Пример #1
0
/* ----------------
 *		index_fetch_heap - get the scan's next heap tuple
 *
 * The result is a visible heap tuple associated with the index TID most
 * recently fetched by index_getnext_tid, or NULL if no more matching tuples
 * exist.  (There can be more than one matching tuple because of HOT chains,
 * although when using an MVCC snapshot it should be impossible for more than
 * one such tuple to exist.)
 *
 * On success, the buffer containing the heap tup is pinned (the pin will be
 * dropped in a future index_getnext_tid, index_fetch_heap or index_endscan
 * call).
 *
 * Note: caller must check scan->xs_recheck, and perform rechecking of the
 * scan keys if required.  We do not do that here because we don't have
 * enough information to do it efficiently in the general case.
 * ----------------
 */
HeapTuple
index_fetch_heap(IndexScanDesc scan)
{
	ItemPointer tid = &scan->xs_ctup.t_self;
	bool		all_dead = false;
	bool		got_heap_tuple;

	/* We can skip the buffer-switching logic if we're in mid-HOT chain. */
	if (!scan->xs_continue_hot)
	{
		/* Switch to correct buffer if we don't have it already */
		Buffer		prev_buf = scan->xs_cbuf;

		scan->xs_cbuf = ReleaseAndReadBuffer(scan->xs_cbuf,
											 scan->heapRelation,
											 ItemPointerGetBlockNumber(tid));

		/*
		 * Prune page, but only if we weren't already on this page
		 */
		if (prev_buf != scan->xs_cbuf)
			heap_page_prune_opt(scan->heapRelation, scan->xs_cbuf);
	}

	/* Obtain share-lock on the buffer so we can examine visibility */
	LockBuffer(scan->xs_cbuf, BUFFER_LOCK_SHARE);
	got_heap_tuple = heap_hot_search_buffer(tid, scan->heapRelation,
											scan->xs_cbuf,
											scan->xs_snapshot,
											&scan->xs_ctup,
											&all_dead,
											!scan->xs_continue_hot);
	LockBuffer(scan->xs_cbuf, BUFFER_LOCK_UNLOCK);

	if (got_heap_tuple)
	{
		/*
		 * Only in a non-MVCC snapshot can more than one member of the HOT
		 * chain be visible.
		 */
		scan->xs_continue_hot = !IsMVCCSnapshot(scan->xs_snapshot);
		pgstat_count_heap_fetch(scan->indexRelation);
		return &scan->xs_ctup;
	}

	/* We've reached the end of the HOT chain. */
	scan->xs_continue_hot = false;

	/*
	 * If we scanned a whole HOT chain and found only dead tuples, tell index
	 * AM to kill its entry for that TID (this will take effect in the next
	 * amgettuple call, in index_getnext_tid).  We do not do this when in
	 * recovery because it may violate MVCC to do so.  See comments in
	 * RelationGetIndexScan().
	 */
	if (!scan->xactStartedInRecovery)
		scan->kill_prior_tuple = all_dead;

	return NULL;
}
Пример #2
0
/*
 * bitgetpage - subroutine for BitmapHeapNext()
 *
 * This routine reads and pins the specified page of the relation, then
 * builds an array indicating which tuples on the page are both potentially
 * interesting according to the bitmap, and visible according to the snapshot.
 */
static void
bitgetpage(HeapScanDesc scan, TBMIterateResult *tbmres)
{
	BlockNumber page = tbmres->blockno;
	Buffer		buffer;
	Snapshot	snapshot;
	int			ntup;

	/*
	 * Acquire pin on the target heap page, trading in any pin we held before.
	 */
	Assert(page < scan->rs_nblocks);

	scan->rs_cbuf = ReleaseAndReadBuffer(scan->rs_cbuf,
										 scan->rs_rd,
										 page);
	buffer = scan->rs_cbuf;
	snapshot = scan->rs_snapshot;

	ntup = 0;

	/*
	 * Prune and repair fragmentation for the whole page, if possible.
	 */
	Assert(TransactionIdIsValid(RecentGlobalXmin));
	heap_page_prune_opt(scan->rs_rd, buffer, RecentGlobalXmin);

	/*
	 * We must hold share lock on the buffer content while examining tuple
	 * visibility.	Afterwards, however, the tuples we have found to be
	 * visible are guaranteed good as long as we hold the buffer pin.
	 */
	LockBuffer(buffer, BUFFER_LOCK_SHARE);

	/*
	 * We need two separate strategies for lossy and non-lossy cases.
	 */
	if (tbmres->ntuples >= 0)
	{
		/*
		 * Bitmap is non-lossy, so we just look through the offsets listed in
		 * tbmres; but we have to follow any HOT chain starting at each such
		 * offset.
		 */
		int			curslot;

		for (curslot = 0; curslot < tbmres->ntuples; curslot++)
		{
			OffsetNumber offnum = tbmres->offsets[curslot];
			ItemPointerData tid;

