Fix memory leak in indexUnchanged hint mechanism.

Commit 9dc718b added a "logically unchanged by UPDATE" hinting
mechanism, which is currently used within nbtree indexes only (see
commit d168b66).  This mechanism determined whether or not the incoming
item is a logically unchanged duplicate (a duplicate needed only for
MVCC versioning purposes) once per row updated per non-HOT update.  This
approach led to memory leaks which were noticeable with an UPDATE
statement that updated sufficiently many rows, at least on tables that
happen to have an expression index.

On HEAD, fix the issue by adding a cache to the executor's per-index
IndexInfo struct.

Take a different approach on Postgres 14 to avoid an ABI break: simply
pass down the hint to all indexes unconditionally with non-HOT UPDATEs.
This is deemed acceptable because the hint is currently interpreted
within btinsert() as "perform a bottom-up index deletion pass if and
when the only alternative is splitting the leaf page -- prefer to delete
any LP_DEAD-set items first".  nbtree must always treat the hint as a
noisy signal about what might work, as a strategy of last resort, with
costs imposed on non-HOT updaters.  (The same thing might not be true
within another index AM that applies the hint, which is why the original
behavior is preserved on HEAD.)

Author: Peter Geoghegan <pg@bowt.ie>
Reported-By: Klaudie Willis <Klaudie.Willis@protonmail.com>
Diagnosed-By: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/261065.1639497535@sss.pgh.pa.us
Backpatch: 14-, where the hinting mechanism was added.

Author: petergeoghegan

src/backend/catalog/toasting.c

@@ -302,6 +302,8 @@ create_toast_table(Relation rel, Oid toastOid, Oid toastIndexOid,
 	indexInfo->ii_OpclassOptions = NULL;
 	indexInfo->ii_Unique = true;
 	indexInfo->ii_ReadyForInserts = true;
+	indexInfo->ii_CheckedUnchanged = false;
+	indexInfo->ii_IndexUnchanged = false;
 	indexInfo->ii_Concurrent = false;
 	indexInfo->ii_BrokenHotChain = false;
 	indexInfo->ii_ParallelWorkers = 0;

src/backend/executor/execIndexing.c

@@ -935,19 +935,32 @@ static bool
 index_unchanged_by_update(ResultRelInfo *resultRelInfo, EState *estate,
 						  IndexInfo *indexInfo, Relation indexRelation)
 {
-	Bitmapset  *updatedCols = ExecGetUpdatedCols(resultRelInfo, estate);
-	Bitmapset  *extraUpdatedCols = ExecGetExtraUpdatedCols(resultRelInfo, estate);
+	Bitmapset  *updatedCols;
+	Bitmapset  *extraUpdatedCols;
 	Bitmapset  *allUpdatedCols;
 	bool		hasexpression = false;
 	List	   *idxExprs;
 
+	/*
+	 * Check cache first
+	 */
+	if (indexInfo->ii_CheckedUnchanged)
+		return indexInfo->ii_IndexUnchanged;
+	indexInfo->ii_CheckedUnchanged = true;
+
 	/*
 	 * Check for indexed attribute overlap with updated columns.
 	 *
 	 * Only do this for key columns.  A change to a non-key column within an
 	 * INCLUDE index should not be counted here.  Non-key column values are
 	 * opaque payload state to the index AM, a little like an extra table TID.
+	 *
+	 * Note that row-level BEFORE triggers won't affect our behavior, since
+	 * they don't affect the updatedCols bitmaps generally.  It doesn't seem
+	 * worth the trouble of checking which attributes were changed directly.
 	 */
+	updatedCols = ExecGetUpdatedCols(resultRelInfo, estate);
+	extraUpdatedCols = ExecGetExtraUpdatedCols(resultRelInfo, estate);
 	for (int attr = 0; attr < indexInfo->ii_NumIndexKeyAttrs; attr++)
 	{
 		int			keycol = indexInfo->ii_IndexAttrNumbers[attr];
@@ -968,6 +981,7 @@ index_unchanged_by_update(ResultRelInfo *resultRelInfo, EState *estate,
 						  extraUpdatedCols))
 		{
 			/* Changed key column -- don't hint for this index */
+			indexInfo->ii_IndexUnchanged = false;
 			return false;
 		}
 	}
@@ -981,7 +995,10 @@ index_unchanged_by_update(ResultRelInfo *resultRelInfo, EState *estate,
 	 * shows that the index as a whole is logically unchanged by UPDATE.
 	 */
 	if (!hasexpression)
+	{
+		indexInfo->ii_IndexUnchanged = true;
 		return true;
+	}
 
 	/*
 	 * Need to pass only one bms to expression_tree_walker helper function.
@@ -1008,8 +1025,18 @@ index_unchanged_by_update(ResultRelInfo *resultRelInfo, EState *estate,
 		bms_free(allUpdatedCols);
 
 	if (hasexpression)
+	{
+		indexInfo->ii_IndexUnchanged = false;
 		return false;
+	}
 
+	/*
+	 * Deliberately don't consider index predicates.  We should even give the
+	 * hint when result rel's "updated tuple" has no corresponding index
+	 * tuple, which is possible with a partial index (provided the usual
+	 * conditions are met).
+	 */
+	indexInfo->ii_IndexUnchanged = true;
 	return true;
 }
 

src/backend/nodes/makefuncs.c

@@ -751,6 +751,8 @@ makeIndexInfo(int numattrs, int numkeyattrs, Oid amoid, List *expressions,
 	Assert(n->ii_NumIndexKeyAttrs <= n->ii_NumIndexAttrs);
 	n->ii_Unique = unique;
 	n->ii_ReadyForInserts = isready;
+	n->ii_CheckedUnchanged = false;
+	n->ii_IndexUnchanged = false;
 	n->ii_Concurrent = concurrent;
 
 	/* expressions */

src/include/nodes/execnodes.h

@@ -142,6 +142,8 @@ typedef struct ExprState
  *		Unique				is it a unique index?
  *		OpclassOptions		opclass-specific options, or NULL if none
  *		ReadyForInserts		is it valid for inserts?
+ *		CheckedUnchanged	IndexUnchanged status determined yet?
+ *		IndexUnchanged		aminsert hint, cached for retail inserts
  *		Concurrent			are we doing a concurrent index build?
  *		BrokenHotChain		did we detect any broken HOT chains?
  *		ParallelWorkers		# of workers requested (excludes leader)
@@ -172,6 +174,8 @@ typedef struct IndexInfo
 	Datum	   *ii_OpclassOptions;	/* array with one entry per column */
 	bool		ii_Unique;
 	bool		ii_ReadyForInserts;
+	bool		ii_CheckedUnchanged;
+	bool		ii_IndexUnchanged;
 	bool		ii_Concurrent;
 	bool		ii_BrokenHotChain;
 	int			ii_ParallelWorkers;