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

/*-------------------------------------------------------------------------
 *
 * nbtree.c
 *	  Implementation of Lehman and Yao's btree management algorithm for
 *	  Postgres.
 *
 * NOTES
 *	  This file contains only the public interface routines.
 *
 *
 * Portions Copyright (c) 1996-2015, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *	  src/backend/access/nbtree/nbtree.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "access/nbtree.h"
#include "access/relscan.h"
#include "access/xlog.h"
#include "catalog/index.h"
#include "commands/vacuum.h"
#include "storage/indexfsm.h"
#include "storage/ipc.h"
#include "storage/lmgr.h"
#include "storage/smgr.h"
#include "tcop/tcopprot.h"
#include "utils/memutils.h"


/* Working state for btbuild and its callback */
typedef struct
{
	bool		isUnique;
	bool		haveDead;
	Relation	heapRel;
	BTSpool    *spool;

	/*
	 * spool2 is needed only when the index is a unique index. Dead tuples
	 * are put into spool2 instead of spool in order to avoid uniqueness
	 * check.
	 */
	BTSpool    *spool2;
	double		indtuples;
} BTBuildState;

/* Working state needed by btvacuumpage */
typedef struct
{
	IndexVacuumInfo *info;
	IndexBulkDeleteResult *stats;
	IndexBulkDeleteCallback callback;
	void	   *callback_state;
	BTCycleId	cycleid;
	BlockNumber lastBlockVacuumed;		/* highest blkno actually vacuumed */
	BlockNumber lastBlockLocked;	/* highest blkno we've cleanup-locked */
	BlockNumber totFreePages;	/* true total # of free pages */
	MemoryContext pagedelcontext;
} BTVacState;


static void btbuildCallback(Relation index,
				HeapTuple htup,
				Datum *values,
				bool *isnull,
				bool tupleIsAlive,
				void *state);
static void btvacuumscan(IndexVacuumInfo *info, IndexBulkDeleteResult *stats,
			 IndexBulkDeleteCallback callback, void *callback_state,
			 BTCycleId cycleid);
static void btvacuumpage(BTVacState *vstate, BlockNumber blkno,
			 BlockNumber orig_blkno);


/*
 *	btbuild() -- build a new btree index.
 */
Datum
btbuild(PG_FUNCTION_ARGS)
{
	Relation	heap = (Relation) PG_GETARG_POINTER(0);
	Relation	index = (Relation) PG_GETARG_POINTER(1);
	IndexInfo  *indexInfo = (IndexInfo *) PG_GETARG_POINTER(2);
	IndexBuildResult *result;
	double		reltuples;
	BTBuildState buildstate;

	buildstate.isUnique = indexInfo->ii_Unique;
	buildstate.haveDead = false;
	buildstate.heapRel = heap;
	buildstate.spool = NULL;
	buildstate.spool2 = NULL;
	buildstate.indtuples = 0;

#ifdef BTREE_BUILD_STATS
	if (log_btree_build_stats)
		ResetUsage();
#endif   /* BTREE_BUILD_STATS */

	/*
	 * We expect to be called exactly once for any index relation. If that's
	 * not the case, big trouble's what we have.
	 */
	if (RelationGetNumberOfBlocks(index) != 0)
		elog(ERROR, "index \"%s\" already contains data",
			 RelationGetRelationName(index));

	buildstate.spool = _bt_spoolinit(heap, index, indexInfo->ii_Unique, false);

	/*
	 * If building a unique index, put dead tuples in a second spool to keep
	 * them out of the uniqueness check.
	 */
	if (indexInfo->ii_Unique)
		buildstate.spool2 = _bt_spoolinit(heap, index, false, true);

	/* do the heap scan */
	reltuples = IndexBuildHeapScan(heap, index, indexInfo, true,
								   btbuildCallback, (void *) &buildstate);

	/* okay, all heap tuples are indexed */
	if (buildstate.spool2 && !buildstate.haveDead)
	{
		/* spool2 turns out to be unnecessary */
		_bt_spooldestroy(buildstate.spool2);
		buildstate.spool2 = NULL;
	}

	/*
	 * Finish the build by (1) completing the sort of the spool file, (2)
	 * inserting the sorted tuples into btree pages and (3) building the upper
	 * levels.
	 */
	_bt_leafbuild(buildstate.spool, buildstate.spool2);
	_bt_spooldestroy(buildstate.spool);
	if (buildstate.spool2)
		_bt_spooldestroy(buildstate.spool2);

#ifdef BTREE_BUILD_STATS
	if (log_btree_build_stats)
	{
		ShowUsage("BTREE BUILD STATS");
		ResetUsage();
	}
#endif   /* BTREE_BUILD_STATS */

	/*
	 * Return statistics
	 */
	result = (IndexBuildResult *) palloc(sizeof(IndexBuildResult));

	result->heap_tuples = reltuples;
	result->index_tuples = buildstate.indtuples;

