Revise _bt_binsrch() so that its binary search loop takes
care of equal-key cases, eliminating bt_firsteq(). The linear search formerly done by bt_firsteq() took a lot of time in the case where many equal keys appear on the same page.
This commit is contained in:
Tom Lane
parent
9679cb3910
commit
bc9236bc01
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@ -7,7 +7,7 @@
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*
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*
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* IDENTIFICATION
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* $Header: /cvsroot/pgsql/src/backend/access/nbtree/nbtsearch.c,v 1.50 1999/07/16 04:58:30 momjian Exp $
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* $Header: /cvsroot/pgsql/src/backend/access/nbtree/nbtsearch.c,v 1.51 1999/07/16 22:17:06 tgl Exp $
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*
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*-------------------------------------------------------------------------
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*/
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@ -26,8 +26,6 @@
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static BTStack _bt_searchr(Relation rel, int keysz, ScanKey scankey,
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Buffer *bufP, BTStack stack_in);
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static OffsetNumber _bt_firsteq(Relation rel, TupleDesc itupdesc, Page page,
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Size keysz, ScanKey scankey, OffsetNumber offnum);
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static int _bt_compare(Relation rel, TupleDesc itupdesc, Page page,
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int keysz, ScanKey scankey, OffsetNumber offnum);
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static bool
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@ -368,7 +366,9 @@ _bt_skeycmp(Relation rel,
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* comparison for every key in the scankey. _bt_binsrch() returns
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* the OffsetNumber of the first matching key on the page, or the
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* OffsetNumber at which the matching key would appear if it were
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* on this page.
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* on this page. (NOTE: in particular, this means it is possible to
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* return a value 1 greater than the number of keys on the page, if
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* the scankey is > all keys on the page.)
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*
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* By the time this procedure is called, we're sure we're looking
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* at the right page -- don't need to walk right. _bt_binsrch() has
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@ -385,8 +385,8 @@ _bt_binsrch(Relation rel,
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Page page;
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BTPageOpaque opaque;
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OffsetNumber low,
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mid,
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high;
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bool haveEq;
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int natts = rel->rd_rel->relnatts;
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int result;
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@ -395,148 +395,112 @@ _bt_binsrch(Relation rel,
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opaque = (BTPageOpaque) PageGetSpecialPointer(page);
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/* by convention, item 1 on any non-rightmost page is the high key */
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low = mid = P_RIGHTMOST(opaque) ? P_HIKEY : P_FIRSTKEY;
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low = P_RIGHTMOST(opaque) ? P_HIKEY : P_FIRSTKEY;
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high = PageGetMaxOffsetNumber(page);
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/*
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* Since for non-rightmost pages, the first item on the page is the
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* high key, there are two notions of emptiness. One is if nothing
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* appears on the page. The other is if nothing but the high key
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* does. The reason we test high <= low, rather than high == low, is
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* that after vacuuming there may be nothing *but* the high key on a
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* page. In that case, given the scheme above, low = 2 and high = 1.
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* If there are no keys on the page, return the first available slot.
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* Note this covers two cases: the page is really empty (no keys),
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* or it contains only a high key. The latter case is possible after
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* vacuuming.
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*/
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if (PageIsEmpty(page))
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if (high < low)
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return low;
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if ((!P_RIGHTMOST(opaque) && high <= low))
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{
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if (high < low ||
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(srchtype == BT_DESCENT && !(opaque->btpo_flags & BTP_LEAF)))
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return low;
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/* It's insertion and high == low == 2 */
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result = _bt_compare(rel, itupdesc, page, keysz, scankey, low);
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if (result > 0)
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return OffsetNumberNext(low);
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return low;
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}
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while ((high - low) > 1)
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/*
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* Binary search to find the first key on the page >= scan key.
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* Loop invariant: all slots before 'low' are < scan key, all slots
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* at or after 'high' are >= scan key. Also, haveEq is true if the
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* tuple at 'high' is == scan key.
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* We can fall out when high == low.
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*/
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high++; /* establish the loop invariant for high */
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haveEq = false;
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while (high > low)
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{
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mid = low + ((high - low) / 2);
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OffsetNumber mid = low + ((high - low) / 2);
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/* We have low <= mid < high, so mid points at a real slot */
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result = _bt_compare(rel, itupdesc, page, keysz, scankey, mid);
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if (result > 0)
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low = mid;
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else if (result < 0)
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high = mid - 1;
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low = mid + 1;
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else
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{
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mid = _bt_firsteq(rel, itupdesc, page, keysz, scankey, mid);
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/*
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* NOTE for multi-column indices: we may do scan using keys
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* not for all attrs. But we handle duplicates using all attrs
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* in _bt_insert/_bt_spool code. And so while searching on
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* internal pages having number of attrs > keysize we want to
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* point at the last item < the scankey, not at the first item
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* = the scankey (!!!), and let _bt_moveright decide later
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* whether to move right or not (see comments and example
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* there). Note also that INSERTions are not affected by this
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* code (natts == keysz). - vadim 04/15/97
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*/
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if (natts == keysz || opaque->btpo_flags & BTP_LEAF)
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return mid;
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low = P_RIGHTMOST(opaque) ? P_HIKEY : P_FIRSTKEY;
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if (mid == low)
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return mid;
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return OffsetNumberPrev(mid);
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high = mid;
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haveEq = (result == 0);
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}
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}
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/*
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* We terminated because the endpoints got too close together. There
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* are two cases to take care of.