			ItemPointerSet(&tid, page, offnum);
			if (heap_hot_search_buffer(&tid, scan->rs_rd, buffer, snapshot, NULL))
				scan->rs_vistuples[ntup++] = ItemPointerGetOffsetNumber(&tid);
		}
	}
	else
	{
		/*
		 * Bitmap is lossy, so we must examine each item pointer on the page.
		 * But we can ignore HOT chains, since we'll check each tuple anyway.
		 */
		Page		dp = (Page) BufferGetPage(buffer);
		OffsetNumber maxoff = PageGetMaxOffsetNumber(dp);
		OffsetNumber offnum;

		for (offnum = FirstOffsetNumber; offnum <= maxoff; offnum = OffsetNumberNext(offnum))
		{
			ItemId		lp;
			HeapTupleData loctup;
			bool		valid;

			lp = PageGetItemId(dp, offnum);
			if (!ItemIdIsNormal(lp))
				continue;
			loctup.t_data = (HeapTupleHeader) PageGetItem((Page) dp, lp);
			loctup.t_len = ItemIdGetLength(lp);
			loctup.t_tableOid = scan->rs_rd->rd_id;
			ItemPointerSet(&loctup.t_self, page, offnum);
			valid = HeapTupleSatisfiesVisibility(&loctup, snapshot, buffer);
			if (valid)
			{
				scan->rs_vistuples[ntup++] = offnum;
				PredicateLockTuple(scan->rs_rd, &loctup, snapshot);
			}
			CheckForSerializableConflictOut(valid, scan->rs_rd, &loctup,
											buffer, snapshot);
		}
	}

	LockBuffer(buffer, BUFFER_LOCK_UNLOCK);

	Assert(ntup <= MaxHeapTuplesPerPage);
	scan->rs_ntuples = ntup;
}
Пример #3
0
/* ----------------
 *		index_getnext - get the next heap tuple from a scan
 *
 * The result is the next heap tuple satisfying the scan keys and the
 * snapshot, or NULL if no more matching tuples exist.	On success,
 * the buffer containing the heap tuple is pinned (the pin will be dropped
 * at the next index_getnext or index_endscan).
 *
 * Note: caller must check scan->xs_recheck, and perform rechecking of the
 * scan keys if required.  We do not do that here because we don't have
 * enough information to do it efficiently in the general case.
 * ----------------
 */
HeapTuple
index_getnext(IndexScanDesc scan, ScanDirection direction)
{
	HeapTuple	heapTuple = &scan->xs_ctup;
	ItemPointer tid = &heapTuple->t_self;
	FmgrInfo   *procedure;
	bool		all_dead = false;

	SCAN_CHECKS;
	GET_SCAN_PROCEDURE(amgettuple);

	Assert(TransactionIdIsValid(RecentGlobalXmin));

	for (;;)
	{
		bool	got_heap_tuple;

		if (scan->xs_continue_hot)
		{
			/*
			 * We are resuming scan of a HOT chain after having returned an
			 * earlier member.	Must still hold pin on current heap page.
			 */
			Assert(BufferIsValid(scan->xs_cbuf));
			Assert(ItemPointerGetBlockNumber(tid) ==
				   BufferGetBlockNumber(scan->xs_cbuf));
		}
		else
		{
			bool		found;
			Buffer		prev_buf;

			/*
			 * If we scanned a whole HOT chain and found only dead tuples,
			 * tell index AM to kill its entry for that TID. We do not do this
			 * when in recovery because it may violate MVCC to do so. see
			 * comments in RelationGetIndexScan().
			 */
			if (!scan->xactStartedInRecovery)
				scan->kill_prior_tuple = all_dead;

			/*
			 * The AM's gettuple proc finds the next index entry matching the
			 * scan keys, and puts the TID in xs_ctup.t_self (ie, *tid). It
			 * should also set scan->xs_recheck, though we pay no attention to
			 * that here.
			 */
			found = DatumGetBool(FunctionCall2(procedure,
											   PointerGetDatum(scan),
											   Int32GetDatum(direction)));

			/* Reset kill flag immediately for safety */
			scan->kill_prior_tuple = false;

			/* If we're out of index entries, break out of outer loop */
			if (!found)
				break;

			pgstat_count_index_tuples(scan->indexRelation, 1);

			/* Switch to correct buffer if we don't have it already */
			prev_buf = scan->xs_cbuf;
			scan->xs_cbuf = ReleaseAndReadBuffer(scan->xs_cbuf,
												 scan->heapRelation,
											 ItemPointerGetBlockNumber(tid));

			/*
			 * Prune page, but only if we weren't already on this page
			 */
			if (prev_buf != scan->xs_cbuf)
				heap_page_prune_opt(scan->heapRelation, scan->xs_cbuf,
									RecentGlobalXmin);
		}

		/* Obtain share-lock on the buffer so we can examine visibility */
		LockBuffer(scan->xs_cbuf, BUFFER_LOCK_SHARE);
		got_heap_tuple = heap_hot_search_buffer(tid, scan->heapRelation,
												scan->xs_cbuf,
												scan->xs_snapshot,
												&scan->xs_ctup,
												&all_dead,
												!scan->xs_continue_hot);
		LockBuffer(scan->xs_cbuf, BUFFER_LOCK_UNLOCK);

		if (got_heap_tuple)
		{
			/*
			 * Only in a non-MVCC snapshot can more than one member of the
			 * HOT chain be visible.
			 */
			scan->xs_continue_hot = !IsMVCCSnapshot(scan->xs_snapshot);
			pgstat_count_heap_fetch(scan->indexRelation);
			return heapTuple;
		}

		/* Loop around to ask index AM for another TID */
		scan->xs_continue_hot = false;
	}

	/* Release any held pin on a heap page */
	if (BufferIsValid(scan->xs_cbuf))
	{
		ReleaseBuffer(scan->xs_cbuf);
		scan->xs_cbuf = InvalidBuffer;
	}

	return NULL;				/* failure exit */
}