	PG_RETURN_POINTER(result);
}

/*
 * Per-tuple callback from IndexBuildHeapScan
 */
static void
btbuildCallback(Relation index,
				HeapTuple htup,
				Datum *values,
				bool *isnull,
				bool tupleIsAlive,
				void *state)
{
	BTBuildState *buildstate = (BTBuildState *) state;

	/*
	 * insert the index tuple into the appropriate spool file for subsequent
	 * processing
	 */
	if (tupleIsAlive || buildstate->spool2 == NULL)
		_bt_spool(buildstate->spool, &htup->t_self, values, isnull);
	else
	{
		/* dead tuples are put into spool2 */
		buildstate->haveDead = true;
		_bt_spool(buildstate->spool2, &htup->t_self, values, isnull);
	}

	buildstate->indtuples += 1;
}

/*
 *	btbuildempty() -- build an empty btree index in the initialization fork
 */
Datum
btbuildempty(PG_FUNCTION_ARGS)
{
	Relation	index = (Relation) PG_GETARG_POINTER(0);
	Page		metapage;

	/* Construct metapage. */
	metapage = (Page) palloc(BLCKSZ);
	_bt_initmetapage(metapage, P_NONE, 0);

	/* Write the page.  If archiving/streaming, XLOG it. */
	PageSetChecksumInplace(metapage, BTREE_METAPAGE);
	smgrwrite(index->rd_smgr, INIT_FORKNUM, BTREE_METAPAGE,
			  (char *) metapage, true);
	if (XLogIsNeeded())
		log_newpage(&index->rd_smgr->smgr_rnode.node, INIT_FORKNUM,
					BTREE_METAPAGE, metapage, false);

	/*
	 * An immediate sync is required even if we xlog'd the page, because the
	 * write did not go through shared_buffers and therefore a concurrent
	 * checkpoint may have moved the redo pointer past our xlog record.
	 */
	smgrimmedsync(index->rd_smgr, INIT_FORKNUM);

	PG_RETURN_VOID();
}

/*
 *	btinsert() -- insert an index tuple into a btree.
 *
 *		Descend the tree recursively, find the appropriate location for our
 *		new tuple, and put it there.
 */
Datum
btinsert(PG_FUNCTION_ARGS)
{
	Relation	rel = (Relation) PG_GETARG_POINTER(0);
	Datum	   *values = (Datum *) PG_GETARG_POINTER(1);
	bool	   *isnull = (bool *) PG_GETARG_POINTER(2);
	ItemPointer ht_ctid = (ItemPointer) PG_GETARG_POINTER(3);
	Relation	heapRel = (Relation) PG_GETARG_POINTER(4);
	IndexUniqueCheck checkUnique = (IndexUniqueCheck) PG_GETARG_INT32(5);
	bool		result;
	IndexTuple	itup;

	/* generate an index tuple */
	itup = index_form_tuple(RelationGetDescr(rel), values, isnull);
	itup->t_tid = *ht_ctid;

	result = _bt_doinsert(rel, itup, checkUnique, heapRel);

	pfree(itup);

	PG_RETURN_BOOL(result);
}

/*
 *	btgettuple() -- Get the next tuple in the scan.
 */
Datum
btgettuple(PG_FUNCTION_ARGS)
{
	IndexScanDesc scan = (IndexScanDesc) PG_GETARG_POINTER(0);
	ScanDirection dir = (ScanDirection) PG_GETARG_INT32(1);
	BTScanOpaque so = (BTScanOpaque) scan->opaque;
	bool		res;

	/* btree indexes are never lossy */
	scan->xs_recheck = false;

	/*
	 * If we have any array keys, initialize them during first call for a
	 * scan.  We can't do this in btrescan because we don't know the scan
	 * direction at that time.
	 */
	if (so->numArrayKeys && !BTScanPosIsValid(so->currPos))
	{
		/* punt if we have any unsatisfiable array keys */
		if (so->numArrayKeys < 0)
			PG_RETURN_BOOL(false);

		_bt_start_array_keys(scan, dir);
	}

	/* This loop handles advancing to the next array elements, if any */
	do
	{
		/*
		 * If we've already initialized this scan, we can just advance it in
		 * the appropriate direction.  If we haven't done so yet, we call
		 * _bt_first() to get the first item in the scan.
		 */
		if (!BTScanPosIsValid(so->currPos))
			res = _bt_first(scan, dir);
		else
		{
			/*
			 * Check to see if we should kill the previously-fetched tuple.
			 */
			if (scan->kill_prior_tuple)
			{
				/*
				 * Yes, remember it for later. (We'll deal with all such
				 * tuples at once right before leaving the index page.)  The
				 * test for numKilled overrun is not just paranoia: if the
				 * caller reverses direction in the indexscan then the same
				 * item might get entered multiple times. It's not worth
				 * trying to optimize that, so we don't detect it, but instead
				 * just forget any excess entries.
				 */
				if (so->killedItems == NULL)
					so->killedItems = (int *)
						palloc(MaxIndexTuplesPerPage * sizeof(int));
				if (so->numKilled < MaxIndexTuplesPerPage)
					so->killedItems[so->numKilled++] = so->currPos.itemIndex;
			}

