Remove unnecessary checks for indexes for REPLICA IDENTITY FULL tables.

Previously, when selecting an usable index for update/delete for the
REPLICA IDENTITY FULL table, in IsIndexOnlyExpression(), we used to
check if all index fields are not expressions. However, it was not
necessary, because it is enough to check if only the leftmost index
field is not an expression (and references the remote table column)
and this check has already been done by
RemoteRelContainsLeftMostColumnOnIdx().

This commit removes IsIndexOnlyExpression() and
RemoteRelContainsLeftMostColumnOnIdx() and all checks for usable
indexes for REPLICA IDENTITY FULL tables are now performed by
IsIndexUsableForReplicaIdentityFull().

Backpatch this to remain the code consistent.

Reported-by: Peter Smith
Reviewed-by: Amit Kapila, Önder Kalacı
Discussion: https://postgr.es/m/CAHut%2BPsGRE5WSsY0jcLHJEoA17MrbP9yy8FxdjC_ZOAACxbt%2BQ%40mail.gmail.com
Backpatch-through: 16

src/backend/executor/execReplication.c

@@ -222,16 +222,7 @@ retry:
 		if (!isIdxSafeToSkipDuplicates)
 		{
 			if (eq == NULL)
-			{
-#ifdef USE_ASSERT_CHECKING
-				/* apply assertions only once for the input idxoid */
-				IndexInfo  *indexInfo = BuildIndexInfo(idxrel);
-
-				Assert(IsIndexUsableForReplicaIdentityFull(indexInfo));
-#endif
-
 				eq = palloc0(sizeof(*eq) * outslot->tts_tupleDescriptor->natts);
-			}
 
 			if (!tuples_equal(outslot, searchslot, eq))
 				continue;

src/backend/replication/logical/relation.c

@@ -734,71 +734,9 @@ logicalrep_partition_open(LogicalRepRelMapEntry *root,
 	return entry;
 }
 
-/*
- * Returns true if the given index consists only of expressions such as:
- * 	CREATE INDEX idx ON table(foo(col));
- *
- * Returns false even if there is one column reference:
- * 	 CREATE INDEX idx ON table(foo(col), col_2);
- */
-static bool
-IsIndexOnlyOnExpression(IndexInfo *indexInfo)
-{
-	for (int i = 0; i < indexInfo->ii_NumIndexKeyAttrs; i++)
-	{
-		AttrNumber	attnum = indexInfo->ii_IndexAttrNumbers[i];
-
-		if (AttributeNumberIsValid(attnum))
-			return false;
-	}
-
-	return true;
-}
-
-/*
- * Returns true if the attrmap contains the leftmost column of the index.
- * Otherwise returns false.
- *
- * attrmap is a map of local attributes to remote ones. We can consult this
- * map to check whether the local index attribute has a corresponding remote
- * attribute.
- */
-static bool
-RemoteRelContainsLeftMostColumnOnIdx(IndexInfo *indexInfo, AttrMap *attrmap)
-{
-	AttrNumber	keycol;
-
-	Assert(indexInfo->ii_NumIndexAttrs >= 1);
-
-	keycol = indexInfo->ii_IndexAttrNumbers[0];
-	if (!AttributeNumberIsValid(keycol))
-		return false;
-
-	if (attrmap->maplen <= AttrNumberGetAttrOffset(keycol))
-		return false;
-
-	return attrmap->attnums[AttrNumberGetAttrOffset(keycol)] >= 0;
-}
-
 /*
  * Returns the oid of an index that can be used by the apply worker to scan
- * the relation. The index must be btree or hash, non-partial, and the leftmost
- * field must be a column (not an expression) that references the remote
- * relation column. These limitations help to keep the index scan similar
- * to PK/RI index scans.
- *
- * Note that the limitations of index scans for replica identity full only
- * adheres to a subset of the limitations of PK/RI. For example, we support
- * columns that are marked as [NULL] or we are not interested in the [NOT
- * DEFERRABLE] aspect of constraints here. It works for us because we always
- * compare the tuples for non-PK/RI index scans. See
- * RelationFindReplTupleByIndex().
- *
- * XXX: See IsIndexUsableForReplicaIdentityFull() to know the challenges in
- * supporting indexes other than btree and hash. For partial indexes, the
- * required changes are likely to be larger. If none of the tuples satisfy
- * the expression for the index scan, we fall-back to sequential execution,
- * which might not be a good idea in some cases.
+ * the relation.
  *
  * We expect to call this function when REPLICA IDENTITY FULL is defined for
  * the remote relation.
@@ -815,19 +753,16 @@ FindUsableIndexForReplicaIdentityFull(Relation localrel, AttrMap *attrmap)
 	{
 		Oid			idxoid = lfirst_oid(lc);
 		bool		isUsableIdx;
-		bool		containsLeftMostCol;
 		Relation	idxRel;
 		IndexInfo  *idxInfo;
 