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/*--------------------
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* At this point we have high == low, but be careful: they could point
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* past the last slot on the page. We also know that haveEq is true
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* if and only if there is an equal key (in which case high&low point
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* at the first equal key).
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*
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* For non-insertion searches on internal pages, we want to point at the
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* last key <, or first key =, the scankey on the page. This
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* guarantees that we'll descend the tree correctly. (NOTE comments
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* above for multi-column indices).
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*
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* For all other cases, we want to point at the first key >= the scankey
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* on the page. This guarantees that scans and insertions will happen
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* correctly.
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* On a leaf page, we always return the first key >= scan key
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* (which could be the last slot + 1).
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*--------------------
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*/
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if (!(opaque->btpo_flags & BTP_LEAF) && srchtype == BT_DESCENT)
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{ /* We want the last key <, or first key
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* ==, the scan key. */
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result = _bt_compare(rel, itupdesc, page, keysz, scankey, high);
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if (opaque->btpo_flags & BTP_LEAF)
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return low;
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if (result == 0)
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{
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mid = _bt_firsteq(rel, itupdesc, page, keysz, scankey, high);
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/*--------------------
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* On a non-leaf page, there are special cases:
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*
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* For an insertion (srchtype != BT_DESCENT and natts == keysz)
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* always return first key >= scan key (which could be off the end).
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*
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* For a standard search (srchtype == BT_DESCENT and natts == keysz)
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* return the first equal key if one exists, else the last lesser key
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* if one exists, else the first slot on the page.
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*
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* For a partial-match search (srchtype == BT_DESCENT and natts < keysz)
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* return the last lesser key if one exists, else the first slot.
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*
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* Old comments:
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* For multi-column indices, we may scan using keys
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* not for all attrs. But we handle duplicates using all attrs
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* in _bt_insert/_bt_spool code. And so while searching on
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* internal pages having number of attrs > keysize we want to
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* point at the last item < the scankey, not at the first item
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* = the scankey (!!!), and let _bt_moveright decide later
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* whether to move right or not (see comments and example
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* there). Note also that INSERTions are not affected by this
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* code (since natts == keysz for inserts). - vadim 04/15/97
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*--------------------
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*/
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/*
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* If natts > keysz we want last item < the scan key. See
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* comments above for multi-column indices.
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*/
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if (natts == keysz)
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return mid;
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low = P_RIGHTMOST(opaque) ? P_HIKEY : P_FIRSTKEY;
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if (mid == low)
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return mid;
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return OffsetNumberPrev(mid);
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}
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else if (result > 0)
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return high;
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else
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return low;
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if (haveEq)
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{
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/*
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* There is an equal key. We return either the first equal key
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* (which we just found), or the last lesser key.
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*
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* We need not check srchtype != BT_DESCENT here, since if that
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* is true then natts == keysz by assumption.
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*/
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if (natts == keysz)
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return low; /* return first equal key */
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}
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else
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/* we want the first key >= the scan key */
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{
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result = _bt_compare(rel, itupdesc, page, keysz, scankey, low);
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if (result <= 0)
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return low;
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else
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{
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if (low == high)
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return OffsetNumberNext(low);
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result = _bt_compare(rel, itupdesc, page, keysz, scankey, high);
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if (result <= 0)
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return high;
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else
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return OffsetNumberNext(high);
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}
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/*
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* There is no equal key. We return either the first greater key
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* (which we just found), or the last lesser key.
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*/
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if (srchtype != BT_DESCENT)
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return low; /* return first greater key */
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}
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}
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static OffsetNumber
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_bt_firsteq(Relation rel,
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TupleDesc itupdesc,
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Page page,
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Size keysz,
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ScanKey scankey,
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OffsetNumber offnum)
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{
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BTPageOpaque opaque;
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OffsetNumber limit;
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opaque = (BTPageOpaque) PageGetSpecialPointer(page);
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if (low == (P_RIGHTMOST(opaque) ? P_HIKEY : P_FIRSTKEY))
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return low; /* there is no prior item */
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/* skip the high key, if any */
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limit = P_RIGHTMOST(opaque) ? P_HIKEY : P_FIRSTKEY;
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/* walk backwards looking for the first key in the chain of duplicates */
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while (offnum > limit
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&& _bt_compare(rel, itupdesc, page,
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keysz, scankey, OffsetNumberPrev(offnum)) == 0)
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offnum = OffsetNumberPrev(offnum);
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return offnum;
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return OffsetNumberPrev(low);
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}
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/*
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@ -571,7 +535,6 @@ _bt_compare(Relation rel,
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{
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Datum datum;
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BTItem btitem;
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ItemId itemid;
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IndexTuple itup;
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BTPageOpaque opaque;
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ScanKey entry;
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@ -589,12 +552,11 @@ _bt_compare(Relation rel,
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*/
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opaque = (BTPageOpaque) PageGetSpecialPointer(page);
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if (!(opaque->btpo_flags & BTP_LEAF)
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&& P_LEFTMOST(opaque)
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&& offnum == P_HIKEY)
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{
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itemid = PageGetItemId(page, offnum);
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/*
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* we just have to believe that this will only be called with
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* offnum == P_HIKEY when P_HIKEY is the OffsetNumber of the first
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@ -621,7 +583,7 @@ _bt_compare(Relation rel,
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* on the page is greater than anything.
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*/
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if (_bt_skeycmp(rel, keysz, scankey, page, itemid,
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if (_bt_skeycmp(rel, keysz, scankey, page, PageGetItemId(page, offnum),
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BTEqualStrategyNumber))
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return 0;
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return 1;
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