Allow the use of indexes other than PK and REPLICA IDENTITY on the subscriber.

Using REPLICA IDENTITY FULL on the publisher can lead to a full table scan
per tuple change on the subscription when REPLICA IDENTITY or PK index is
not available. This makes REPLICA IDENTITY FULL impractical to use apart
from some small number of use cases.

This patch allows using indexes other than PRIMARY KEY or REPLICA
IDENTITY on the subscriber during apply of update/delete. The index that
can be used must be a btree index, not a partial index, and it must have
at least one column reference (i.e. cannot consist of only expressions).
We can uplift these restrictions in the future. There is no smart
mechanism to pick the index. If there is more than one index that
satisfies these requirements, we just pick the first one. We discussed
using some of the optimizer's low-level APIs for this but ruled it out
as that can be a maintenance burden in the long run.

This patch improves the performance in the vast majority of cases and the
improvement is proportional to the amount of data in the table. However,
there could be some regression in a small number of cases where the indexes
have a lot of duplicate and dead rows. It was discussed that those are
mostly impractical cases but we can provide a table or subscription level
option to disable this feature if required.

Author: Onder Kalaci, Amit Kapila
Reviewed-by: Peter Smith, Shi yu, Hou Zhijie, Vignesh C, Kuroda Hayato, Amit Kapila
Discussion: https://postgr.es/m/CACawEhVLqmAAyPXdHEPv1ssU2c=dqOniiGz7G73HfyS7+nGV4w@mail.gmail.com

1 files changed, 210 insertions, 2 deletions

diff --git a/src/backend/replication/logical/relation.c b/src/backend/replication/logical/relation.c
index 55bfa078711..57ad22b48a1 100644
--- a/src/backend/replication/logical/relation.c
+++ b/src/backend/replication/logical/relation.c
@@ -17,8 +17,10 @@
 
 #include "postgres.h"
 
+#include "access/genam.h"
 #include "access/table.h"
 #include "catalog/namespace.h"
+#include "catalog/pg_am_d.h"
 #include "catalog/pg_subscription_rel.h"
 #include "executor/executor.h"
 #include "nodes/makefuncs.h"
@@ -50,6 +52,9 @@ typedef struct LogicalRepPartMapEntry
 	LogicalRepRelMapEntry relmapentry;
 } LogicalRepPartMapEntry;
 
+static Oid	FindLogicalRepLocalIndex(Relation localrel, LogicalRepRelation *remoterel,
+									 AttrMap *attrMap);
+
 /*
  * Relcache invalidation callback for our relation map cache.
  */
@@ -439,6 +444,15 @@ logicalrep_rel_open(LogicalRepRelId remoteid, LOCKMODE lockmode)
 		 */
 		logicalrep_rel_mark_updatable(entry);
 
+		/*
+		 * Finding a usable index is an infrequent task. It occurs when an
+		 * operation is first performed on the relation, or after invalidation
+		 * of the relation cache entry (such as ANALYZE or CREATE/DROP index
+		 * on the relation).
+		 */
+		entry->localindexoid = FindLogicalRepLocalIndex(entry->localrel, remoterel,
+														entry->attrmap);
+
 		entry->localrelvalid = true;
 	}
 
@@ -697,10 +711,204 @@ logicalrep_partition_open(LogicalRepRelMapEntry *root,
 	/* Set if the table's replica identity is enough to apply update/delete. */
 	logicalrep_rel_mark_updatable(entry);
 
-	entry->localrelvalid = true;
-
 	/* state and statelsn are left set to 0. */
 	MemoryContextSwitchTo(oldctx);
 
