffer;
page = BufferGetPage(buf);
TestForOldSnapshot(snapshot, rel, page);
opaque = (BTPageOpaque) PageGetSpecialPointer(page);
for (;;)
{
/*
* If we landed on a deleted page, step right to find a live page
* (there must be one). Also, if we want the rightmost page, step
* right if needed to get to it (this could happen if the page split
* since we obtained a pointer to it).
*/
while (P_IGNORE(opaque) ||
(rightmost && !P_RIGHTMOST(opaque)))
{
blkno = opaque->btpo_next;
if (blkno == P_NONE)
elog(ERROR, "fell off the end of index \"%s\"",
RelationGetRelationName(rel));
buf = _bt_relandgetbuf(rel, buf, blkno, BT_READ);
page = BufferGetPage(buf);
TestForOldSnapshot(snapshot, rel, page);
opaque = (BTPageOpaque) PageGetSpecialPointer(page);
}
/* Done? */
if (opaque->btpo.level == level)
break;
if (opaque->btpo.level < level)
elog(ERROR, "btree level %u not found in index \"%s\"",
level, RelationGetRelationName(rel));
/* Descend to leftmost or rightmost child page */
if (rightmost)
offnum = PageGetMaxOffsetNumber(page);
else
offnum = P_FIRSTDATAKEY(opaque);
itup = (IndexTuple) PageGetItem(page, PageGetItemId(page, offnum));
blkno = BTreeInnerTupleGetDownLink(itup);
buf = _bt_relandgetbuf(rel, buf, blkno, BT_READ);
page = BufferGetPage(buf);
opaque = (BTPageOpaque) PageGetSpecialPointer(page);
}
return buf;
}
/*
* _bt_endpoint() -- Find the first or last page in the index, and scan
* from there to the first key satisfying all the quals.
*
* This is used by _bt_first() to set up a scan when we've determined
* that the scan must start at the beginning or end of the index (for
* a forward or backward scan respectively). Exit conditions are the
* same as for _bt_first().
*/
static bool
_bt_endpoint(IndexScanDesc scan, ScanDirection dir)
{
Relation rel = scan->indexRelation;
BTScanOpaque so = (BTScanOpaque) scan->opaque;
Buffer buf;
Page page;
BTPageOpaque opaque;
OffsetNumber start;
BTScanPosItem *currItem;
/*
* Scan down to the leftmost or rightmost leaf page. This is a simplified
* version of _bt_search(). We don't maintain a stack since we know we
* won't need it.
*/
buf = _bt_get_endpoint(rel, 0, ScanDirectionIsBackward(dir), scan->xs_snapshot);
if (!BufferIsValid(buf))
{
/*
* Empty index. Lock the whole relation, as nothing finer to lock
* exists.
*/
PredicateLockRelation(rel, scan->xs_snapshot);
BTScanPosInvalidate(so->currPos);
return false;
}
PredicateLockPage(rel, BufferGetBlockNumber(buf), scan->xs_snapshot);
page = BufferGetPage(buf);
opaque = (BTPageOpaque) PageGetSpecialPointer(page);
Assert(P_ISLEAF(opaque));
if (ScanDirectionIsForward(dir))
{
/* There could be dead pages to the left, so not this: */
/* Assert(P_LEFTMOST(opaque)); */
start = P_FIRSTDATAKEY(opaque);
}
else if (ScanDirectionIsBackward(dir))
{
Assert(P_RIGHTMOST(opaque));
start = PageGetMaxOffsetNumber(page);
}
else
{
elog(ERROR, "invalid scan direction: %d", (int) dir);
start = 0; /* keep compiler quiet */
}
/* remember which buffer we have pinned */
so->currPos.buf = buf;
_bt_initialize_more_data(so, dir);
/*
* Now load data from the first page of the scan.
*/
if (!_bt_readpage(scan, dir, start))
{
/*
* There's no actually-matching data on this page. Try to advance to
* the next page. Return false if there's no matching data at all.
*/
LockBuffer(so->currPos.buf, BUFFER_LOCK_UNLOCK);
if (!_bt_steppage(scan, dir))
return false;
}
else
{
/* Drop the lock, and maybe the pin, on the current page */
_bt_drop_lock_and_maybe_pin(scan, &so->currPos);
}
/* OK, itemIndex says what to return */
currItem = &so->currPos.items[so->currPos.itemIndex];
scan->xs_heaptid = currItem->heapTid;
if (scan->xs_want_itup)
scan->xs_itup = (IndexTuple) (so->currTuples + currItem->tupleOffset);
return true;
}
/*
* _bt_initialize_more_data() -- initialize moreLeft/moreRight appropriately
* for scan direction
*/
static inline void
_bt_initialize_more_data(BTScanOpaque so, ScanDirection dir)
{
/* initialize moreLeft/moreRight appropriately for scan direction */
if (ScanDirectionIsForward(dir))
{
so->currPos.moreLeft = false;
so->currPos.moreRight = true;
}
else
{
so->currPos.moreLeft = true;
so->currPos.moreRight = false;
}
so->numKilled = 0; /* just paranoia */
so->markItemIndex = -1; /* ditto */
}
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