			/*
			 * Now continue the scan.
			 */
			res = _bt_next(scan, dir);
		}

		/* If we have a tuple, return it ... */
		if (res)
			break;
		/* ... otherwise see if we have more array keys to deal with */
	} while (so->numArrayKeys && _bt_advance_array_keys(scan, dir));

	PG_RETURN_BOOL(res);
}

/*
 * btgetbitmap() -- gets all matching tuples, and adds them to a bitmap
 */
Datum
btgetbitmap(PG_FUNCTION_ARGS)
{
	IndexScanDesc scan = (IndexScanDesc) PG_GETARG_POINTER(0);
	TIDBitmap  *tbm = (TIDBitmap *) PG_GETARG_POINTER(1);
	BTScanOpaque so = (BTScanOpaque) scan->opaque;
	int64		ntids = 0;
	ItemPointer heapTid;

	/*
	 * If we have any array keys, initialize them.
	 */
	if (so->numArrayKeys)
	{
		/* punt if we have any unsatisfiable array keys */
		if (so->numArrayKeys < 0)
			PG_RETURN_INT64(ntids);

		_bt_start_array_keys(scan, ForwardScanDirection);
	}

	/* This loop handles advancing to the next array elements, if any */
	do
	{
		/* Fetch the first page & tuple */
		if (_bt_first(scan, ForwardScanDirection))
		{
			/* Save tuple ID, and continue scanning */
			heapTid = &scan->xs_ctup.t_self;
			tbm_add_tuples(tbm, heapTid, 1, false);
			ntids++;

			for (;;)
			{
				/*
				 * Advance to next tuple within page.  This is the same as the
				 * easy case in _bt_next().
				 */
				if (++so->currPos.itemIndex > so->currPos.lastItem)
				{
					/* let _bt_next do the heavy lifting */
					if (!_bt_next(scan, ForwardScanDirection))
						break;
				}

				/* Save tuple ID, and continue scanning */
				heapTid = &so->currPos.items[so->currPos.itemIndex].heapTid;
				tbm_add_tuples(tbm, heapTid, 1, false);
				ntids++;
			}
		}
		/* Now see if we have more array keys to deal with */
	} while (so->numArrayKeys && _bt_advance_array_keys(scan, ForwardScanDirection));

	PG_RETURN_INT64(ntids);
}

/*
 *	btbeginscan() -- start a scan on a btree index
 */
Datum
btbeginscan(PG_FUNCTION_ARGS)
{
	Relation	rel = (Relation) PG_GETARG_POINTER(0);
	int			nkeys = PG_GETARG_INT32(1);
	int			norderbys = PG_GETARG_INT32(2);
	IndexScanDesc scan;
	BTScanOpaque so;

	/* no order by operators allowed */
	Assert(norderbys == 0);

	/* get the scan */
	scan = RelationGetIndexScan(rel, nkeys, norderbys);

	/* allocate private workspace */
	so = (BTScanOpaque) palloc(sizeof(BTScanOpaqueData));
	BTScanPosInvalidate(so->currPos);
	BTScanPosInvalidate(so->markPos);
	if (scan->numberOfKeys > 0)
		so->keyData = (ScanKey) palloc(scan->numberOfKeys * sizeof(ScanKeyData));
	else
		so->keyData = NULL;

	so->arrayKeyData = NULL;	/* assume no array keys for now */
	so->numArrayKeys = 0;
	so->arrayKeys = NULL;
	so->arrayContext = NULL;

	so->killedItems = NULL;		/* until needed */
	so->numKilled = 0;

	/*
	 * We don't know yet whether the scan will be index-only, so we do not
	 * allocate the tuple workspace arrays until btrescan.  However, we set up
	 * scan->xs_itupdesc whether we'll need it or not, since that's so cheap.
	 */
	so->currTuples = so->markTuples = NULL;

	scan->xs_itupdesc = RelationGetDescr(rel);

	scan->opaque = so;

	PG_RETURN_POINTER(scan);
}

/*
 *	btrescan() -- rescan an index relation
 */
Datum
btrescan(PG_FUNCTION_ARGS)
{
	IndexScanDesc scan = (IndexScanDesc) PG_GETARG_POINTER(0);
	ScanKey		scankey = (ScanKey) PG_GETARG_POINTER(1);

	/* remaining arguments are ignored */
	BTScanOpaque so = (BTScanOpaque) scan->opaque;

	/* we aren't holding any read locks, but gotta drop the pins */
	if (BTScanPosIsValid(so->currPos))
	{
		/* Before leaving current page, deal with any killed items */
		if (so->numKilled > 0)
			_bt_killitems(scan);
		BTScanPosUnpinIfPinned(so->currPos);
		BTScanPosInvalidate(so->currPos);
	}

	so->markItemIndex = -1;
	BTScanPosUnpinIfPinned(so->markPos);
	BTScanPosInvalidate(so->markPos);

