tableam: VACUUM and ANALYZE support.

This is a relatively straightforward move of the current
implementation to sit below tableam. As the current analyze sampling
implementation is pretty inherently block based, the tableam analyze
interface is as well. It might make sense to generalize that at some
point, but that seems like a larger project that shouldn't be
undertaken at the same time as the introduction of tableam.

Author: Andres Freund
Discussion: https://postgr.es/m/20180703070645.wchpu5muyto5n647@alap3.anarazel.de

src/backend/access/heap/heapam_handler.c

@@ -921,6 +921,173 @@ heapam_relation_copy_for_cluster(Relation OldHeap, Relation NewHeap,
 	pfree(isnull);
 }
 
+static bool
+heapam_scan_analyze_next_block(TableScanDesc sscan, BlockNumber blockno,
+							   BufferAccessStrategy bstrategy)
+{
+	HeapScanDesc scan = (HeapScanDesc) sscan;
+
+	/*
+	 * We must maintain a pin on the target page's buffer to ensure that
+	 * concurrent activity - e.g. HOT pruning - doesn't delete tuples out from
+	 * under us.  Hence, pin the page until we are done looking at it.  We
+	 * also choose to hold sharelock on the buffer throughout --- we could
+	 * release and re-acquire sharelock for each tuple, but since we aren't
+	 * doing much work per tuple, the extra lock traffic is probably better
+	 * avoided.
+	 */
+	scan->rs_cblock = blockno;
+	scan->rs_cindex = FirstOffsetNumber;
+	scan->rs_cbuf = ReadBufferExtended(scan->rs_base.rs_rd, MAIN_FORKNUM,
+									   blockno, RBM_NORMAL, bstrategy);
+	LockBuffer(scan->rs_cbuf, BUFFER_LOCK_SHARE);
+
+	/* in heap all blocks can contain tuples, so always return true */
+	return true;
+}
+
+static bool
+heapam_scan_analyze_next_tuple(TableScanDesc sscan, TransactionId OldestXmin,
+							   double *liverows, double *deadrows,
+							   TupleTableSlot *slot)
+{
+	HeapScanDesc scan = (HeapScanDesc) sscan;
+	Page		targpage;
+	OffsetNumber maxoffset;
+	BufferHeapTupleTableSlot *hslot;
+
+	Assert(TTS_IS_BUFFERTUPLE(slot));
+
+	hslot = (BufferHeapTupleTableSlot *) slot;
+	targpage = BufferGetPage(scan->rs_cbuf);
+	maxoffset = PageGetMaxOffsetNumber(targpage);
+
+	/* Inner loop over all tuples on the selected page */
+	for (; scan->rs_cindex <= maxoffset; scan->rs_cindex++)
+	{
+		ItemId		itemid;
+		HeapTuple	targtuple = &hslot->base.tupdata;
+		bool		sample_it = false;
+
+		itemid = PageGetItemId(targpage, scan->rs_cindex);
+
+		/*
+		 * We ignore unused and redirect line pointers.  DEAD line pointers
+		 * should be counted as dead, because we need vacuum to run to get rid
+		 * of them.  Note that this rule agrees with the way that
+		 * heap_page_prune() counts things.
+		 */
+		if (!ItemIdIsNormal(itemid))
+		{
+			if (ItemIdIsDead(itemid))
+				*deadrows += 1;
+			continue;
+		}
+
+		ItemPointerSet(&targtuple->t_self, scan->rs_cblock, scan->rs_cindex);
+
+		targtuple->t_tableOid = RelationGetRelid(scan->rs_base.rs_rd);
+		targtuple->t_data = (HeapTupleHeader) PageGetItem(targpage, itemid);
+		targtuple->t_len = ItemIdGetLength(itemid);
+
+		switch (HeapTupleSatisfiesVacuum(targtuple, OldestXmin, scan->rs_cbuf))
+		{
+			case HEAPTUPLE_LIVE:
+				sample_it = true;
+				*liverows += 1;
+				break;
+
+			case HEAPTUPLE_DEAD:
+			case HEAPTUPLE_RECENTLY_DEAD:
+				/* Count dead and recently-dead rows */
+				*deadrows += 1;
+				break;
+
+			case HEAPTUPLE_INSERT_IN_PROGRESS:
+
+				/*
+				 * Insert-in-progress rows are not counted.  We assume that
