heapam.c

/*-------------------------------------------------------------------------
 *
 * heapam.c--
 *	  heap access method code
 *
 * Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *	  $Header: /cvsroot/pgsql/src/backend/access/heap/heapam.c,v 1.37 1998/10/12 00:53:30 momjian Exp $
 *
 *
 * INTERFACE ROUTINES
 *		heapgettup		- fetch next heap tuple from a scan
 *		heap_open		- open a heap relation by relationId
 *		heap_openr		- open a heap relation by name
 *		heap_close		- close a heap relation
 *		heap_beginscan	- begin relation scan
 *		heap_rescan		- restart a relation scan
 *		heap_endscan	- end relation scan
 *		heap_getnext	- retrieve next tuple in scan
 *		heap_fetch		- retrive tuple with tid
 *		heap_insert		- insert tuple into a relation
 *		heap_delete		- delete a tuple from a relation
 *		heap_replace	- replace a tuple in a relation with another tuple
 *		heap_markpos	- mark scan position
 *		heap_restrpos	- restore position to marked location
 *
 * NOTES
 *	  This file contains the heap_ routines which implement
 *	  the POSTGRES heap access method used for all POSTGRES
 *	  relations.
 *
 * OLD COMMENTS
 *		struct relscan hints:  (struct should be made AM independent?)
 *
 *		rs_ctid is the tid of the last tuple returned by getnext.
 *		rs_ptid and rs_ntid are the tids of the previous and next tuples
 *		returned by getnext, respectively.	NULL indicates an end of
 *		scan (either direction); NON indicates an unknow value.
 *
 *		possible combinations:
 *		rs_p	rs_c	rs_n			interpretation
 *		NULL	NULL	NULL			empty scan
 *		NULL	NULL	NON				at begining of scan
 *		NULL	NULL	t1				at begining of scan (with cached tid)
 *		NON		NULL	NULL			at end of scan
 *		t1		NULL	NULL			at end of scan (with cached tid)
 *		NULL	t1		NULL			just returned only tuple
 *		NULL	t1		NON				just returned first tuple
 *		NULL	t1		t2				returned first tuple (with cached tid)
 *		NON		t1		NULL			just returned last tuple
 *		t2		t1		NULL			returned last tuple (with cached tid)
 *		t1		t2		NON				in the middle of a forward scan
 *		NON		t2		t1				in the middle of a reverse scan
 *		ti		tj		tk				in the middle of a scan (w cached tid)
 *
 *		Here NULL is ...tup == NULL && ...buf == InvalidBuffer,
 *		and NON is ...tup == NULL && ...buf == UnknownBuffer.
 *
 *		Currently, the NONTID values are not cached with their actual
 *		values by getnext.	Values may be cached by markpos since it stores
 *		all three tids.
 *
 *		NOTE:  the calls to elog() must stop.  Should decide on an interface
 *		between the general and specific AM calls.
 *
 *		XXX probably do not need a free tuple routine for heaps.
 *		Huh?  Free tuple is not necessary for tuples returned by scans, but
 *		is necessary for tuples which are returned by
 *		RelationGetTupleByItemPointer. -hirohama
 *
 *-------------------------------------------------------------------------
 */

#include <postgres.h>

#include <storage/bufpage.h>
#include <access/heapam.h>
#include <miscadmin.h>
#include <utils/relcache.h>
#include <access/valid.h>
#include <access/hio.h>
#include <storage/lmgr.h>
#include <storage/smgr.h>
#include <catalog/catalog.h>
#include <access/transam.h>
#include <access/xact.h>
#include <utils/inval.h>
#include <utils/memutils.h>


#ifndef HAVE_MEMMOVE
#include <regex/utils.h>
#else
#include <string.h>
#endif

static void doinsert(Relation relation, HeapTuple tup);

/* ----------------------------------------------------------------
 *						 heap support routines
 * ----------------------------------------------------------------
 */

/* ----------------
 *		initscan - scan code common to heap_beginscan and heap_rescan
 * ----------------
 */
static void
initscan(HeapScanDesc scan,
		 Relation relation,
		 int atend,
		 unsigned nkeys,
		 ScanKey key)
{
	if (!RelationGetNumberOfBlocks(relation))
	{
		/* ----------------
		 *	relation is empty
		 * ----------------
		 */
		scan->rs_ntup = scan->rs_ctup = scan->rs_ptup = NULL;
		scan->rs_nbuf = scan->rs_cbuf = scan->rs_pbuf = InvalidBuffer;
	}
	else if (atend)
	{
		/* ----------------
		 *	reverse scan
		 * ----------------
		 */
		scan->rs_ntup = scan->rs_ctup = NULL;
		scan->rs_nbuf = scan->rs_cbuf = InvalidBuffer;
		scan->rs_ptup = NULL;
		scan->rs_pbuf = UnknownBuffer;
	}
	else
	{
		/* ----------------
		 *	forward scan
		 * ----------------
		 */
		scan->rs_ctup = scan->rs_ptup = NULL;
		scan->rs_cbuf = scan->rs_pbuf = InvalidBuffer;
		scan->rs_ntup = NULL;
		scan->rs_nbuf = UnknownBuffer;
	}							/* invalid too */

	/* we don't have a marked position... */
	ItemPointerSetInvalid(&(scan->rs_mptid));
	ItemPointerSetInvalid(&(scan->rs_mctid));
	ItemPointerSetInvalid(&(scan->rs_mntid));
	ItemPointerSetInvalid(&(scan->rs_mcd));

	/* ----------------
	 *	copy the scan key, if appropriate
	 * ----------------
	 */
	if (key != NULL)
		memmove(scan->rs_key, key, nkeys * sizeof(ScanKeyData));
}

/* ----------------
 *		unpinscan - code common to heap_rescan and heap_endscan
 * ----------------
 */
static void
unpinscan(HeapScanDesc scan)
{
	if (BufferIsValid(scan->rs_pbuf))
		ReleaseBuffer(scan->rs_pbuf);