+	/*
+	 * Finding a usable index is an infrequent task. It occurs when an
+	 * operation is first performed on the relation, or after invalidation of
+	 * the relation cache entry (such as ANALYZE or CREATE/DROP index on the
+	 * relation).
+	 *
+	 * We also prefer to run this code on the oldctx so that we do not leak
+	 * anything in the LogicalRepPartMapContext (hence CacheMemoryContext).
+	 */
+	entry->localindexoid = FindLogicalRepLocalIndex(partrel, remoterel,
+													entry->attrmap);
+
+	entry->localrelvalid = true;
+
 	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, non-partial, and have at least
+ * one column reference (i.e. cannot consist of only expressions). 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: There are no fundamental problems for supporting non-btree indexes.
+ * We mostly need to relax the limitations in RelationFindReplTupleByIndex().
+ * 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.
+ *
+ * We also skip indexes if the remote relation does not contain the leftmost
+ * column of the index. This is because in most such cases sequential scan is
+ * favorable over index scan.
+ *
+ * We expect to call this function when REPLICA IDENTITY FULL is defined for
+ * the remote relation.
+ *
+ * If no suitable index is found, returns InvalidOid.
+ */
+static Oid
+FindUsableIndexForReplicaIdentityFull(Relation localrel, AttrMap *attrmap)
+{
+	List	   *idxlist = RelationGetIndexList(localrel);
+	ListCell   *lc;
+
+	foreach(lc, idxlist)
+	{
+		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);
+		index_close(idxRel, AccessShareLock);
+
+		/* Return the first eligible index found */
+		if (isUsableIdx && containsLeftMostCol)
+			return idxoid;
+	}
+
+	return InvalidOid;
+}
+
+/*
+ * Returns true if the index is usable for replica identity full. For details,
+ * see FindUsableIndexForReplicaIdentityFull.
+ */
+bool
+IsIndexUsableForReplicaIdentityFull(IndexInfo *indexInfo)
+{
+	bool		is_btree = (indexInfo->ii_Am == BTREE_AM_OID);
+	bool		is_partial = (indexInfo->ii_Predicate != NIL);
+	bool		is_only_on_expression = IsIndexOnlyOnExpression(indexInfo);
+
+	return is_btree && !is_partial && !is_only_on_expression;
+}
+
+/*
+ * Get replica identity index or if it is not defined a primary key.
+ *
+ * If neither is defined, returns InvalidOid
+ */
+Oid
+GetRelationIdentityOrPK(Relation rel)
+{
+	Oid			idxoid;
+
+	idxoid = RelationGetReplicaIndex(rel);
+
+	if (!OidIsValid(idxoid))
+		idxoid = RelationGetPrimaryKeyIndex(rel);
+
+	return idxoid;
+}
+
+/*
+ * Returns the index oid if we can use an index for subscriber. Otherwise,
+ * returns InvalidOid.
+ */
+static Oid
+FindLogicalRepLocalIndex(Relation localrel, LogicalRepRelation *remoterel,
+						 AttrMap *attrMap)
+{
+	Oid			idxoid;
+
+	/*
+	 * We never need index oid for partitioned tables, always rely on leaf
+	 * partition's index.
+	 */
+	if (localrel->rd_rel->relkind == RELKIND_PARTITIONED_TABLE)
+		return InvalidOid;
+
+	/*
+	 * Simple case, we already have a primary key or a replica identity index.
+	 */
+	idxoid = GetRelationIdentityOrPK(localrel);
+	if (OidIsValid(idxoid))
+		return idxoid;
+
+	if (remoterel->replident == REPLICA_IDENTITY_FULL)
+	{
+		/*
+		 * We are looking for one more opportunity for using an index. If
+		 * there are any indexes defined on the local relation, try to pick a
+		 * suitable index.
+		 *
+		 * The index selection safely assumes that all the columns are going
+		 * to be available for the index scan given that remote relation has
+		 * replica identity full.
+		 *
+		 * Note that we are not using the planner to find the cheapest method
+		 * to scan the relation as that would require us to either use lower
+		 * level planner functions which would be a maintenance burden in the
+		 * long run or use the full-fledged planner which could cause
+		 * overhead.
+		 */
+		return FindUsableIndexForReplicaIdentityFull(localrel, attrMap);
+	}
+
+	return InvalidOid;
+}