	/*
	 * Allocate tuple workspace arrays, if needed for an index-only scan and
	 * not already done in a previous rescan call.  To save on palloc
	 * overhead, both workspaces are allocated as one palloc block; only this
	 * function and btendscan know that.
	 *
	 * NOTE: this data structure also makes it safe to return data from a
	 * "name" column, even though btree name_ops uses an underlying storage
	 * datatype of cstring.  The risk there is that "name" is supposed to be
	 * padded to NAMEDATALEN, but the actual index tuple is probably shorter.
	 * However, since we only return data out of tuples sitting in the
	 * currTuples array, a fetch of NAMEDATALEN bytes can at worst pull some
	 * data out of the markTuples array --- running off the end of memory for
	 * a SIGSEGV is not possible.  Yeah, this is ugly as sin, but it beats
	 * adding special-case treatment for name_ops elsewhere.
	 */
	if (scan->xs_want_itup && so->currTuples == NULL)
	{
		so->currTuples = (char *) palloc(BLCKSZ * 2);
		so->markTuples = so->currTuples + BLCKSZ;
	}

	/*
	 * Reset the scan keys. Note that keys ordering stuff moved to _bt_first.
	 * - vadim 05/05/97
	 */
	if (scankey && scan->numberOfKeys > 0)
		memmove(scan->keyData,
				scankey,
				scan->numberOfKeys * sizeof(ScanKeyData));
	so->numberOfKeys = 0;		/* until _bt_preprocess_keys sets it */

	/* If any keys are SK_SEARCHARRAY type, set up array-key info */
	_bt_preprocess_array_keys(scan);

	PG_RETURN_VOID();
}

/*
 *	btendscan() -- close down a scan
 */
Datum
btendscan(PG_FUNCTION_ARGS)
{
	IndexScanDesc scan = (IndexScanDesc) PG_GETARG_POINTER(0);
	BTScanOpaque so = (BTScanOpaque) scan->opaque;

	/* we aren't holding any read locks, but gotta drop the pins */
	if (BTScanPosIsValid(so->currPos))
	{
		/* Before leaving current page, deal with any killed items */
		if (so->numKilled > 0)
			_bt_killitems(scan);
		BTScanPosUnpinIfPinned(so->currPos);
	}

	so->markItemIndex = -1;
	BTScanPosUnpinIfPinned(so->markPos);

	/* No need to invalidate positions, the RAM is about to be freed. */

	/* Release storage */
	if (so->keyData != NULL)
		pfree(so->keyData);
	/* so->arrayKeyData and so->arrayKeys are in arrayContext */
	if (so->arrayContext != NULL)
		MemoryContextDelete(so->arrayContext);
	if (so->killedItems != NULL)
		pfree(so->killedItems);
	if (so->currTuples != NULL)
		pfree(so->currTuples);
	/* so->markTuples should not be pfree'd, see btrescan */
	pfree(so);

	PG_RETURN_VOID();
}

/*
 *	btmarkpos() -- save current scan position
 */
Datum
btmarkpos(PG_FUNCTION_ARGS)
{
	IndexScanDesc scan = (IndexScanDesc) PG_GETARG_POINTER(0);
	BTScanOpaque so = (BTScanOpaque) scan->opaque;

	/* There may be an old mark with a pin (but no lock). */
	BTScanPosUnpinIfPinned(so->markPos);

	/*
	 * Just record the current itemIndex.  If we later step to next page
	 * before releasing the marked position, _bt_steppage makes a full copy of
	 * the currPos struct in markPos.  If (as often happens) the mark is moved
	 * before we leave the page, we don't have to do that work.
	 */
	if (BTScanPosIsValid(so->currPos))
		so->markItemIndex = so->currPos.itemIndex;
	else
	{
		BTScanPosInvalidate(so->markPos);
		so->markItemIndex = -1;
	}

	/* Also record the current positions of any array keys */
	if (so->numArrayKeys)
		_bt_mark_array_keys(scan);

	PG_RETURN_VOID();
}

/*
 *	btrestrpos() -- restore scan to last saved position
 */
Datum
btrestrpos(PG_FUNCTION_ARGS)
{
	IndexScanDesc scan = (IndexScanDesc) PG_GETARG_POINTER(0);
	BTScanOpaque so = (BTScanOpaque) scan->opaque;