	/* ------------------------------------
	 *	Scan will pin buffer one for each non-NULL tuple pointer
	 *	(ptup, ctup, ntup), so they have to be unpinned multiple
	 *	times.
	 * ------------------------------------
	 */
	if (BufferIsValid(scan->rs_cbuf))
		ReleaseBuffer(scan->rs_cbuf);

	if (BufferIsValid(scan->rs_nbuf))
		ReleaseBuffer(scan->rs_nbuf);
}

/* ------------------------------------------
 *		nextpage
 *
 *		figure out the next page to scan after the current page
 *		taking into account of possible adjustment of degrees of
 *		parallelism
 * ------------------------------------------
 */
static int
nextpage(int page, int dir)
{
	return (dir < 0) ? page - 1 : page + 1;
}

/* ----------------
 *		heapgettup - fetch next heap tuple
 *
 *		routine used by heap_getnext() which does most of the
 *		real work in scanning tuples.
 *
 *		The scan routines handle their own buffer lock/unlocking, so
 *		there is no reason to request the buffer number unless
 *		to want to perform some other operation with the result,
 *		like pass it to another function.
 * ----------------
 */
static HeapTuple
heapgettup(Relation relation,
		   ItemPointer tid,
		   int dir,
		   Buffer *buf,
		   Snapshot snapshot,
		   int nkeys,
		   ScanKey key)
{
	ItemId		lpp;
	Page		dp;
	int			page;
	int			pages;
	int			lines;
	HeapTuple	rtup;
	OffsetNumber lineoff;
	int			linesleft;

	/* ----------------
	 *	increment access statistics
	 * ----------------
	 */
	IncrHeapAccessStat(local_heapgettup);
	IncrHeapAccessStat(global_heapgettup);

	/* ----------------
	 *	debugging stuff
	 *
	 * check validity of arguments, here and for other functions too
	 * Note: no locking manipulations needed--this is a local function
	 * ----------------
	 */
#ifdef	HEAPDEBUGALL
	if (ItemPointerIsValid(tid))
	{
		elog(DEBUG, "heapgettup(%s, tid=0x%x[%d,%d], dir=%d, ...)",
			 RelationGetRelationName(relation), tid, tid->ip_blkid,
			 tid->ip_posid, dir);
	}
	else
	{
		elog(DEBUG, "heapgettup(%s, tid=0x%x, dir=%d, ...)",
			 RelationGetRelationName(relation), tid, dir);
	}
	elog(DEBUG, "heapgettup(..., b=0x%x, nkeys=%d, key=0x%x", buf, nkeys, key);

	elog(DEBUG, "heapgettup: relation(%c)=`%s', %p",
		 relation->rd_rel->relkind, &relation->rd_rel->relname,
		 snapshot);
#endif	 /* !defined(HEAPDEBUGALL) */

	if (!ItemPointerIsValid(tid))
		Assert(!PointerIsValid(tid));

	/* ----------------
	 *	return null immediately if relation is empty
	 * ----------------
	 */
	if (!(pages = relation->rd_nblocks))
		return NULL;

	/* ----------------
	 *	calculate next starting lineoff, given scan direction
	 * ----------------
	 */
	if (!dir)
	{
		/* ----------------
		 * ``no movement'' scan direction
		 * ----------------
		 */
		/* assume it is a valid TID XXX */
		if (ItemPointerIsValid(tid) == false)
		{
			*buf = InvalidBuffer;
			return NULL;
		}
		*buf = RelationGetBufferWithBuffer(relation,
										   ItemPointerGetBlockNumber(tid),
										   *buf);

#ifndef NO_BUFFERISVALID
		if (!BufferIsValid(*buf))
			elog(ERROR, "heapgettup: failed ReadBuffer");
#endif

		dp = (Page) BufferGetPage(*buf);
		lineoff = ItemPointerGetOffsetNumber(tid);
		lpp = PageGetItemId(dp, lineoff);

		rtup = (HeapTuple) PageGetItem((Page) dp, lpp);
		return rtup;

	}
	else if (dir < 0)
	{
		/* ----------------
		 *	reverse scan direction
		 * ----------------
		 */
		if (ItemPointerIsValid(tid) == false)
			tid = NULL;
		if (tid == NULL)
		{
			page = pages - 1;	/* final page */
		}
		else
		{
			page = ItemPointerGetBlockNumber(tid);		/* current page */
		}
		if (page < 0)
		{
			*buf = InvalidBuffer;
			return NULL;
		}

		*buf = RelationGetBufferWithBuffer(relation, page, *buf);
#ifndef NO_BUFFERISVALID
		if (!BufferIsValid(*buf))
			elog(ERROR, "heapgettup: failed ReadBuffer");
#endif

		dp = (Page) BufferGetPage(*buf);
		lines = PageGetMaxOffsetNumber(dp);
		if (tid == NULL)
		{
			lineoff = lines;	/* final offnum */
		}
		else
		{
			lineoff =			/* previous offnum */
				OffsetNumberPrev(ItemPointerGetOffsetNumber(tid));
		}
		/* page and lineoff now reference the physically previous tid */

	}
	else
	{
		/* ----------------
		 *	forward scan direction
		 * ----------------
		 */
		if (ItemPointerIsValid(tid) == false)
		{
			page = 0;			/* first page */
			lineoff = FirstOffsetNumber;		/* first offnum */
		}
		else
		{
			page = ItemPointerGetBlockNumber(tid);		/* current page */
			lineoff =			/* next offnum */
				OffsetNumberNext(ItemPointerGetOffsetNumber(tid));
		}

		if (page >= pages)
		{
			*buf = InvalidBuffer;
			return NULL;
		}
		/* page and lineoff now reference the physically next tid */

