postgres/src/backend/access/gin/ginget.c

/*-------------------------------------------------------------------------
 *
 * ginget.c
 *	  fetch tuples from a GIN scan.
 *
 *
 * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *			$PostgreSQL: pgsql/src/backend/access/gin/ginget.c,v 1.17 2008/06/19 00:46:03 alvherre Exp $
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include "access/gin.h"
#include "access/relscan.h"
#include "catalog/index.h"
#include "miscadmin.h"
#include "storage/bufmgr.h"
#include "utils/datum.h"
#include "utils/memutils.h"


/*
 * Tries to refind previously taken ItemPointer on page.
 */
static bool
findItemInPage(Page page, ItemPointer item, OffsetNumber *off)
{
	OffsetNumber maxoff = GinPageGetOpaque(page)->maxoff;
	int			res;

	if (GinPageGetOpaque(page)->flags & GIN_DELETED)
		/* page was deleted by concurrent  vacuum */
		return false;

	/*
	 * scan page to find equal or first greater value
	 */

	for (*off = FirstOffsetNumber; *off <= maxoff; (*off)++)
	{
		res = compareItemPointers(item, (ItemPointer) GinDataPageGetItem(page, *off));

		if (res <= 0)
			return true;
	}

	return false;
}

/*
 * Goes to the next page if current offset is outside of bounds
 */
static bool
moveRightIfItNeeded( GinBtreeData *btree, GinBtreeStack *stack )
{
	Page page = BufferGetPage(stack->buffer);

	if ( stack->off > PageGetMaxOffsetNumber(page) )
	{
		/*
		 * We scanned the whole page, so we should take right page
		 */
		stack->blkno = GinPageGetOpaque(page)->rightlink;

		if ( GinPageRightMost(page) )
			return false;  /* no more pages */

		LockBuffer(stack->buffer, GIN_UNLOCK);
		stack->buffer = ReleaseAndReadBuffer(stack->buffer, btree->index, stack->blkno);
		LockBuffer(stack->buffer, GIN_SHARE);
		stack->off = FirstOffsetNumber;
	}

	return true;
}

/*
 * Does fullscan of posting tree and saves ItemPointers
 * in scanEntry->partialMatch TIDBitmap
 */
static void
scanForItems( Relation index, GinScanEntry scanEntry, BlockNumber rootPostingTree )
{
	GinPostingTreeScan *gdi;
	Buffer				buffer;
	Page				page;
	BlockNumber			blkno;

	gdi = prepareScanPostingTree(index, rootPostingTree, TRUE);

	buffer = scanBeginPostingTree(gdi);
	IncrBufferRefCount(buffer); /* prevent unpin in freeGinBtreeStack */

	freeGinBtreeStack(gdi->stack);
	pfree(gdi);

	/*
	 * Goes through all leaves
	 */
	for(;;)
	{
		page = BufferGetPage(buffer);

		if ((GinPageGetOpaque(page)->flags & GIN_DELETED) == 0 && GinPageGetOpaque(page)->maxoff >= FirstOffsetNumber )
		{
			tbm_add_tuples( scanEntry->partialMatch,
							(ItemPointer)GinDataPageGetItem(page, FirstOffsetNumber),
						 	GinPageGetOpaque(page)->maxoff, false);
			scanEntry->predictNumberResult += GinPageGetOpaque(page)->maxoff;
		}

		blkno = GinPageGetOpaque(page)->rightlink;
		if ( GinPageRightMost(page) )
		{
			UnlockReleaseBuffer(buffer);
			return;  /* no more pages */
		}

		LockBuffer(buffer, GIN_UNLOCK);
		buffer = ReleaseAndReadBuffer(buffer, index, blkno);
		LockBuffer(buffer, GIN_SHARE);
	}
}

/*
 * Collects all ItemPointer into the TIDBitmap struct
 * for entries partially matched to search entry.
 *
 * Returns true if done, false if it's needed to restart scan from scratch
 */
static bool
computePartialMatchList( GinBtreeData *btree, GinBtreeStack *stack, GinScanEntry scanEntry )
{
	Page 		page;
	IndexTuple  itup;
	Datum		idatum;
	bool		isnull;
	int32		cmp;

	scanEntry->partialMatch = tbm_create( work_mem * 1024L );

	for(;;)
	{
		/*
		 * stack->off points to the interested entry, buffer is already locked
		 */
		if ( moveRightIfItNeeded(btree, stack) == false )
			return true;

		page = BufferGetPage(stack->buffer);
		itup = (IndexTuple) PageGetItem(page, PageGetItemId(page, stack->off));
		idatum = index_getattr(itup, 1, btree->ginstate->tupdesc, &isnull);
		Assert(!isnull);