	/* Restore the marked positions of any array keys */
	if (so->numArrayKeys)
		_bt_restore_array_keys(scan);

	if (so->markItemIndex >= 0)
	{
		/*
		 * The scan has never moved to a new page since the last mark.  Just
		 * restore the itemIndex.
		 *
		 * NB: In this case we can't count on anything in so->markPos to be
		 * accurate.
		 */
		so->currPos.itemIndex = so->markItemIndex;
	}
	else if (so->currPos.currPage == so->markPos.currPage)
	{
		/*
		 * so->markItemIndex < 0 but mark and current positions are on the
		 * same page.  This would be an unusual case, where the scan moved to
		 * a new index page after the mark, restored, and later restored again
		 * without moving off the marked page.  It is not clear that this code
		 * can currently be reached, but it seems better to make this function
		 * robust for this case than to Assert() or elog() that it can't
		 * happen.
		 *
		 * We neither want to set so->markItemIndex >= 0 (because that could
		 * cause a later move to a new page to redo the memcpy() executions)
		 * nor re-execute the memcpy() functions for a restore within the same
		 * page.  The previous restore to this page already set everything
		 * except markPos as it should be.
		 */
		so->currPos.itemIndex = so->markPos.itemIndex;
	}
	else
	{
		/*
		 * The scan moved to a new page after last mark or restore, and we are
		 * now restoring to the marked page.  We aren't holding any read
		 * locks, but if we're still holding the pin for the current position,
		 * we must drop it.
		 */
		if (BTScanPosIsValid(so->currPos))
		{
			/* Before leaving current page, deal with any killed items */
			if (so->numKilled > 0)
				_bt_killitems(scan);
			BTScanPosUnpinIfPinned(so->currPos);
		}

		if (BTScanPosIsValid(so->markPos))
		{
			/* bump pin on mark buffer for assignment to current buffer */
			if (BTScanPosIsPinned(so->markPos))
				IncrBufferRefCount(so->markPos.buf);
			memcpy(&so->currPos, &so->markPos,
				   offsetof(BTScanPosData, items[1]) +
				   so->markPos.lastItem * sizeof(BTScanPosItem));
			if (so->currTuples)
				memcpy(so->currTuples, so->markTuples,
					   so->markPos.nextTupleOffset);
		}
		else
			BTScanPosInvalidate(so->currPos);
	}

	PG_RETURN_VOID();
}

/*
 * Bulk deletion of all index entries pointing to a set of heap tuples.
 * The set of target tuples is specified via a callback routine that tells
 * whether any given heap tuple (identified by ItemPointer) is being deleted.
 *
 * Result: a palloc'd struct containing statistical info for VACUUM displays.
 */
Datum
btbulkdelete(PG_FUNCTION_ARGS)
{
	IndexVacuumInfo *info = (IndexVacuumInfo *) PG_GETARG_POINTER(0);
	IndexBulkDeleteResult *volatile stats = (IndexBulkDeleteResult *) PG_GETARG_POINTER(1);
	IndexBulkDeleteCallback callback = (IndexBulkDeleteCallback) PG_GETARG_POINTER(2);
	void	   *callback_state = (void *) PG_GETARG_POINTER(3);
	Relation	rel = info->index;
	BTCycleId	cycleid;

	/* allocate stats if first time through, else re-use existing struct */
	if (stats == NULL)
		stats = (IndexBulkDeleteResult *) palloc0(sizeof(IndexBulkDeleteResult));

	/* Establish the vacuum cycle ID to use for this scan */
	/* The ENSURE stuff ensures we clean up shared memory on failure */
	PG_ENSURE_ERROR_CLEANUP(_bt_end_vacuum_callback, PointerGetDatum(rel));
	{
		cycleid = _bt_start_vacuum(rel);

		btvacuumscan(info, stats, callback, callback_state, cycleid);
	}
	PG_END_ENSURE_ERROR_CLEANUP(_bt_end_vacuum_callback, PointerGetDatum(rel));
	_bt_end_vacuum(rel);

	PG_RETURN_POINTER(stats);
}

/*
 * Post-VACUUM cleanup.
 *
 * Result: a palloc'd struct containing statistical info for VACUUM displays.
 */
Datum
btvacuumcleanup(PG_FUNCTION_ARGS)
{
	IndexVacuumInfo *info = (IndexVacuumInfo *) PG_GETARG_POINTER(0);
	IndexBulkDeleteResult *stats = (IndexBulkDeleteResult *) PG_GETARG_POINTER(1);

	/* No-op in ANALYZE ONLY mode */
	if (info->analyze_only)
		PG_RETURN_POINTER(stats);

	/*
	 * If btbulkdelete was called, we need not do anything, just return the
	 * stats from the latest btbulkdelete call.  If it wasn't called, we must
	 * still do a pass over the index, to recycle any newly-recyclable pages
	 * and to obtain index statistics.
	 *
	 * Since we aren't going to actually delete any leaf items, there's no
	 * need to go through all the vacuum-cycle-ID pushups.
	 */
	if (stats == NULL)
	{
		stats = (IndexBulkDeleteResult *) palloc0(sizeof(IndexBulkDeleteResult));
		btvacuumscan(info, stats, NULL, NULL, 0);
	}

	/* Finally, vacuum the FSM */
	IndexFreeSpaceMapVacuum(info->index);

	/*
	 * It's quite possible for us to be fooled by concurrent page splits into
	 * double-counting some index tuples, so disbelieve any total that exceeds
	 * the underlying heap's count ... if we know that accurately.  Otherwise
	 * this might just make matters worse.
	 */
	if (!info->estimated_count)
	{
		if (stats->num_index_tuples > info->num_heap_tuples)
			stats->num_index_tuples = info->num_heap_tuples;
	}

