tableam: Add and use scan APIs.

Too allow table accesses to be not directly dependent on heap, several
new abstractions are needed. Specifically:

1) Heap scans need to be generalized into table scans. Do this by
   introducing TableScanDesc, which will be the "base class" for
   individual AMs. This contains the AM independent fields from
   HeapScanDesc.

   The previous heap_{beginscan,rescan,endscan} et al. have been
   replaced with a table_ version.

   There's no direct replacement for heap_getnext(), as that returned
   a HeapTuple, which is undesirable for a other AMs. Instead there's
   table_scan_getnextslot().  But note that heap_getnext() lives on,
   it's still used widely to access catalog tables.

   This is achieved by new scan_begin, scan_end, scan_rescan,
   scan_getnextslot callbacks.

2) The portion of parallel scans that's shared between backends need
   to be able to do so without the user doing per-AM work. To achieve
   that new parallelscan_{estimate, initialize, reinitialize}
   callbacks are introduced, which operate on a new
   ParallelTableScanDesc, which again can be subclassed by AMs.

   As it is likely that several AMs are going to be block oriented,
   block oriented callbacks that can be shared between such AMs are
   provided and used by heap. table_block_parallelscan_{estimate,
   intiialize, reinitialize} as callbacks, and
   table_block_parallelscan_{nextpage, init} for use in AMs. These
   operate on a ParallelBlockTableScanDesc.

3) Index scans need to be able to access tables to return a tuple, and
   there needs to be state across individual accesses to the heap to
   store state like buffers. That's now handled by introducing a
   sort-of-scan IndexFetchTable, which again is intended to be
   subclassed by individual AMs (for heap IndexFetchHeap).

   The relevant callbacks for an AM are index_fetch_{end, begin,
   reset} to create the necessary state, and index_fetch_tuple to
   retrieve an indexed tuple.  Note that index_fetch_tuple
   implementations need to be smarter than just blindly fetching the
   tuples for AMs that have optimizations similar to heap's HOT - the
   currently alive tuple in the update chain needs to be fetched if
   appropriate.

   Similar to table_scan_getnextslot(), it's undesirable to continue
   to return HeapTuples. Thus index_fetch_heap (might want to rename
   that later) now accepts a slot as an argument. Core code doesn't
   have a lot of call sites performing index scans without going
   through the systable_* API (in contrast to loads of heap_getnext
   calls and working directly with HeapTuples).

   Index scans now store the result of a search in
   IndexScanDesc->xs_heaptid, rather than xs_ctup->t_self. As the
   target is not generally a HeapTuple anymore that seems cleaner.

To be able to sensible adapt code to use the above, two further
callbacks have been introduced:

a) slot_callbacks returns a TupleTableSlotOps* suitable for creating
   slots capable of holding a tuple of the AMs
   type. table_slot_callbacks() and table_slot_create() are based
   upon that, but have additional logic to deal with views, foreign
   tables, etc.

   While this change could have been done separately, nearly all the
   call sites that needed to be adapted for the rest of this commit
   also would have been needed to be adapted for
   table_slot_callbacks(), making separation not worthwhile.

b) tuple_satisfies_snapshot checks whether the tuple in a slot is
   currently visible according to a snapshot. That's required as a few
   places now don't have a buffer + HeapTuple around, but a
   slot (which in heap's case internally has that information).

Additionally a few infrastructure changes were needed:

I) SysScanDesc, as used by systable_{beginscan, getnext} et al. now
   internally uses a slot to keep track of tuples. While
   systable_getnext() still returns HeapTuples, and will so for the
   foreseeable future, the index API (see 1) above) now only deals with
   slots.

The remainder, and largest part, of this commit is then adjusting all
scans in postgres to use the new APIs.

Author: Andres Freund, Haribabu Kommi, Alvaro Herrera
Discussion:
    https://postgr.es/m/20180703070645.wchpu5muyto5n647@alap3.anarazel.de
    https://postgr.es/m/20160812231527.GA690404@alvherre.pgsql