		/*----------
		 * Check of partial match.
		 * case cmp == 0 => match
		 * case cmp > 0 => not match and finish scan
		 * case cmp < 0 => not match and continue scan
		 *----------
		 */
    	cmp = DatumGetInt32(FunctionCall3(&btree->ginstate->comparePartialFn,
										  scanEntry->entry,
										  idatum,
										  UInt16GetDatum(scanEntry->strategy)));

		if ( cmp > 0 )
			return true;
		else if ( cmp < 0 )
		{
			stack->off++;
			continue;
		}

		if ( GinIsPostingTree(itup) )
		{
			BlockNumber rootPostingTree = GinGetPostingTree(itup);
			Datum		newDatum,
						savedDatum = datumCopy (
										idatum,
										btree->ginstate->tupdesc->attrs[0]->attbyval,
										btree->ginstate->tupdesc->attrs[0]->attlen
									);
			/*
			 * We should unlock current page (but not unpin) during
			 * tree scan to prevent deadlock with vacuum processes.
			 *
			 * We save current entry value (savedDatum) to be able to refind
			 * our tuple after re-locking
			 */
			LockBuffer(stack->buffer, GIN_UNLOCK);
			scanForItems( btree->index, scanEntry, rootPostingTree );

			/*
			 * We lock again the entry page and while it was unlocked
			 * insert might occured, so we need to refind our position
			 */
			LockBuffer(stack->buffer, GIN_SHARE);
			page = BufferGetPage(stack->buffer);
			if ( !GinPageIsLeaf(page) )
			{
				/*
				 * Root page becomes non-leaf while we unlock it. We
				 * will start again, this situation doesn't cause
				 * often - root can became a non-leaf only one per
				 * life of index.
				 */

				return false;
			}

			for(;;)
			{
				if ( moveRightIfItNeeded(btree, stack) == false )
					elog(ERROR, "lost saved point in index"); /* must not happen !!! */

				page = BufferGetPage(stack->buffer);
				itup = (IndexTuple) PageGetItem(page, PageGetItemId(page, stack->off));
				newDatum = index_getattr(itup, FirstOffsetNumber, btree->ginstate->tupdesc, &isnull);

				if ( compareEntries(btree->ginstate, newDatum, savedDatum) == 0 )
				{
					/* Found!  */
					if ( btree->ginstate->tupdesc->attrs[0]->attbyval == false )
						pfree( DatumGetPointer(savedDatum) );
					break;
				}

				stack->off++;
			}
		}
		else
		{
			tbm_add_tuples( scanEntry->partialMatch, GinGetPosting(itup),  GinGetNPosting(itup), false);
			scanEntry->predictNumberResult +=  GinGetNPosting(itup);
		}

		/*
		 * Ok, we save ItemPointers, go to the next entry
		 */
		stack->off++;
	}

	return true;
}

/*
 * Start* functions setup begining state of searches: finds correct buffer and pins it.
 */
static void
startScanEntry(Relation index, GinState *ginstate, GinScanEntry entry)
{
	GinBtreeData 	btreeEntry;
	GinBtreeStack  *stackEntry;
	Page			page;
	bool			needUnlock = TRUE;

	if (entry->master != NULL)
	{
		entry->isFinished = entry->master->isFinished;
		return;
	}

	/*
	 * we should find entry, and begin scan of posting tree
	 * or just store posting list in memory
	 */

	prepareEntryScan(&btreeEntry, index, entry->entry, ginstate);
	btreeEntry.searchMode = TRUE;
	stackEntry = ginFindLeafPage(&btreeEntry, NULL);
	page = BufferGetPage(stackEntry->buffer);

	entry->isFinished = TRUE;
	entry->buffer = InvalidBuffer;
	entry->offset = InvalidOffsetNumber;
	entry->list = NULL;
	entry->nlist = 0;
	entry->partialMatch = NULL;
	entry->partialMatchResult = NULL;
	entry->reduceResult = FALSE;
	entry->predictNumberResult = 0;