	PG_RETURN_POINTER(stats);
}

/*
 * btvacuumscan --- scan the index for VACUUMing purposes
 *
 * This combines the functions of looking for leaf tuples that are deletable
 * according to the vacuum callback, looking for empty pages that can be
 * deleted, and looking for old deleted pages that can be recycled.  Both
 * btbulkdelete and btvacuumcleanup invoke this (the latter only if no
 * btbulkdelete call occurred).
 *
 * The caller is responsible for initially allocating/zeroing a stats struct
 * and for obtaining a vacuum cycle ID if necessary.
 */
static void
btvacuumscan(IndexVacuumInfo *info, IndexBulkDeleteResult *stats,
			 IndexBulkDeleteCallback callback, void *callback_state,
			 BTCycleId cycleid)
{
	Relation	rel = info->index;
	BTVacState	vstate;
	BlockNumber num_pages;
	BlockNumber blkno;
	bool		needLock;

	/*
	 * Reset counts that will be incremented during the scan; needed in case
	 * of multiple scans during a single VACUUM command
	 */
	stats->estimated_count = false;
	stats->num_index_tuples = 0;
	stats->pages_deleted = 0;

	/* Set up info to pass down to btvacuumpage */
	vstate.info = info;
	vstate.stats = stats;
	vstate.callback = callback;
	vstate.callback_state = callback_state;
	vstate.cycleid = cycleid;
	vstate.lastBlockVacuumed = BTREE_METAPAGE;	/* Initialise at first block */
	vstate.lastBlockLocked = BTREE_METAPAGE;
	vstate.totFreePages = 0;

	/* Create a temporary memory context to run _bt_pagedel in */
	vstate.pagedelcontext = AllocSetContextCreate(CurrentMemoryContext,
												  "_bt_pagedel",
												  ALLOCSET_DEFAULT_MINSIZE,
												  ALLOCSET_DEFAULT_INITSIZE,
												  ALLOCSET_DEFAULT_MAXSIZE);

	/*
	 * The outer loop iterates over all index pages except the metapage, in
	 * physical order (we hope the kernel will cooperate in providing
	 * read-ahead for speed).  It is critical that we visit all leaf pages,
	 * including ones added after we start the scan, else we might fail to
	 * delete some deletable tuples.  Hence, we must repeatedly check the
	 * relation length.  We must acquire the relation-extension lock while
	 * doing so to avoid a race condition: if someone else is extending the
	 * relation, there is a window where bufmgr/smgr have created a new
	 * all-zero page but it hasn't yet been write-locked by _bt_getbuf(). If
	 * we manage to scan such a page here, we'll improperly assume it can be
	 * recycled.  Taking the lock synchronizes things enough to prevent a
	 * problem: either num_pages won't include the new page, or _bt_getbuf
	 * already has write lock on the buffer and it will be fully initialized
	 * before we can examine it.  (See also vacuumlazy.c, which has the same
	 * issue.)	Also, we need not worry if a page is added immediately after
	 * we look; the page splitting code already has write-lock on the left
	 * page before it adds a right page, so we must already have processed any
	 * tuples due to be moved into such a page.
	 *
	 * We can skip locking for new or temp relations, however, since no one
	 * else could be accessing them.
	 */
	needLock = !RELATION_IS_LOCAL(rel);

	blkno = BTREE_METAPAGE + 1;
	for (;;)
	{
		/* Get the current relation length */
		if (needLock)
			LockRelationForExtension(rel, ExclusiveLock);
		num_pages = RelationGetNumberOfBlocks(rel);
		if (needLock)
			UnlockRelationForExtension(rel, ExclusiveLock);

		/* Quit if we've scanned the whole relation */
		if (blkno >= num_pages)
			break;
		/* Iterate over pages, then loop back to recheck length */
		for (; blkno < num_pages; blkno++)
		{
			btvacuumpage(&vstate, blkno, blkno);
		}
	}

	/*
	 * If the WAL is replayed in hot standby, the replay process needs to get
	 * cleanup locks on all index leaf pages, just as we've been doing here.
	 * However, we won't issue any WAL records about pages that have no items
	 * to be deleted.  For pages between pages we've vacuumed, the replay code
	 * will take locks under the direction of the lastBlockVacuumed fields in
	 * the XLOG_BTREE_VACUUM WAL records.  To cover pages after the last one
	 * we vacuum, we need to issue a dummy XLOG_BTREE_VACUUM WAL record
	 * against the last leaf page in the index, if that one wasn't vacuumed.
	 */
	if (XLogStandbyInfoActive() &&
		vstate.lastBlockVacuumed < vstate.lastBlockLocked)
	{
		Buffer		buf;