+				 * when the inserting transaction commits or aborts, it will
+				 * send a stats message to increment the proper count.  This
+				 * works right only if that transaction ends after we finish
+				 * analyzing the table; if things happen in the other order,
+				 * its stats update will be overwritten by ours.  However, the
+				 * error will be large only if the other transaction runs long
+				 * enough to insert many tuples, so assuming it will finish
+				 * after us is the safer option.
+				 *
+				 * A special case is that the inserting transaction might be
+				 * our own.  In this case we should count and sample the row,
+				 * to accommodate users who load a table and analyze it in one
+				 * transaction.  (pgstat_report_analyze has to adjust the
+				 * numbers we send to the stats collector to make this come
+				 * out right.)
+				 */
+				if (TransactionIdIsCurrentTransactionId(HeapTupleHeaderGetXmin(targtuple->t_data)))
+				{
+					sample_it = true;
+					*liverows += 1;
+				}
+				break;
+
+			case HEAPTUPLE_DELETE_IN_PROGRESS:
+
+				/*
+				 * We count and sample delete-in-progress rows the same as
+				 * live ones, so that the stats counters come out right if the
+				 * deleting transaction commits after us, per the same
+				 * reasoning given above.
+				 *
+				 * If the delete was done by our own transaction, however, we
+				 * must count the row as dead to make pgstat_report_analyze's
+				 * stats adjustments come out right.  (Note: this works out
+				 * properly when the row was both inserted and deleted in our
+				 * xact.)
+				 *
+				 * The net effect of these choices is that we act as though an
+				 * IN_PROGRESS transaction hasn't happened yet, except if it
+				 * is our own transaction, which we assume has happened.
+				 *
+				 * This approach ensures that we behave sanely if we see both
+				 * the pre-image and post-image rows for a row being updated
+				 * by a concurrent transaction: we will sample the pre-image
+				 * but not the post-image.  We also get sane results if the
+				 * concurrent transaction never commits.
+				 */
+				if (TransactionIdIsCurrentTransactionId(HeapTupleHeaderGetUpdateXid(targtuple->t_data)))
+					deadrows += 1;
+				else
+				{
+					sample_it = true;
+					liverows += 1;
+				}
+				break;
+
+			default:
+				elog(ERROR, "unexpected HeapTupleSatisfiesVacuum result");
+				break;
+		}
+
+		if (sample_it)
+		{
+			ExecStoreBufferHeapTuple(targtuple, slot, scan->rs_cbuf);
+			scan->rs_cindex++;
+
+			/* note that we leave the buffer locked here! */
+			return true;
+		}
+	}
+
+	/* Now release the lock and pin on the page */
+	UnlockReleaseBuffer(scan->rs_cbuf);
+	scan->rs_cbuf = InvalidBuffer;
+
+	/* also prevent old slot contents from having pin on page */
+	ExecClearTuple(slot);
+
+	return false;
+}
+
 static double
 heapam_index_build_range_scan(Relation heapRelation,
 							  Relation indexRelation,
@@ -1743,6 +1910,9 @@ static const TableAmRoutine heapam_methods = {
 	.relation_nontransactional_truncate = heapam_relation_nontransactional_truncate,
 	.relation_copy_data = heapam_relation_copy_data,
 	.relation_copy_for_cluster = heapam_relation_copy_for_cluster,
+	.relation_vacuum = heap_vacuum_rel,
+	.scan_analyze_next_block = heapam_scan_analyze_next_block,
+	.scan_analyze_next_tuple = heapam_scan_analyze_next_tuple,
 	.index_build_range_scan = heapam_index_build_range_scan,
 	.index_validate_scan = heapam_index_validate_scan,
 };