	if ( entry->isPartialMatch )
	{
		/*
		 * btreeEntry.findItem points to the first equal or greater value
		 * than needed. So we will scan further and collect all
	 	 * ItemPointers
		 */
		btreeEntry.findItem(&btreeEntry, stackEntry);
		if ( computePartialMatchList( &btreeEntry, stackEntry, entry ) == false )
		{
			/*
			 * GIN tree was seriously restructured, so we will
			 * cleanup all found data and rescan. See comments near
			 * 'return false' in computePartialMatchList()
			 */
			if ( entry->partialMatch )
			{
				tbm_free( entry->partialMatch );
				entry->partialMatch = NULL;
			}
			LockBuffer(stackEntry->buffer, GIN_UNLOCK);
			freeGinBtreeStack(stackEntry);

			startScanEntry(index, ginstate, entry);
			return;
		}

		if ( entry->partialMatch && !tbm_is_empty(entry->partialMatch) )
		{
			tbm_begin_iterate(entry->partialMatch);
			entry->isFinished = FALSE;
		}
	}
	else if (btreeEntry.findItem(&btreeEntry, stackEntry))
	{
		IndexTuple	itup = (IndexTuple) PageGetItem(page, PageGetItemId(page, stackEntry->off));

		if (GinIsPostingTree(itup))
		{
			BlockNumber rootPostingTree = GinGetPostingTree(itup);
			GinPostingTreeScan *gdi;
			Page		page;

			/*
			 * We should unlock entry page before make deal with
			 * posting tree to prevent deadlocks with vacuum processes.
			 * Because entry is never deleted from page and posting tree is
			 * never reduced to the posting list, we can unlock page after
			 * getting BlockNumber of root of posting tree.
			 */
			LockBuffer(stackEntry->buffer, GIN_UNLOCK);
			needUnlock = FALSE;
			gdi = prepareScanPostingTree(index, rootPostingTree, TRUE);

			entry->buffer = scanBeginPostingTree(gdi);
			/*
			 * We keep buffer pinned because we need to prevent deletition
			 * page during scan. See GIN's vacuum implementation. RefCount
			 * is increased to keep buffer pinned after freeGinBtreeStack() call.
			 */
			IncrBufferRefCount(entry->buffer);

			page = BufferGetPage(entry->buffer);
			entry->predictNumberResult = gdi->stack->predictNumber * GinPageGetOpaque(page)->maxoff;

			/*
			 * Keep page content in memory to prevent durable page locking
			 */
			entry->list = (ItemPointerData *) palloc( BLCKSZ );
			entry->nlist = GinPageGetOpaque(page)->maxoff;
			memcpy( entry->list, GinDataPageGetItem(page, FirstOffsetNumber),
						GinPageGetOpaque(page)->maxoff * sizeof(ItemPointerData) );

			LockBuffer(entry->buffer, GIN_UNLOCK);
			freeGinBtreeStack(gdi->stack);
			pfree(gdi);
			entry->isFinished = FALSE;
		}
		else if (GinGetNPosting(itup) > 0)
		{
			entry->nlist = GinGetNPosting(itup);
			entry->list = (ItemPointerData *) palloc(sizeof(ItemPointerData) * entry->nlist);
			memcpy(entry->list, GinGetPosting(itup), sizeof(ItemPointerData) * entry->nlist);
			entry->isFinished = FALSE;
		}
	}

	if (needUnlock)
			LockBuffer(stackEntry->buffer, GIN_UNLOCK);
	freeGinBtreeStack(stackEntry);
}

static void
startScanKey(Relation index, GinState *ginstate, GinScanKey key)
{
	uint32		i;

	if (!key->firstCall)
		return;

	for (i = 0; i < key->nentries; i++)
	{
		startScanEntry(index, ginstate, key->scanEntry + i);
		/*
		 * Copy strategy number to each entry of key to
		 * use in comparePartialFn call
		 */
		key->scanEntry[i].strategy = key->strategy;
	}

	memset(key->entryRes, TRUE, sizeof(bool) * key->nentries);
	key->isFinished = FALSE;
	key->firstCall = FALSE;

	if (GinFuzzySearchLimit > 0)
	{
		/*
		 * If all of keys more than threshold we will try to reduce
		 * result, we hope (and only hope, for intersection operation of
		 * array our supposition isn't true), that total result will not
		 * more than minimal predictNumberResult.
		 */