		/*
		 * The page should be valid, but we can't use _bt_getbuf() because we
		 * want to use a nondefault buffer access strategy.  Since we aren't
		 * going to delete any items, getting cleanup lock again is probably
		 * overkill, but for consistency do that anyway.
		 */
		buf = ReadBufferExtended(rel, MAIN_FORKNUM, vstate.lastBlockLocked,
								 RBM_NORMAL, info->strategy);
		LockBufferForCleanup(buf);
		_bt_checkpage(rel, buf);
		_bt_delitems_vacuum(rel, buf, NULL, 0, vstate.lastBlockVacuumed);
		_bt_relbuf(rel, buf);
	}

	MemoryContextDelete(vstate.pagedelcontext);

	/* update statistics */
	stats->num_pages = num_pages;
	stats->pages_free = vstate.totFreePages;
}

/*
 * btvacuumpage --- VACUUM one page
 *
 * This processes a single page for btvacuumscan().  In some cases we
 * must go back and re-examine previously-scanned pages; this routine
 * recurses when necessary to handle that case.
 *
 * blkno is the page to process.  orig_blkno is the highest block number
 * reached by the outer btvacuumscan loop (the same as blkno, unless we
 * are recursing to re-examine a previous page).
 */
static void
btvacuumpage(BTVacState *vstate, BlockNumber blkno, BlockNumber orig_blkno)
{
	IndexVacuumInfo *info = vstate->info;
	IndexBulkDeleteResult *stats = vstate->stats;
	IndexBulkDeleteCallback callback = vstate->callback;
	void	   *callback_state = vstate->callback_state;
	Relation	rel = info->index;
	bool		delete_now;
	BlockNumber recurse_to;
	Buffer		buf;
	Page		page;
	BTPageOpaque opaque;

restart:
	delete_now = false;
	recurse_to = P_NONE;

	/* call vacuum_delay_point while not holding any buffer lock */
	vacuum_delay_point();

	/*
	 * We can't use _bt_getbuf() here because it always applies
	 * _bt_checkpage(), which will barf on an all-zero page. We want to
	 * recycle all-zero pages, not fail.  Also, we want to use a nondefault
	 * buffer access strategy.
	 */
	buf = ReadBufferExtended(rel, MAIN_FORKNUM, blkno, RBM_NORMAL,
							 info->strategy);
	LockBuffer(buf, BT_READ);
	page = BufferGetPage(buf);
	opaque = (BTPageOpaque) PageGetSpecialPointer(page);
	if (!PageIsNew(page))
		_bt_checkpage(rel, buf);

	/*
	 * If we are recursing, the only case we want to do anything with is a
	 * live leaf page having the current vacuum cycle ID.  Any other state
	 * implies we already saw the page (eg, deleted it as being empty).
	 */
	if (blkno != orig_blkno)
	{
		if (_bt_page_recyclable(page) ||
			P_IGNORE(opaque) ||
			!P_ISLEAF(opaque) ||
			opaque->btpo_cycleid != vstate->cycleid)
		{
			_bt_relbuf(rel, buf);
			return;
		}
	}

	/* Page is valid, see what to do with it */
	if (_bt_page_recyclable(page))
	{
		/* Okay to recycle this page */
		RecordFreeIndexPage(rel, blkno);
		vstate->totFreePages++;
		stats->pages_deleted++;
	}
	else if (P_ISDELETED(opaque))
	{
		/* Already deleted, but can't recycle yet */
		stats->pages_deleted++;
	}
	else if (P_ISHALFDEAD(opaque))
	{
		/* Half-dead, try to delete */
		delete_now = true;
	}
	else if (P_ISLEAF(opaque))
	{
		OffsetNumber deletable[MaxOffsetNumber];
		int			ndeletable;
		OffsetNumber offnum,
					minoff,
					maxoff;

		/*
		 * Trade in the initial read lock for a super-exclusive write lock on
		 * this page.  We must get such a lock on every leaf page over the
		 * course of the vacuum scan, whether or not it actually contains any
		 * deletable tuples --- see nbtree/README.
		 */
		LockBuffer(buf, BUFFER_LOCK_UNLOCK);
		LockBufferForCleanup(buf);

		/*
		 * Remember highest leaf page number we've taken cleanup lock on; see
		 * notes in btvacuumscan
		 */
		if (blkno > vstate->lastBlockLocked)
			vstate->lastBlockLocked = blkno;

		/*
		 * Check whether we need to recurse back to earlier pages.  What we
		 * are concerned about is a page split that happened since we started
		 * the vacuum scan.  If the split moved some tuples to a lower page
		 * then we might have missed 'em.  If so, set up for tail recursion.
		 * (Must do this before possibly clearing btpo_cycleid below!)
		 */
		if (vstate->cycleid != 0 &&
			opaque->btpo_cycleid == vstate->cycleid &&
			!(opaque->btpo_flags & BTP_SPLIT_END) &&
			!P_RIGHTMOST(opaque) &&
			opaque->btpo_next < orig_blkno)
			recurse_to = opaque->btpo_next;