		*buf = RelationGetBufferWithBuffer(relation, page, *buf);
#ifndef NO_BUFFERISVALID
		if (!BufferIsValid(*buf))
			elog(ERROR, "heapgettup: failed ReadBuffer");
#endif

		dp = (Page) BufferGetPage(*buf);
		lines = PageGetMaxOffsetNumber(dp);
	}

	/* 'dir' is now non-zero */

	/* ----------------
	 *	calculate line pointer and number of remaining items
	 *	to check on this page.
	 * ----------------
	 */
	lpp = PageGetItemId(dp, lineoff);
	if (dir < 0)
		linesleft = lineoff - 1;
	else
		linesleft = lines - lineoff;

	/* ----------------
	 *	advance the scan until we find a qualifying tuple or
	 *	run out of stuff to scan
	 * ----------------
	 */
	for (;;)
	{
		while (linesleft >= 0)
		{
			/* ----------------
			 *	if current tuple qualifies, return it.
			 * ----------------
			 */
			HeapTupleSatisfies(lpp, relation, *buf, (PageHeader) dp,
							   snapshot, nkeys, key, rtup);
			if (rtup != NULL)
			{
				ItemPointer iptr = &(rtup->t_ctid);

				if (ItemPointerGetBlockNumber(iptr) != page)
				{

					/*
					 * set block id to the correct page number --- this is
					 * a hack to support the virtual fragment concept
					 */
					ItemPointerSetBlockNumber(iptr, page);
				}
				return rtup;
			}

			/* ----------------
			 *	otherwise move to the next item on the page
			 * ----------------
			 */
			--linesleft;
			if (dir < 0)
			{
				--lpp;			/* move back in this page's ItemId array */
			}
			else
			{
				++lpp;			/* move forward in this page's ItemId
								 * array */
			}
		}

		/* ----------------
		 *	if we get here, it means we've exhausted the items on
		 *	this page and it's time to move to the next..
		 * ----------------
		 */
		page = nextpage(page, dir);

		/* ----------------
		 *	return NULL if we've exhausted all the pages..
		 * ----------------
		 */
		if (page < 0 || page >= pages)
		{
			if (BufferIsValid(*buf))
				ReleaseBuffer(*buf);
			*buf = InvalidBuffer;
			return NULL;
		}

		*buf = ReleaseAndReadBuffer(*buf, relation, page);

#ifndef NO_BUFFERISVALID
		if (!BufferIsValid(*buf))
			elog(ERROR, "heapgettup: failed ReadBuffer");
#endif
		dp = (Page) BufferGetPage(*buf);
		lines = lineoff = PageGetMaxOffsetNumber((Page) dp);
		linesleft = lines - 1;
		if (dir < 0)
			lpp = PageGetItemId(dp, lineoff);
		else
			lpp = PageGetItemId(dp, FirstOffsetNumber);
	}
}

static void
doinsert(Relation relation, HeapTuple tup)
{
	RelationPutHeapTupleAtEnd(relation, tup);
	return;
}


/* ----------------------------------------------------------------
 *					 heap access method interface
 * ----------------------------------------------------------------
 */
/* ----------------
 *		heap_open - open a heap relation by relationId
 *
 *		presently the relcache routines do all the work we need
 *		to open/close heap relations.
 * ----------------
 */
Relation
heap_open(Oid relationId)
{
	Relation	r;

	/* ----------------
	 *	increment access statistics
	 * ----------------
	 */
	IncrHeapAccessStat(local_open);
	IncrHeapAccessStat(global_open);

	r = (Relation) RelationIdGetRelation(relationId);

	if (RelationIsValid(r) && r->rd_rel->relkind == RELKIND_INDEX)
		elog(ERROR, "%s is an index relation", r->rd_rel->relname.data);

	return r;
}

/* ----------------
 *		heap_openr - open a heap relation by name
 *
 *		presently the relcache routines do all the work we need
 *		to open/close heap relations.
 * ----------------
 */
Relation
heap_openr(char *relationName)
{
	Relation	r;

	/* ----------------
	 *	increment access statistics
	 * ----------------
	 */
	IncrHeapAccessStat(local_openr);
	IncrHeapAccessStat(global_openr);

	r = RelationNameGetRelation(relationName);

	if (RelationIsValid(r) && r->rd_rel->relkind == RELKIND_INDEX)
		elog(ERROR, "%s is an index relation", r->rd_rel->relname.data);

	return r;
}

/* ----------------
 *		heap_close - close a heap relation
 *
 *		presently the relcache routines do all the work we need
 *		to open/close heap relations.
 * ----------------
 */
void
heap_close(Relation relation)
{
	/* ----------------
	 *	increment access statistics
	 * ----------------
	 */
	IncrHeapAccessStat(local_close);
	IncrHeapAccessStat(global_close);

	RelationClose(relation);
}


/* ----------------
 *		heap_beginscan	- begin relation scan
 * ----------------
 */
HeapScanDesc
heap_beginscan(Relation relation,
			   int atend,
			   Snapshot snapshot,
			   unsigned nkeys,
			   ScanKey key)
{
	HeapScanDesc scan;

	/* ----------------
	 *	increment access statistics
	 * ----------------
	 */
	IncrHeapAccessStat(local_beginscan);
	IncrHeapAccessStat(global_beginscan);

	/* ----------------
	 *	sanity checks
	 * ----------------
	 */
	if (RelationIsValid(relation) == false)
		elog(ERROR, "heap_beginscan: !RelationIsValid(relation)");

	/* ----------------
	 * set relation level read lock
	 * ----------------
	 */
	RelationSetLockForRead(relation);

	/* XXX someday assert SelfTimeQual if relkind == RELKIND_UNCATALOGED */
	if (relation->rd_rel->relkind == RELKIND_UNCATALOGED)
		snapshot = SnapshotSelf;

	/* ----------------
	 *	increment relation ref count while scanning relation
	 * ----------------
	 */
	RelationIncrementReferenceCount(relation);