		for (i = 0; i < key->nentries; i++)
			if (key->scanEntry[i].predictNumberResult <= key->nentries * GinFuzzySearchLimit)
				return;

		for (i = 0; i < key->nentries; i++)
			if (key->scanEntry[i].predictNumberResult > key->nentries * GinFuzzySearchLimit)
			{
				key->scanEntry[i].predictNumberResult /= key->nentries;
				key->scanEntry[i].reduceResult = TRUE;
			}
	}
}

static void
startScan(IndexScanDesc scan)
{
	uint32		i;
	GinScanOpaque so = (GinScanOpaque) scan->opaque;

	for (i = 0; i < so->nkeys; i++)
		startScanKey(scan->indexRelation, &so->ginstate, so->keys + i);
}

/*
 * Gets next ItemPointer from PostingTree. Note, that we copy
 * page into GinScanEntry->list array and unlock page, but keep it pinned
 * to prevent interference with vacuum
 */
static void
entryGetNextItem(Relation index, GinScanEntry entry)
{
	Page		page;
	BlockNumber blkno;

	for(;;)
	{
		entry->offset++;

		if (entry->offset <= entry->nlist)
		{
			entry->curItem = entry->list[entry->offset - 1];
			return;
		}

		LockBuffer(entry->buffer, GIN_SHARE);
		page = BufferGetPage(entry->buffer);
		for(;;)
		{
			/*
			 * It's needed to go by right link. During that we should refind
			 * first ItemPointer greater that stored
			 */

			blkno = GinPageGetOpaque(page)->rightlink;

			LockBuffer(entry->buffer, GIN_UNLOCK);
			if (blkno == InvalidBlockNumber)
			{
				ReleaseBuffer(entry->buffer);
				ItemPointerSet(&entry->curItem, InvalidBlockNumber, InvalidOffsetNumber);
				entry->buffer = InvalidBuffer;
				entry->isFinished = TRUE;
				return;
			}

			entry->buffer = ReleaseAndReadBuffer(entry->buffer, index, blkno);
			LockBuffer(entry->buffer, GIN_SHARE);
			page = BufferGetPage(entry->buffer);

			entry->offset = InvalidOffsetNumber;
			if (!ItemPointerIsValid(&entry->curItem) || findItemInPage(page, &entry->curItem, &entry->offset))
			{
				/*
				 * Found position equal to or greater than stored
				 */
				entry->nlist = GinPageGetOpaque(page)->maxoff;
				memcpy( entry->list, GinDataPageGetItem(page, FirstOffsetNumber),
							GinPageGetOpaque(page)->maxoff * sizeof(ItemPointerData) );

				LockBuffer(entry->buffer, GIN_UNLOCK);

				if ( !ItemPointerIsValid(&entry->curItem) ||
					 compareItemPointers( &entry->curItem, entry->list + entry->offset - 1 ) == 0 )
				{
					/*
					 * First pages are deleted or empty, or we found exact position,
					 * so break inner loop and continue outer one.
					 */

					 break;
				}

				/*
				 * Find greater than entry->curItem position, store it.
				 */
				entry->curItem = entry->list[entry->offset - 1];

				return;
			}
		}
	}
}

#define gin_rand() (((double) random()) / ((double) MAX_RANDOM_VALUE))
#define dropItem(e) ( gin_rand() > ((double)GinFuzzySearchLimit)/((double)((e)->predictNumberResult)) )

/*
 * Sets entry->curItem to new found heap item pointer for one
 * entry of one scan key
 */
static bool
entryGetItem(Relation index, GinScanEntry entry)
{
	if (entry->master)
	{
		entry->isFinished = entry->master->isFinished;
		entry->curItem = entry->master->curItem;
	}
	else if ( entry->partialMatch )
	{
		do
		{
			if ( entry->partialMatchResult == NULL || entry->offset >= entry->partialMatchResult->ntuples )
			{
				entry->partialMatchResult = tbm_iterate( entry->partialMatch );

				if ( entry->partialMatchResult == NULL )
				{
					ItemPointerSet(&entry->curItem, InvalidBlockNumber, InvalidOffsetNumber);
					entry->isFinished = TRUE;
					break;
				}
				else if ( entry->partialMatchResult->ntuples < 0 )
				{
					/* bitmap became lossy */
					ereport(ERROR,
							(errcode(ERRCODE_OUT_OF_MEMORY),
							errmsg("not enough memory to store result of partial match operator" ),
							errhint("Increase the \"work_mem\" parameter.")));
				}
				entry->offset = 0;
			}