		/*
		 * Scan over all items to see which ones need deleted according to the
		 * callback function.
		 */
		ndeletable = 0;
		minoff = P_FIRSTDATAKEY(opaque);
		maxoff = PageGetMaxOffsetNumber(page);
		if (callback)
		{
			for (offnum = minoff;
				 offnum <= maxoff;
				 offnum = OffsetNumberNext(offnum))
			{
				IndexTuple	itup;
				ItemPointer htup;

				itup = (IndexTuple) PageGetItem(page,
												PageGetItemId(page, offnum));
				htup = &(itup->t_tid);

				/*
				 * During Hot Standby we currently assume that
				 * XLOG_BTREE_VACUUM records do not produce conflicts. That is
				 * only true as long as the callback function depends only
				 * upon whether the index tuple refers to heap tuples removed
				 * in the initial heap scan. When vacuum starts it derives a
				 * value of OldestXmin. Backends taking later snapshots could
				 * have a RecentGlobalXmin with a later xid than the vacuum's
				 * OldestXmin, so it is possible that row versions deleted
				 * after OldestXmin could be marked as killed by other
				 * backends. The callback function *could* look at the index
				 * tuple state in isolation and decide to delete the index
				 * tuple, though currently it does not. If it ever did, we
				 * would need to reconsider whether XLOG_BTREE_VACUUM records
				 * should cause conflicts. If they did cause conflicts they
				 * would be fairly harsh conflicts, since we haven't yet
				 * worked out a way to pass a useful value for
				 * latestRemovedXid on the XLOG_BTREE_VACUUM records. This
				 * applies to *any* type of index that marks index tuples as
				 * killed.
				 */
				if (callback(htup, callback_state))
					deletable[ndeletable++] = offnum;
			}
		}

		/*
		 * Apply any needed deletes.  We issue just one _bt_delitems_vacuum()
		 * call per page, so as to minimize WAL traffic.
		 */
		if (ndeletable > 0)
		{
			/*
			 * Notice that the issued XLOG_BTREE_VACUUM WAL record includes an
			 * instruction to the replay code to get cleanup lock on all pages
			 * between the previous lastBlockVacuumed and this page.  This
			 * ensures that WAL replay locks all leaf pages at some point.
			 *
			 * Since we can visit leaf pages out-of-order when recursing,
			 * replay might end up locking such pages an extra time, but it
			 * doesn't seem worth the amount of bookkeeping it'd take to avoid
			 * that.
			 */
			_bt_delitems_vacuum(rel, buf, deletable, ndeletable,
								vstate->lastBlockVacuumed);

			/*
			 * Remember highest leaf page number we've issued a
			 * XLOG_BTREE_VACUUM WAL record for.
			 */
			if (blkno > vstate->lastBlockVacuumed)
				vstate->lastBlockVacuumed = blkno;

			stats->tuples_removed += ndeletable;
			/* must recompute maxoff */
			maxoff = PageGetMaxOffsetNumber(page);
		}
		else
		{
			/*
			 * If the page has been split during this vacuum cycle, it seems
			 * worth expending a write to clear btpo_cycleid even if we don't
			 * have any deletions to do.  (If we do, _bt_delitems_vacuum takes
			 * care of this.)  This ensures we won't process the page again.
			 *
			 * We treat this like a hint-bit update because there's no need to
			 * WAL-log it.
			 */
			if (vstate->cycleid != 0 &&
				opaque->btpo_cycleid == vstate->cycleid)
			{
				opaque->btpo_cycleid = 0;
				MarkBufferDirtyHint(buf, true);
			}
		}

		/*
		 * If it's now empty, try to delete; else count the live tuples. We
		 * don't delete when recursing, though, to avoid putting entries into
		 * freePages out-of-order (doesn't seem worth any extra code to handle
		 * the case).
		 */
		if (minoff > maxoff)
			delete_now = (blkno == orig_blkno);
		else
			stats->num_index_tuples += maxoff - minoff + 1;
	}

	if (delete_now)
	{
		MemoryContext oldcontext;
		int			ndel;

		/* Run pagedel in a temp context to avoid memory leakage */
		MemoryContextReset(vstate->pagedelcontext);
		oldcontext = MemoryContextSwitchTo(vstate->pagedelcontext);

		ndel = _bt_pagedel(rel, buf);

		/* count only this page, else may double-count parent */
		if (ndel)
			stats->pages_deleted++;

		MemoryContextSwitchTo(oldcontext);
		/* pagedel released buffer, so we shouldn't */
	}
	else
		_bt_relbuf(rel, buf);

	/*
	 * This is really tail recursion, but if the compiler is too stupid to
	 * optimize it as such, we'd eat an uncomfortably large amount of stack
	 * space per recursion level (due to the deletable[] array). A failure is
	 * improbable since the number of levels isn't likely to be large ... but
	 * just in case, let's hand-optimize into a loop.
	 */
	if (recurse_to != P_NONE)
	{
		blkno = recurse_to;
		goto restart;
	}
}

/*
 *	btcanreturn() -- Check whether btree indexes support index-only scans.
 *
 * btrees always do, so this is trivial.
 */
Datum
btcanreturn(PG_FUNCTION_ARGS)
{
	PG_RETURN_BOOL(true);
}