	/* ----------------
	 *	allocate and initialize scan descriptor
	 * ----------------
	 */
	scan = (HeapScanDesc) palloc(sizeof(HeapScanDescData));

	relation->rd_nblocks = smgrnblocks(DEFAULT_SMGR, relation);
	scan->rs_rd = relation;

	if (nkeys)

		/*
		 * we do this here instead of in initscan() because heap_rescan
		 * also calls initscan() and we don't want to allocate memory
		 * again
		 */
		scan->rs_key = (ScanKey) palloc(sizeof(ScanKeyData) * nkeys);
	else
		scan->rs_key = NULL;

	initscan(scan, relation, atend, nkeys, key);

	scan->rs_atend = atend;
	scan->rs_snapshot = snapshot;
	scan->rs_nkeys = (short) nkeys;

	return scan;
}

/* ----------------
 *		heap_rescan		- restart a relation scan
 * ----------------
 */
void
heap_rescan(HeapScanDesc scan,
			bool scanFromEnd,
			ScanKey key)
{
	/* ----------------
	 *	increment access statistics
	 * ----------------
	 */
	IncrHeapAccessStat(local_rescan);
	IncrHeapAccessStat(global_rescan);

	/* Note: set relation level read lock is still set */

	/* ----------------
	 *	unpin scan buffers
	 * ----------------
	 */
	unpinscan(scan);

	/* ----------------
	 *	reinitialize scan descriptor
	 * ----------------
	 */
	initscan(scan, scan->rs_rd, scanFromEnd, scan->rs_nkeys, key);
	scan->rs_atend = (bool) scanFromEnd;
}

/* ----------------
 *		heap_endscan	- end relation scan
 *
 *		See how to integrate with index scans.
 *		Check handling if reldesc caching.
 * ----------------
 */
void
heap_endscan(HeapScanDesc scan)
{
	/* ----------------
	 *	increment access statistics
	 * ----------------
	 */
	IncrHeapAccessStat(local_endscan);
	IncrHeapAccessStat(global_endscan);

	/* Note: no locking manipulations needed */

	/* ----------------
	 *	unpin scan buffers
	 * ----------------
	 */
	unpinscan(scan);

	/* ----------------
	 *	decrement relation reference count and free scan descriptor storage
	 * ----------------
	 */
	RelationDecrementReferenceCount(scan->rs_rd);

	/* ----------------
	 * Non 2-phase read locks on catalog relations
	 * ----------------
	 */
	if (IsSystemRelationName(RelationGetRelationName(scan->rs_rd)->data))

		RelationUnsetLockForRead(scan->rs_rd);

	pfree(scan);				/* XXX */
}

/* ----------------
 *		heap_getnext	- retrieve next tuple in scan
 *
 *		Fix to work with index relations.
 *		We don't return the buffer anymore, but you can get it from the
 *		returned HeapTuple.
 * ----------------
 */

#ifdef HEAPDEBUGALL
#define HEAPDEBUG_1 \
elog(DEBUG, "heap_getnext([%s,nkeys=%d],backw=%d) called", \
	 scan->rs_rd->rd_rel->relname.data, scan->rs_nkeys, backw)

#define HEAPDEBUG_2 \
	 elog(DEBUG, "heap_getnext called with backw (no tracing yet)")

#define HEAPDEBUG_3 \
	 elog(DEBUG, "heap_getnext returns NULL at end")

#define HEAPDEBUG_4 \
	 elog(DEBUG, "heap_getnext valid buffer UNPIN'd")

#define HEAPDEBUG_5 \
	 elog(DEBUG, "heap_getnext next tuple was cached")

#define HEAPDEBUG_6 \
	 elog(DEBUG, "heap_getnext returning EOS")

#define HEAPDEBUG_7 \
	 elog(DEBUG, "heap_getnext returning tuple");
#else
#define HEAPDEBUG_1
#define HEAPDEBUG_2
#define HEAPDEBUG_3
#define HEAPDEBUG_4
#define HEAPDEBUG_5
#define HEAPDEBUG_6
#define HEAPDEBUG_7
#endif	 /* !defined(HEAPDEBUGALL) */


HeapTuple
heap_getnext(HeapScanDesc scandesc, int backw)
{
	HeapScanDesc scan = scandesc;

	/* ----------------
	 *	increment access statistics
	 * ----------------
	 */
	IncrHeapAccessStat(local_getnext);
	IncrHeapAccessStat(global_getnext);

	/* Note: no locking manipulations needed */

	/* ----------------
	 *	argument checks
	 * ----------------
	 */
	if (scan == NULL)
		elog(ERROR, "heap_getnext: NULL relscan");

	/* ----------------
	 *	initialize return buffer to InvalidBuffer
	 * ----------------
	 */

	HEAPDEBUG_1;				/* heap_getnext( info ) */

	if (backw)
	{
		/* ----------------
		 *	handle reverse scan
		 * ----------------
		 */
		HEAPDEBUG_2;			/* heap_getnext called with backw */

		if (scan->rs_ptup == scan->rs_ctup &&
			BufferIsInvalid(scan->rs_pbuf))
		{
			if (BufferIsValid(scan->rs_nbuf))
				ReleaseBuffer(scan->rs_nbuf);
			return NULL;
		}

		/*
		 * Copy the "current" tuple/buffer to "next". Pin/unpin the
		 * buffers accordingly
		 */
		if (scan->rs_nbuf != scan->rs_cbuf)
		{
			if (BufferIsValid(scan->rs_nbuf))
				ReleaseBuffer(scan->rs_nbuf);
			if (BufferIsValid(scan->rs_cbuf))
				IncrBufferRefCount(scan->rs_cbuf);
		}
		scan->rs_ntup = scan->rs_ctup;
		scan->rs_nbuf = scan->rs_cbuf;

		if (scan->rs_ptup != NULL)
		{
			if (scan->rs_cbuf != scan->rs_pbuf)
			{
				if (BufferIsValid(scan->rs_cbuf))
					ReleaseBuffer(scan->rs_cbuf);
				if (BufferIsValid(scan->rs_pbuf))
					IncrBufferRefCount(scan->rs_pbuf);
			}
			scan->rs_ctup = scan->rs_ptup;
			scan->rs_cbuf = scan->rs_pbuf;
		}
		else
		{						/* NONTUP */
			ItemPointer iptr;

			iptr = (scan->rs_ctup != NULL) ?
				&(scan->rs_ctup->t_ctid) : (ItemPointer) NULL;

