tableam: Add helper for indexes to check if a corresponding table tuples exist.

This is, likely exclusively, useful to verify that conflicts detected
in a unique index are with live tuples, rather than dead ones.

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

src/backend/access/nbtree/nbtinsert.c

@@ -15,9 +15,9 @@
 
 #include "postgres.h"
 
-#include "access/heapam.h"
 #include "access/nbtree.h"
 #include "access/nbtxlog.h"
+#include "access/tableam.h"
 #include "access/transam.h"
 #include "access/xloginsert.h"
 #include "miscadmin.h"
@@ -431,12 +431,14 @@ _bt_check_unique(Relation rel, BTInsertState insertstate, Relation heapRel,
 				}
 
 				/*
-				 * We check the whole HOT-chain to see if there is any tuple
-				 * that satisfies SnapshotDirty.  This is necessary because we
-				 * have just a single index entry for the entire chain.
+				 * Check if there's any table tuples for this index entry
+				 * satisfying SnapshotDirty. This is necessary because for AMs
+				 * with optimizations like heap's HOT, we have just a single
+				 * index entry for the entire chain.
 				 */
-				else if (heap_hot_search(&htid, heapRel, &SnapshotDirty,
-										 &all_dead))
+				else if (table_index_fetch_tuple_check(heapRel, &htid,
+													   &SnapshotDirty,
+													   &all_dead))
 				{
 					TransactionId xwait;
 
@@ -489,7 +491,8 @@ _bt_check_unique(Relation rel, BTInsertState insertstate, Relation heapRel,
 					 * entry.
 					 */
 					htid = itup->t_tid;
-					if (heap_hot_search(&htid, heapRel, SnapshotSelf, NULL))
+					if (table_index_fetch_tuple_check(heapRel, &htid,
+													  SnapshotSelf, NULL))
 					{
 						/* Normal case --- it's still live */
 					}

src/backend/access/nbtree/nbtsort.c

@@ -57,10 +57,10 @@
 
 #include "postgres.h"
 
-#include "access/heapam.h"
 #include "access/nbtree.h"
 #include "access/parallel.h"
 #include "access/relscan.h"
+#include "access/table.h"
 #include "access/tableam.h"
 #include "access/xact.h"
 #include "access/xlog.h"

src/backend/access/table/tableam.c

@@ -176,6 +176,40 @@ table_beginscan_parallel(Relation relation, ParallelTableScanDesc parallel_scan)
 }
 
 
+/* ----------------------------------------------------------------------------
+ * Index scan related functions.
+ * ----------------------------------------------------------------------------
+ */
+
+/*
+ * To perform that check simply start an index scan, create the necessary
+ * slot, do the heap lookup, and shut everything down again. This could be
+ * optimized, but is unlikely to matter from a performance POV. If there
+ * frequently are live index pointers also matching a unique index key, the
+ * CPU overhead of this routine is unlikely to matter.
+ */
+bool
+table_index_fetch_tuple_check(Relation rel,
+							  ItemPointer tid,
+							  Snapshot snapshot,
+							  bool *all_dead)
+{
+	IndexFetchTableData *scan;
+	TupleTableSlot *slot;
+	bool		call_again = false;
+	bool		found;
+
+	slot = table_slot_create(rel, NULL);
+	scan = table_index_fetch_begin(rel);
+	found = table_index_fetch_tuple(scan, tid, snapshot, slot, &call_again,
+									all_dead);
+	table_index_fetch_end(scan);
+	ExecDropSingleTupleTableSlot(slot);
+
+	return found;
+}
+
+
 /* ----------------------------------------------------------------------------
  * Functions to make modifications a bit simpler.
  * ----------------------------------------------------------------------------