1 files changed, 62 insertions, 48 deletions

diff --git a/src/backend/access/index/genam.c b/src/backend/access/index/genam.c
index e0a5ea42d52..5222966e510 100644
--- a/src/backend/access/index/genam.c
+++ b/src/backend/access/index/genam.c
@@ -22,6 +22,7 @@
 #include "access/genam.h"
 #include "access/heapam.h"
 #include "access/relscan.h"
+#include "access/tableam.h"
 #include "access/transam.h"
 #include "catalog/index.h"
 #include "lib/stringinfo.h"
@@ -83,6 +84,7 @@ RelationGetIndexScan(Relation indexRelation, int nkeys, int norderbys)
 	scan = (IndexScanDesc) palloc(sizeof(IndexScanDescData));
 
 	scan->heapRelation = NULL;	/* may be set later */
+	scan->xs_heapfetch = NULL;
 	scan->indexRelation = indexRelation;
 	scan->xs_snapshot = InvalidSnapshot;	/* caller must initialize this */
 	scan->numberOfKeys = nkeys;
@@ -123,11 +125,6 @@ RelationGetIndexScan(Relation indexRelation, int nkeys, int norderbys)
 	scan->xs_hitup = NULL;
 	scan->xs_hitupdesc = NULL;
 
-	ItemPointerSetInvalid(&scan->xs_ctup.t_self);
-	scan->xs_ctup.t_data = NULL;
-	scan->xs_cbuf = InvalidBuffer;
-	scan->xs_continue_hot = false;
-
 	return scan;
 }
 
@@ -335,6 +332,7 @@ systable_beginscan(Relation heapRelation,
 
 	sysscan->heap_rel = heapRelation;
 	sysscan->irel = irel;
+	sysscan->slot = table_slot_create(heapRelation, NULL);
 
 	if (snapshot == NULL)
 	{
@@ -384,9 +382,9 @@ systable_beginscan(Relation heapRelation,
 		 * disadvantage; and there are no compensating advantages, because
 		 * it's unlikely that such scans will occur in parallel.
 		 */
-		sysscan->scan = heap_beginscan_strat(heapRelation, snapshot,
-											 nkeys, key,
-											 true, false);
+		sysscan->scan = table_beginscan_strat(heapRelation, snapshot,
+											  nkeys, key,
+											  true, false);
 		sysscan->iscan = NULL;
 	}
 
@@ -401,28 +399,46 @@ systable_beginscan(Relation heapRelation,
  * Note that returned tuple is a reference to data in a disk buffer;
  * it must not be modified, and should be presumed inaccessible after
  * next getnext() or endscan() call.
+ *
+ * XXX: It'd probably make sense to offer a slot based interface, at least
+ * optionally.
  */
 HeapTuple
 systable_getnext(SysScanDesc sysscan)
 {
-	HeapTuple	htup;
+	HeapTuple	htup = NULL;
 
 	if (sysscan->irel)
 	{
-		htup = index_getnext(sysscan->iscan, ForwardScanDirection);
+		if (index_getnext_slot(sysscan->iscan, ForwardScanDirection, sysscan->slot))
+		{
+			bool		shouldFree;
 
-		/*
-		 * We currently don't need to support lossy index operators for any
-		 * system catalog scan.  It could be done here, using the scan keys to
-		 * drive the operator calls, if we arranged to save the heap attnums
-		 * during systable_beginscan(); this is practical because we still
-		 * wouldn't need to support indexes on expressions.
-		 */
-		if (htup && sysscan->iscan->xs_recheck)
-			elog(ERROR, "system catalog scans with lossy index conditions are not implemented");
+			htup = ExecFetchSlotHeapTuple(sysscan->slot, false, &shouldFree);
+			Assert(!shouldFree);
+
+			/*
+			 * We currently don't need to support lossy index operators for
+			 * any system catalog scan.  It could be done here, using the scan
+			 * keys to drive the operator calls, if we arranged to save the
+			 * heap attnums during systable_beginscan(); this is practical
+			 * because we still wouldn't need to support indexes on
+			 * expressions.
+			 */
+			if (sysscan->iscan->xs_recheck)
+				elog(ERROR, "system catalog scans with lossy index conditions are not implemented");
+		}
 	}
 	else
-		htup = heap_getnext(sysscan->scan, ForwardScanDirection);
+	{
+		if (table_scan_getnextslot(sysscan->scan, ForwardScanDirection, sysscan->slot))
+		{
+			bool		shouldFree;
+
+			htup = ExecFetchSlotHeapTuple(sysscan->slot, false, &shouldFree);
+			Assert(!shouldFree);
+		}
+	}
 
 	return htup;
 }
@@ -446,37 +462,20 @@ systable_recheck_tuple(SysScanDesc sysscan, HeapTuple tup)
 	Snapshot	freshsnap;
 	bool		result;
 