			/*
			 * Don't release scan->rs_cbuf at this point, because
			 * heapgettup doesn't increase PrivateRefCount if it is
			 * already set. On a backward scan, both rs_ctup and rs_ntup
			 * usually point to the same buffer page, so
			 * PrivateRefCount[rs_cbuf] should be 2 (or more, if for
			 * instance ctup is stored in a TupleTableSlot).  - 01/09/94
			 */

			scan->rs_ctup = (HeapTuple)
				heapgettup(scan->rs_rd,
						   iptr,
						   -1,
						   &(scan->rs_cbuf),
						   scan->rs_snapshot,
						   scan->rs_nkeys,
						   scan->rs_key);
		}

		if (scan->rs_ctup == NULL && !BufferIsValid(scan->rs_cbuf))
		{
			if (BufferIsValid(scan->rs_pbuf))
				ReleaseBuffer(scan->rs_pbuf);
			scan->rs_ptup = NULL;
			scan->rs_pbuf = InvalidBuffer;
			if (BufferIsValid(scan->rs_nbuf))
				ReleaseBuffer(scan->rs_nbuf);
			scan->rs_ntup = NULL;
			scan->rs_nbuf = InvalidBuffer;
			return NULL;
		}

		if (BufferIsValid(scan->rs_pbuf))
			ReleaseBuffer(scan->rs_pbuf);
		scan->rs_ptup = NULL;
		scan->rs_pbuf = UnknownBuffer;

	}
	else
	{
		/* ----------------
		 *	handle forward scan
		 * ----------------
		 */
		if (scan->rs_ctup == scan->rs_ntup &&
			BufferIsInvalid(scan->rs_nbuf))
		{
			if (BufferIsValid(scan->rs_pbuf))
				ReleaseBuffer(scan->rs_pbuf);
			HEAPDEBUG_3;		/* heap_getnext returns NULL at end */
			return NULL;
		}

		/*
		 * Copy the "current" tuple/buffer to "previous". Pin/unpin the
		 * buffers accordingly
		 */
		if (scan->rs_pbuf != scan->rs_cbuf)
		{
			if (BufferIsValid(scan->rs_pbuf))
				ReleaseBuffer(scan->rs_pbuf);
			if (BufferIsValid(scan->rs_cbuf))
				IncrBufferRefCount(scan->rs_cbuf);
		}
		scan->rs_ptup = scan->rs_ctup;
		scan->rs_pbuf = scan->rs_cbuf;

		if (scan->rs_ntup != NULL)
		{
			if (scan->rs_cbuf != scan->rs_nbuf)
			{
				if (BufferIsValid(scan->rs_cbuf))
					ReleaseBuffer(scan->rs_cbuf);
				if (BufferIsValid(scan->rs_nbuf))
					IncrBufferRefCount(scan->rs_nbuf);
			}
			scan->rs_ctup = scan->rs_ntup;
			scan->rs_cbuf = scan->rs_nbuf;
			HEAPDEBUG_5;		/* heap_getnext next tuple was cached */
		}
		else
		{						/* NONTUP */
			ItemPointer iptr;

			iptr = (scan->rs_ctup != NULL) ?
				&scan->rs_ctup->t_ctid : (ItemPointer) NULL;

			/*
			 * Don't release scan->rs_cbuf at this point, because
			 * heapgettup doesn't increase PrivateRefCount if it is
			 * already set. On a forward scan, both rs_ctup and rs_ptup
			 * usually point to the same buffer page, so
			 * PrivateRefCount[rs_cbuf] should be 2 (or more, if for
			 * instance ctup is stored in a TupleTableSlot).  - 01/09/93
			 */

			scan->rs_ctup = (HeapTuple)
				heapgettup(scan->rs_rd,
						   iptr,
						   1,
						   &scan->rs_cbuf,
						   scan->rs_snapshot,
						   scan->rs_nkeys,
						   scan->rs_key);
		}

		if (scan->rs_ctup == NULL && !BufferIsValid(scan->rs_cbuf))
		{
			if (BufferIsValid(scan->rs_nbuf))
				ReleaseBuffer(scan->rs_nbuf);
			scan->rs_ntup = NULL;
			scan->rs_nbuf = InvalidBuffer;
			if (BufferIsValid(scan->rs_pbuf))
				ReleaseBuffer(scan->rs_pbuf);
			scan->rs_ptup = NULL;
			scan->rs_pbuf = InvalidBuffer;
			HEAPDEBUG_6;		/* heap_getnext returning EOS */
			return NULL;
		}

		if (BufferIsValid(scan->rs_nbuf))
			ReleaseBuffer(scan->rs_nbuf);
		scan->rs_ntup = NULL;
		scan->rs_nbuf = UnknownBuffer;
	}

	/* ----------------
	 *	if we get here it means we have a new current scan tuple, so
	 *	point to the proper return buffer and return the tuple.
	 * ----------------
	 */

	HEAPDEBUG_7;				/* heap_getnext returning tuple */

	return scan->rs_ctup;
}

/* ----------------
 *		heap_fetch		- retrive tuple with tid
 *
 *		Currently ignores LP_IVALID during processing!
 *
 *		Because this is not part of a scan, there is no way to
 *		automatically lock/unlock the shared buffers.
 *		For this reason, we require that the user retrieve the buffer
 *		value, and they are required to BuffferRelease() it when they
 *		are done.  If they want to make a copy of it before releasing it,
 *		they can call heap_copytyple().