			ItemPointerSet(&entry->curItem,
							entry->partialMatchResult->blockno,
							entry->partialMatchResult->offsets[ entry->offset ]);
			entry->offset ++;

		} while (entry->isFinished == FALSE && entry->reduceResult == TRUE && dropItem(entry));
	}
	else if (!BufferIsValid(entry->buffer))
	{
		entry->offset++;
		if (entry->offset <= entry->nlist)
			entry->curItem = entry->list[entry->offset - 1];
		else
		{
			ItemPointerSet(&entry->curItem, InvalidBlockNumber, InvalidOffsetNumber);
			entry->isFinished = TRUE;
		}
	}
	else
	{
		do
		{
			entryGetNextItem(index, entry);
		} while (entry->isFinished == FALSE && entry->reduceResult == TRUE && dropItem(entry));
	}

	return entry->isFinished;
}

/*
 * restart from saved position. Actually it's needed only for
 * partial match. function is called only by ginrestpos()
 */
void
ginrestartentry(GinScanEntry entry)
{
	ItemPointerData stopItem = entry->curItem;
	bool savedReduceResult;

	if ( entry->master || entry->partialMatch == NULL )
		return; /* entry is slave or not a partial match type*/

	if ( entry->isFinished )
		return; /* entry was finished before ginmarkpos() call */

	if ( ItemPointerGetBlockNumber(&stopItem) == InvalidBlockNumber )
		return; /* entry  wasn't began before ginmarkpos() call */

	/*
	 * Reset iterator
	 */
	tbm_begin_iterate( entry->partialMatch );
	entry->partialMatchResult = NULL;
	entry->offset = 0;

	/*
	 * Temporary reset reduceResult flag to guarantee refinding
	 * of curItem
	 */
	savedReduceResult = entry->reduceResult;
	entry->reduceResult = FALSE;

	do
	{
		/*
		 * We can use null instead of index because
		 * partial match doesn't use it
		 */
		if ( entryGetItem( NULL, entry ) == false )
			elog(ERROR, "cannot refind scan position"); /* must not be here! */
	} while( compareItemPointers( &stopItem, &entry->curItem ) != 0 );

	Assert( entry->isFinished == FALSE );

	entry->reduceResult = savedReduceResult;
}

/*
 * Sets key->curItem to new found heap item pointer for one scan key
 * Returns isFinished, ie TRUE means we did NOT get a new item pointer!
 * Also, *keyrecheck is set true if recheck is needed for this scan key.
 */
static bool
keyGetItem(Relation index, GinState *ginstate, MemoryContext tempCtx,
		   GinScanKey key, bool *keyrecheck)
{
	uint32		i;
	GinScanEntry entry;
	bool		res;
	MemoryContext oldCtx;

	if (key->isFinished)
		return TRUE;

	do
	{
		/*
		 * move forward from previously value and set new curItem, which is
		 * minimal from entries->curItems
		 */
		ItemPointerSetMax(&key->curItem);
		for (i = 0; i < key->nentries; i++)
		{
			entry = key->scanEntry + i;

			if (key->entryRes[i])
			{
				if (entry->isFinished == FALSE && entryGetItem(index, entry) == FALSE)
				{
					if (compareItemPointers(&entry->curItem, &key->curItem) < 0)
						key->curItem = entry->curItem;
				}
				else
					key->entryRes[i] = FALSE;
			}
			else if (entry->isFinished == FALSE)
			{
				if (compareItemPointers(&entry->curItem, &key->curItem) < 0)
					key->curItem = entry->curItem;
			}
		}

		if (ItemPointerIsMax(&key->curItem))
		{
			/* all entries are finished */
			key->isFinished = TRUE;
			return TRUE;
		}

		/*
		 * if key->nentries == 1 then the consistentFn should always succeed,
		 * but we must call it anyway to find out the recheck status.
		 */

		/* setting up array for consistentFn */
		for (i = 0; i < key->nentries; i++)
		{
			entry = key->scanEntry + i;

			if (entry->isFinished == FALSE &&
				compareItemPointers(&entry->curItem, &key->curItem) == 0)
				key->entryRes[i] = TRUE;
			else
				key->entryRes[i] = FALSE;
		}