 		idxRel = index_open(idxoid, AccessShareLock);
 		idxInfo = BuildIndexInfo(idxRel);
-		isUsableIdx = IsIndexUsableForReplicaIdentityFull(idxInfo);
-		containsLeftMostCol =
-			RemoteRelContainsLeftMostColumnOnIdx(idxInfo, attrmap);
+		isUsableIdx = IsIndexUsableForReplicaIdentityFull(idxInfo, attrmap);
 		index_close(idxRel, AccessShareLock);
 
 		/* Return the first eligible index found */
-		if (isUsableIdx && containsLeftMostCol)
+		if (isUsableIdx)
 			return idxoid;
 	}
 
@@ -835,11 +770,24 @@ FindUsableIndexForReplicaIdentityFull(Relation localrel, AttrMap *attrmap)
 }
 
 /*
- * Returns true if the index is usable for replica identity full. For details,
- * see FindUsableIndexForReplicaIdentityFull.
+ * Returns true if the index is usable for replica identity full.
  *
- * Currently, only Btree and Hash indexes can be returned as usable. This
- * is due to following reasons:
+ * The index must be btree or hash, non-partial, and the leftmost field must be
+ * a column (not an expression) that references the remote relation column. These
+ * limitations help to keep the index scan similar to PK/RI index scans.
+ *
+ * attrmap is a map of local attributes to remote ones. We can consult this
+ * map to check whether the local index attribute has a corresponding remote
+ * attribute.
+ *
+ * Note that the limitations of index scans for replica identity full only
+ * adheres to a subset of the limitations of PK/RI. For example, we support
+ * columns that are marked as [NULL] or we are not interested in the [NOT
+ * DEFERRABLE] aspect of constraints here. It works for us because we always
+ * compare the tuples for non-PK/RI index scans. See
+ * RelationFindReplTupleByIndex().
+ *
+ * The reasons why only Btree and Hash indexes can be considered as usable are:
  *
  * 1) Other index access methods don't have a fixed strategy for equality
  * operation. Refer get_equal_strategy_number_for_am().
@@ -851,16 +799,38 @@ FindUsableIndexForReplicaIdentityFull(Relation localrel, AttrMap *attrmap)
  *
  * XXX: Note that BRIN and GIN indexes do not implement "amgettuple" which
  * will be used later to fetch the tuples. See RelationFindReplTupleByIndex().
+ *
+ * XXX: To support partial indexes, the required changes are likely to be larger.
+ * If none of the tuples satisfy the expression for the index scan, we fall-back
+ * to sequential execution, which might not be a good idea in some cases.
  */
 bool
-IsIndexUsableForReplicaIdentityFull(IndexInfo *indexInfo)
+IsIndexUsableForReplicaIdentityFull(IndexInfo *indexInfo, AttrMap *attrmap)
 {
+	AttrNumber	keycol;
+
 	/* Ensure that the index access method has a valid equal strategy */
 	if (get_equal_strategy_number_for_am(indexInfo->ii_Am) == InvalidStrategy)
 		return false;
+
+	/* The index must not be a partial index */
 	if (indexInfo->ii_Predicate != NIL)
 		return false;
-	if (IsIndexOnlyOnExpression(indexInfo))
+
+	Assert(indexInfo->ii_NumIndexAttrs >= 1);
+
+	/* The leftmost index field must not be an expression */
+	keycol = indexInfo->ii_IndexAttrNumbers[0];
+	if (!AttributeNumberIsValid(keycol))
+		return false;
+
+	/*
+	 * And the leftmost index field must reference the remote relation column.
+	 * This is because if it doesn't, the sequential scan is favorable over
+	 * index scan in most cases.
+	 */
+	if (attrmap->maplen <= AttrNumberGetAttrOffset(keycol) ||
+		attrmap->attnums[AttrNumberGetAttrOffset(keycol)] < 0)
 		return false;
 