 * ----------------
 */
HeapTuple
heap_fetch(Relation relation,
		   Snapshot snapshot,
		   ItemPointer tid,
		   Buffer *userbuf)
{
	ItemId		lp;
	Buffer		buffer;
	PageHeader	dp;
	HeapTuple	tuple;
	OffsetNumber offnum;

	AssertMacro(PointerIsValid(userbuf));		/* see comments above */

	/* ----------------
	 *	increment access statistics
	 * ----------------
	 */
	IncrHeapAccessStat(local_fetch);
	IncrHeapAccessStat(global_fetch);

	/*
	 * Note: This is collosally expensive - does two system calls per
	 * indexscan tuple fetch.  Not good, and since we should be doing page
	 * level locking by the scanner anyway, it is commented out.
	 */

	/* RelationSetLockForTupleRead(relation, tid); */

	/* ----------------
	 *	get the buffer from the relation descriptor
	 *	Note that this does a buffer pin.
	 * ----------------
	 */

	buffer = ReadBuffer(relation, ItemPointerGetBlockNumber(tid));

#ifndef NO_BUFFERISVALID
	if (!BufferIsValid(buffer))
	{
		elog(ERROR, "heap_fetch: %s relation: ReadBuffer(%lx) failed",
			 &relation->rd_rel->relname, (long) tid);
	}
#endif

	/* ----------------
	 *	get the item line pointer corresponding to the requested tid
	 * ----------------
	 */
	dp = (PageHeader) BufferGetPage(buffer);
	offnum = ItemPointerGetOffsetNumber(tid);
	lp = PageGetItemId(dp, offnum);

	/* ----------------
	 *	more sanity checks
	 * ----------------
	 */

	Assert(ItemIdIsUsed(lp));

	/* ----------------
	 *	check time qualification of tid
	 * ----------------
	 */

	HeapTupleSatisfies(lp, relation, buffer, dp,
					   snapshot, 0, (ScanKey) NULL, tuple);

	if (tuple == NULL)
	{
		ReleaseBuffer(buffer);
		return NULL;
	}

	/* ----------------
	 *	all checks passed, now either return a copy of the tuple
	 *	or pin the buffer page and return a pointer, depending on
	 *	whether caller gave us a valid buf.
	 * ----------------
	 */

	*userbuf = buffer;			/* user is required to ReleaseBuffer()
								 * this */

	return tuple;
}

/* ----------------
 *		heap_insert		- insert tuple
 *
 *		The assignment of t_min (and thus the others) should be
 *		removed eventually.
 *
 *		Currently places the tuple onto the last page.	If there is no room,
 *		it is placed on new pages.	(Heap relations)
 *		Note that concurrent inserts during a scan will probably have
 *		unexpected results, though this will be fixed eventually.
 *
 *		Fix to work with indexes.
 * ----------------
 */
Oid
heap_insert(Relation relation, HeapTuple tup)
{
	/* ----------------
	 *	increment access statistics
	 * ----------------
	 */
	IncrHeapAccessStat(local_insert);
	IncrHeapAccessStat(global_insert);

	/* ----------------
	 *	set relation level write lock. If this is a "local" relation (not
	 *	visible to others), we don't need to set a write lock.
	 * ----------------
	 */
	if (!relation->rd_islocal)
		RelationSetLockForWrite(relation);

	/* ----------------
	 *	If the object id of this tuple has already been assigned, trust
	 *	the caller.  There are a couple of ways this can happen.  At initial
	 *	db creation, the backend program sets oids for tuples.	When we
	 *	define an index, we set the oid.  Finally, in the future, we may
	 *	allow users to set their own object ids in order to support a
	 *	persistent object store (objects need to contain pointers to one
	 *	another).
	 * ----------------
	 */
	if (!OidIsValid(tup->t_oid))
	{
		tup->t_oid = newoid();
		LastOidProcessed = tup->t_oid;
	}
	else
		CheckMaxObjectId(tup->t_oid);

	TransactionIdStore(GetCurrentTransactionId(), &(tup->t_xmin));
	tup->t_cmin = GetCurrentCommandId();
	StoreInvalidTransactionId(&(tup->t_xmax));
	tup->t_infomask &= ~(HEAP_XACT_MASK);
	tup->t_infomask |= HEAP_XMAX_INVALID;

	doinsert(relation, tup);

	if (IsSystemRelationName(RelationGetRelationName(relation)->data))
	{
		RelationUnsetLockForWrite(relation);

		/* ----------------
		 *		invalidate caches (only works for system relations)
		 * ----------------
		 */
		RelationInvalidateHeapTuple(relation, tup);
	}

	return tup->t_oid;
}

/* ----------------
 *		heap_delete		- delete a tuple
 *
 *		Must decide how to handle errors.
 * ----------------
 */
int
heap_delete(Relation relation, ItemPointer tid)
{
	ItemId		lp;
	HeapTuple	tp;
	PageHeader	dp;
	Buffer		buf;

	/* ----------------
	 *	increment access statistics
	 * ----------------
	 */
	IncrHeapAccessStat(local_delete);
	IncrHeapAccessStat(global_delete);

	/* ----------------
	 *	sanity check
	 * ----------------
	 */
	Assert(ItemPointerIsValid(tid));

	/* ----------------
	 *	set relation level write lock
	 * ----------------
	 */
	RelationSetLockForWrite(relation);

	buf = ReadBuffer(relation, ItemPointerGetBlockNumber(tid));

#ifndef NO_BUFFERISVALID
	if (!BufferIsValid(buf))
	{							/* XXX L_SH better ??? */
		elog(ERROR, "heap_delete: failed ReadBuffer");
	}
#endif	 /* NO_BUFFERISVALID */

	dp = (PageHeader) BufferGetPage(buf);
	lp = PageGetItemId(dp, ItemPointerGetOffsetNumber(tid));