+	Assert(tup == ExecFetchSlotHeapTuple(sysscan->slot, false, NULL));
+
 	/*
-	 * Trust that LockBuffer() and HeapTupleSatisfiesMVCC() do not themselves
+	 * Trust that table_tuple_satisfies_snapshot() and its subsidiaries
+	 * (commonly LockBuffer() and HeapTupleSatisfiesMVCC()) do not themselves
 	 * acquire snapshots, so we need not register the snapshot.  Those
 	 * facilities are too low-level to have any business scanning tables.
 	 */
 	freshsnap = GetCatalogSnapshot(RelationGetRelid(sysscan->heap_rel));
 
-	if (sysscan->irel)
-	{
-		IndexScanDesc scan = sysscan->iscan;
-
-		Assert(IsMVCCSnapshot(scan->xs_snapshot));
-		Assert(tup == &scan->xs_ctup);
-		Assert(BufferIsValid(scan->xs_cbuf));
-		/* must hold a buffer lock to call HeapTupleSatisfiesVisibility */
-		LockBuffer(scan->xs_cbuf, BUFFER_LOCK_SHARE);
-		result = HeapTupleSatisfiesVisibility(tup, freshsnap, scan->xs_cbuf);
-		LockBuffer(scan->xs_cbuf, BUFFER_LOCK_UNLOCK);
-	}
-	else
-	{
-		HeapScanDesc scan = sysscan->scan;
-
-		Assert(IsMVCCSnapshot(scan->rs_snapshot));
-		Assert(tup == &scan->rs_ctup);
-		Assert(BufferIsValid(scan->rs_cbuf));
-		/* must hold a buffer lock to call HeapTupleSatisfiesVisibility */
-		LockBuffer(scan->rs_cbuf, BUFFER_LOCK_SHARE);
-		result = HeapTupleSatisfiesVisibility(tup, freshsnap, scan->rs_cbuf);
-		LockBuffer(scan->rs_cbuf, BUFFER_LOCK_UNLOCK);
-	}
+	result = table_tuple_satisfies_snapshot(sysscan->heap_rel,
+											sysscan->slot,
+											freshsnap);
+
 	return result;
 }
 
@@ -488,13 +487,19 @@ systable_recheck_tuple(SysScanDesc sysscan, HeapTuple tup)
 void
 systable_endscan(SysScanDesc sysscan)
 {
+	if (sysscan->slot)
+	{
+		ExecDropSingleTupleTableSlot(sysscan->slot);
+		sysscan->slot = NULL;
+	}
+
 	if (sysscan->irel)
 	{
 		index_endscan(sysscan->iscan);
 		index_close(sysscan->irel, AccessShareLock);
 	}
 	else
-		heap_endscan(sysscan->scan);
+		table_endscan(sysscan->scan);
 
 	if (sysscan->snapshot)
 		UnregisterSnapshot(sysscan->snapshot);
@@ -541,6 +546,7 @@ systable_beginscan_ordered(Relation heapRelation,
 
 	sysscan->heap_rel = heapRelation;
 	sysscan->irel = indexRelation;
+	sysscan->slot = table_slot_create(heapRelation, NULL);
 
 	if (snapshot == NULL)
 	{
@@ -586,10 +592,12 @@ systable_beginscan_ordered(Relation heapRelation,
 HeapTuple
 systable_getnext_ordered(SysScanDesc sysscan, ScanDirection direction)
 {
-	HeapTuple	htup;
+	HeapTuple	htup = NULL;
 
 	Assert(sysscan->irel);
-	htup = index_getnext(sysscan->iscan, direction);
+	if (index_getnext_slot(sysscan->iscan, direction, sysscan->slot))
+		htup = ExecFetchSlotHeapTuple(sysscan->slot, false, NULL);
+
 	/* See notes in systable_getnext */
 	if (htup && sysscan->iscan->xs_recheck)
 		elog(ERROR, "system catalog scans with lossy index conditions are not implemented");
@@ -603,6 +611,12 @@ systable_getnext_ordered(SysScanDesc sysscan, ScanDirection direction)
 void
 systable_endscan_ordered(SysScanDesc sysscan)
 {
+	if (sysscan->slot)
+	{
+		ExecDropSingleTupleTableSlot(sysscan->slot);
+		sysscan->slot = NULL;
+	}
+
 	Assert(sysscan->irel);
 	index_endscan(sysscan->iscan);
 	if (sysscan->snapshot)