		/*
		 * Initialize *keyrecheck in case the consistentFn doesn't know it
		 * should set it.  The safe assumption in that case is to force
		 * recheck.
		 */
		*keyrecheck = true;

		oldCtx = MemoryContextSwitchTo(tempCtx);
		res = DatumGetBool(FunctionCall4(&ginstate->consistentFn,
										 PointerGetDatum(key->entryRes),
										 UInt16GetDatum(key->strategy),
										 key->query,
										 PointerGetDatum(keyrecheck)));
		MemoryContextSwitchTo(oldCtx);
		MemoryContextReset(tempCtx);
	} while (!res);

	return FALSE;
}

/*
 * Get heap item pointer from scan
 * returns true if found
 */
static bool
scanGetItem(IndexScanDesc scan, ItemPointerData *item, bool *recheck)
{
	GinScanOpaque so = (GinScanOpaque) scan->opaque;
	uint32		i;
	bool		keyrecheck;

	/*
	 * We return recheck = true if any of the keyGetItem calls return
	 * keyrecheck = true.  Note that because the second loop might advance
	 * some keys, this could theoretically be too conservative.  In practice
	 * though, we expect that a consistentFn's recheck result will depend
	 * only on the operator and the query, so for any one key it should
	 * stay the same regardless of advancing to new items.  So it's not
	 * worth working harder.
	 */
	*recheck = false;

	ItemPointerSetMin(item);
	for (i = 0; i < so->nkeys; i++)
	{
		GinScanKey	key = so->keys + i;

		if (keyGetItem(scan->indexRelation, &so->ginstate, so->tempCtx,
					   key, &keyrecheck))
			return FALSE;		/* finished one of keys */
		if (compareItemPointers(item, &key->curItem) < 0)
			*item = key->curItem;
		*recheck |= keyrecheck;
	}

	for (i = 1; i <= so->nkeys; i++)
	{
		GinScanKey	key = so->keys + i - 1;

		for (;;)
		{
			int			cmp = compareItemPointers(item, &key->curItem);

			if (cmp == 0)
				break;
			else if (cmp > 0)
			{
				if (keyGetItem(scan->indexRelation, &so->ginstate, so->tempCtx,
							   key, &keyrecheck))
					return FALSE;		/* finished one of keys */
				*recheck |= keyrecheck;
			}
			else
			{					/* returns to begin */
				*item = key->curItem;
				i = 0;
				break;
			}
		}
	}

	return TRUE;
}

#define GinIsNewKey(s)		( ((GinScanOpaque) scan->opaque)->keys == NULL )
#define GinIsVoidRes(s)		( ((GinScanOpaque) scan->opaque)->isVoidRes == true )

Datum
gingetbitmap(PG_FUNCTION_ARGS)
{
	IndexScanDesc scan = (IndexScanDesc) PG_GETARG_POINTER(0);
	TIDBitmap *tbm = (TIDBitmap *) PG_GETARG_POINTER(1);
	int64		ntids;

	if (GinIsNewKey(scan))
		newScanKey(scan);

	if (GinIsVoidRes(scan))
		PG_RETURN_INT64(0);

	startScan(scan);

	ntids = 0;
	for (;;)
	{
		ItemPointerData iptr;
		bool		recheck;

		CHECK_FOR_INTERRUPTS();

		if (!scanGetItem(scan, &iptr, &recheck))
			break;

		tbm_add_tuples(tbm, &iptr, 1, recheck);
		ntids++;
	}

	PG_RETURN_INT64(ntids);
}

Datum
gingettuple(PG_FUNCTION_ARGS)
{
	IndexScanDesc scan = (IndexScanDesc) PG_GETARG_POINTER(0);
	ScanDirection dir = (ScanDirection) PG_GETARG_INT32(1);
	bool		res;

	if (dir != ForwardScanDirection)
		elog(ERROR, "GIN doesn't support other scan directions than forward");

	if (GinIsNewKey(scan))
		newScanKey(scan);

	if (GinIsVoidRes(scan))
		PG_RETURN_BOOL(false);

	startScan(scan);
	res = scanGetItem(scan, &scan->xs_ctup.t_self, &scan->xs_recheck);

	PG_RETURN_BOOL(res);
}