	/*
	 * Just like test against non-functional updates we try to catch
	 * non-functional delete attempts.			- vadim 05/05/97
	 */
	tp = (HeapTuple) PageGetItem((Page) dp, lp);
	Assert(HeapTupleIsValid(tp));
	if (TupleUpdatedByCurXactAndCmd(tp))
	{

		/*
		 * Vadim says this is no longer needed 1998/6/15 elog(NOTICE,
		 * "Non-functional delete, tuple already deleted");
		 */
		if (IsSystemRelationName(RelationGetRelationName(relation)->data))
			RelationUnsetLockForWrite(relation);
		ReleaseBuffer(buf);
		return 1;
	}
	/* ----------------
	 *	check that we're deleteing a valid item
	 * ----------------
	 */
	HeapTupleSatisfies(lp, relation, buf, dp,
					   false, 0, (ScanKey) NULL, tp);
	if (!tp)
	{

		/* XXX call something else */
		ReleaseBuffer(buf);

		elog(ERROR, "heap_delete: (am)invalid tid");
	}

	/* ----------------
	 *	get the tuple and lock tell the buffer manager we want
	 *	exclusive access to the page
	 * ----------------
	 */

	/* ----------------
	 *	store transaction information of xact deleting the tuple
	 * ----------------
	 */
	TransactionIdStore(GetCurrentTransactionId(), &(tp->t_xmax));
	tp->t_cmax = GetCurrentCommandId();
	tp->t_infomask &= ~(HEAP_XMAX_COMMITTED | HEAP_XMAX_INVALID);

	/* ----------------
	 *	invalidate caches
	 * ----------------
	 */
	RelationInvalidateHeapTuple(relation, tp);

	WriteBuffer(buf);
	if (IsSystemRelationName(RelationGetRelationName(relation)->data))
		RelationUnsetLockForWrite(relation);

	return 0;
}

/* ----------------
 *		heap_replace	- replace a tuple
 *
 *		Must decide how to handle errors.
 *
 *		Fix arguments, work with indexes.
 *
 *		12/30/93 - modified the return value to be 1 when
 *				   a non-functional update is detected. This
 *				   prevents the calling routine from updating
 *				   indices unnecessarily. -kw
 *
 * ----------------
 */
int
heap_replace(Relation relation, ItemPointer otid, HeapTuple replace_tuple)
{
	ItemId		lp;
	HeapTuple	old_tuple;
	Page		dp;
	Buffer		buffer;
	HeapTuple	tuple;

	/* ----------------
	 *	increment access statistics
	 * ----------------
	 */
	IncrHeapAccessStat(local_replace);
	IncrHeapAccessStat(global_replace);

	/* ----------------
	 *	sanity checks
	 * ----------------
	 */
	Assert(ItemPointerIsValid(otid));

	/* ----------------
	 *	set relation level write lock
	 * ----------------
	 */
	if (!relation->rd_islocal)
		RelationSetLockForWrite(relation);

	buffer = ReadBuffer(relation, ItemPointerGetBlockNumber(otid));
#ifndef NO_BUFFERISVALID
	if (!BufferIsValid(buffer))
	{
		/* XXX L_SH better ??? */
		elog(ERROR, "amreplace: failed ReadBuffer");
	}
#endif	 /* NO_BUFFERISVALID */

	dp = (Page) BufferGetPage(buffer);
	lp = PageGetItemId(dp, ItemPointerGetOffsetNumber(otid));

	/* ----------------
	 *	logically delete old item
	 * ----------------
	 */

	old_tuple = (HeapTuple) PageGetItem(dp, lp);
	Assert(HeapTupleIsValid(old_tuple));

	/* -----------------
	 *	the following test should be able to catch all non-functional
	 *	update attempts and shut out all ghost tuples.
	 *	XXX In the future, Spyros may need to update the rule lock on a tuple
	 *	more than once within the same command and same transaction.
	 *	He will have to introduce a new flag to override the following check.
	 *	-- Wei
	 *
	 * -----------------
	 */

	if (TupleUpdatedByCurXactAndCmd(old_tuple))
	{
		elog(NOTICE, "Non-functional update, only first update is performed");
		if (IsSystemRelationName(RelationGetRelationName(relation)->data))
			RelationUnsetLockForWrite(relation);
		ReleaseBuffer(buffer);
		return 1;
	}

	/* ----------------
	 *	check that we're replacing a valid item -
	 *
	 *	NOTE that this check must follow the non-functional update test
	 *		 above as it can happen that we try to 'replace' the same tuple
	 *		 twice in a single transaction.  The second time around the
	 *		 tuple will fail the NowTimeQual.  We don't want to abort the
	 *		 xact, we only want to flag the 'non-functional' NOTICE. -mer
	 * ----------------
	 */
	HeapTupleSatisfies(lp,
					   relation,
					   buffer,
					   (PageHeader) dp,
					   false,
					   0,
					   (ScanKey) NULL,
					   tuple);
	if (!tuple)
	{
		ReleaseBuffer(buffer);
		elog(ERROR, "heap_replace: (am)invalid otid");
	}

	/* XXX order problems if not atomic assignment ??? */
	replace_tuple->t_oid = old_tuple->t_oid;
	TransactionIdStore(GetCurrentTransactionId(), &(replace_tuple->t_xmin));
	replace_tuple->t_cmin = GetCurrentCommandId();
	StoreInvalidTransactionId(&(replace_tuple->t_xmax));
	replace_tuple->t_infomask &= ~(HEAP_XACT_MASK);
	replace_tuple->t_infomask |= HEAP_XMAX_INVALID;

	/* ----------------
	 *	insert new item
	 * ----------------
	 */
	if ((unsigned) DOUBLEALIGN(replace_tuple->t_len) <= PageGetFreeSpace((Page) dp))
		RelationPutHeapTuple(relation, BufferGetBlockNumber(buffer), replace_tuple);
	else
	{
		/* ----------------
		 *	new item won't fit on same page as old item, have to look
		 *	for a new place to put it.
		 * ----------------
		 */
		doinsert(relation, replace_tuple);
	}