 #ifdef USE_ASSERT_CHECKING

src/backend/replication/logical/worker.c

@@ -140,6 +140,7 @@
 #include <sys/stat.h>
 #include <unistd.h>
 
+#include "access/genam.h"
 #include "access/table.h"
 #include "access/tableam.h"
 #include "access/twophase.h"
@@ -410,7 +411,7 @@ static void apply_handle_delete_internal(ApplyExecutionData *edata,
 										 ResultRelInfo *relinfo,
 										 TupleTableSlot *remoteslot,
 										 Oid localindexoid);
-static bool FindReplTupleInLocalRel(EState *estate, Relation localrel,
+static bool FindReplTupleInLocalRel(ApplyExecutionData *edata, Relation localrel,
 									LogicalRepRelation *remoterel,
 									Oid localidxoid,
 									TupleTableSlot *remoteslot,
@@ -2663,7 +2664,7 @@ apply_handle_update_internal(ApplyExecutionData *edata,
 	EvalPlanQualInit(&epqstate, estate, NULL, NIL, -1, NIL);
 	ExecOpenIndices(relinfo, false);
 
-	found = FindReplTupleInLocalRel(estate, localrel,
+	found = FindReplTupleInLocalRel(edata, localrel,
 									&relmapentry->remoterel,
 									localindexoid,
 									remoteslot, &localslot);
@@ -2816,7 +2817,7 @@ apply_handle_delete_internal(ApplyExecutionData *edata,
 	EvalPlanQualInit(&epqstate, estate, NULL, NIL, -1, NIL);
 	ExecOpenIndices(relinfo, false);
 
-	found = FindReplTupleInLocalRel(estate, localrel, remoterel, localindexoid,
+	found = FindReplTupleInLocalRel(edata, localrel, remoterel, localindexoid,
 									remoteslot, &localslot);
 
 	/* If found delete it. */
@@ -2855,12 +2856,13 @@ apply_handle_delete_internal(ApplyExecutionData *edata,
  * Local tuple, if found, is returned in '*localslot'.
  */
 static bool
-FindReplTupleInLocalRel(EState *estate, Relation localrel,
+FindReplTupleInLocalRel(ApplyExecutionData *edata, Relation localrel,
 						LogicalRepRelation *remoterel,
 						Oid localidxoid,
 						TupleTableSlot *remoteslot,
 						TupleTableSlot **localslot)
 {
+	EState	   *estate = edata->estate;
 	bool		found;
 
 	/*
@@ -2875,9 +2877,21 @@ FindReplTupleInLocalRel(EState *estate, Relation localrel,
 		   (remoterel->replident == REPLICA_IDENTITY_FULL));
 
 	if (OidIsValid(localidxoid))
+	{
+#ifdef USE_ASSERT_CHECKING
+		Relation	idxrel = index_open(localidxoid, AccessShareLock);
+
+		/* Index must be PK, RI, or usable for REPLICA IDENTITY FULL tables */
+		Assert(GetRelationIdentityOrPK(idxrel) == localidxoid ||
+			   IsIndexUsableForReplicaIdentityFull(BuildIndexInfo(idxrel),
+												   edata->targetRel->attrmap));
+		index_close(idxrel, AccessShareLock);
+#endif
+
 		found = RelationFindReplTupleByIndex(localrel, localidxoid,
 											 LockTupleExclusive,
 											 remoteslot, *localslot);
+	}
 	else
 		found = RelationFindReplTupleSeq(localrel, LockTupleExclusive,
 										 remoteslot, *localslot);
@@ -2995,7 +3009,7 @@ apply_handle_tuple_routing(ApplyExecutionData *edata,
 				bool		found;
 
 				/* Get the matching local tuple from the partition. */
-				found = FindReplTupleInLocalRel(estate, partrel,
+				found = FindReplTupleInLocalRel(edata, partrel,
 												&part_entry->remoterel,
 												part_entry->localindexoid,
 												remoteslot_part, &localslot);

src/include/replication/logicalrelation.h

@@ -48,7 +48,7 @@ extern LogicalRepRelMapEntry *logicalrep_partition_open(LogicalRepRelMapEntry *r
 														Relation partrel, AttrMap *map);
 extern void logicalrep_rel_close(LogicalRepRelMapEntry *rel,
 								 LOCKMODE lockmode);
-extern bool IsIndexUsableForReplicaIdentityFull(IndexInfo *indexInfo);
+extern bool IsIndexUsableForReplicaIdentityFull(IndexInfo *indexInfo, AttrMap *attrmap);
 extern Oid	GetRelationIdentityOrPK(Relation rel);
 
 #endif							/* LOGICALRELATION_H */