	/* ----------------
	 *	new item in place, now record transaction information
	 * ----------------
	 */
	TransactionIdStore(GetCurrentTransactionId(), &(old_tuple->t_xmax));
	old_tuple->t_cmax = GetCurrentCommandId();
	old_tuple->t_infomask &= ~(HEAP_XMAX_COMMITTED | HEAP_XMAX_INVALID);

	/* ----------------
	 *	invalidate caches
	 * ----------------
	 */
	RelationInvalidateHeapTuple(relation, old_tuple);

	WriteBuffer(buffer);

	if (IsSystemRelationName(RelationGetRelationName(relation)->data))
		RelationUnsetLockForWrite(relation);

	return 0;
}

/* ----------------
 *		heap_markpos	- mark scan position
 *
 *		Note:
 *				Should only one mark be maintained per scan at one time.
 *		Check if this can be done generally--say calls to get the
 *		next/previous tuple and NEVER pass struct scandesc to the
 *		user AM's.  Now, the mark is sent to the executor for safekeeping.
 *		Probably can store this info into a GENERAL scan structure.
 *
 *		May be best to change this call to store the marked position
 *		(up to 2?) in the scan structure itself.
 *		Fix to use the proper caching structure.
 * ----------------
 */
void
heap_markpos(HeapScanDesc scan)
{

	/* ----------------
	 *	increment access statistics
	 * ----------------
	 */
	IncrHeapAccessStat(local_markpos);
	IncrHeapAccessStat(global_markpos);

	/* Note: no locking manipulations needed */

	if (scan->rs_ptup == NULL &&
		BufferIsUnknown(scan->rs_pbuf))
	{							/* == NONTUP */
		scan->rs_ptup = (HeapTuple)
			heapgettup(scan->rs_rd,
					   (scan->rs_ctup == NULL) ?
					   (ItemPointer) NULL : &scan->rs_ctup->t_ctid,
					   -1,
					   &scan->rs_pbuf,
					   scan->rs_snapshot,
					   scan->rs_nkeys,
					   scan->rs_key);

	}
	else if (scan->rs_ntup == NULL &&
			 BufferIsUnknown(scan->rs_nbuf))
	{							/* == NONTUP */
		scan->rs_ntup = (HeapTuple)
			heapgettup(scan->rs_rd,
					   (scan->rs_ctup == NULL) ?
					   (ItemPointer) NULL : &scan->rs_ctup->t_ctid,
					   1,
					   &scan->rs_nbuf,
					   scan->rs_snapshot,
					   scan->rs_nkeys,
					   scan->rs_key);
	}

	/* ----------------
	 * Should not unpin the buffer pages.  They may still be in use.
	 * ----------------
	 */
	if (scan->rs_ptup != NULL)
		scan->rs_mptid = scan->rs_ptup->t_ctid;
	else
		ItemPointerSetInvalid(&scan->rs_mptid);
	if (scan->rs_ctup != NULL)
		scan->rs_mctid = scan->rs_ctup->t_ctid;
	else
		ItemPointerSetInvalid(&scan->rs_mctid);
	if (scan->rs_ntup != NULL)
		scan->rs_mntid = scan->rs_ntup->t_ctid;
	else
		ItemPointerSetInvalid(&scan->rs_mntid);
}

/* ----------------
 *		heap_restrpos	- restore position to marked location
 *
 *		Note:  there are bad side effects here.  If we were past the end
 *		of a relation when heapmarkpos is called, then if the relation is
 *		extended via insert, then the next call to heaprestrpos will set
 *		cause the added tuples to be visible when the scan continues.
 *		Problems also arise if the TID's are rearranged!!!
 *
 *		Now pins buffer once for each valid tuple pointer (rs_ptup,
 *		rs_ctup, rs_ntup) referencing it.
 *		 - 01/13/94
 *
 * XXX	might be better to do direct access instead of
 *		using the generality of heapgettup().
 *
 * XXX It is very possible that when a scan is restored, that a tuple
 * XXX which previously qualified may fail for time range purposes, unless
 * XXX some form of locking exists (ie., portals currently can act funny.
 * ----------------
 */
void
heap_restrpos(HeapScanDesc scan)
{
	/* ----------------
	 *	increment access statistics
	 * ----------------
	 */
	IncrHeapAccessStat(local_restrpos);
	IncrHeapAccessStat(global_restrpos);

	/* XXX no amrestrpos checking that ammarkpos called */

	/* Note: no locking manipulations needed */

	unpinscan(scan);

	/* force heapgettup to pin buffer for each loaded tuple */
	scan->rs_pbuf = InvalidBuffer;
	scan->rs_cbuf = InvalidBuffer;
	scan->rs_nbuf = InvalidBuffer;

	if (!ItemPointerIsValid(&scan->rs_mptid))
		scan->rs_ptup = NULL;
	else
	{
		scan->rs_ptup = (HeapTuple)
			heapgettup(scan->rs_rd,
					   &scan->rs_mptid,
					   0,
					   &scan->rs_pbuf,
					   false,
					   0,
					   (ScanKey) NULL);
	}

	if (!ItemPointerIsValid(&scan->rs_mctid))
		scan->rs_ctup = NULL;
	else
	{
		scan->rs_ctup = (HeapTuple)
			heapgettup(scan->rs_rd,
					   &scan->rs_mctid,
					   0,
					   &scan->rs_cbuf,
					   false,
					   0,
					   (ScanKey) NULL);
	}

	if (!ItemPointerIsValid(&scan->rs_mntid))
		scan->rs_ntup = NULL;
	else
	{
		scan->rs_ntup = (HeapTuple)
			heapgettup(scan->rs_rd,
					   &scan->rs_mntid,
					   0,
					   &scan->rs_nbuf,
					   false,
					   0,
					   (ScanKey) NULL);
	}
}