Avoid changing an index's indcheckxmin horizon during REINDEX.

There can never be a need to push the indcheckxmin horizon forward, since
any HOT chains that are actually broken with respect to the index must
pre-date its original creation.  So we can just avoid changing pg_index
altogether during a REINDEX operation.

This offers a cleaner solution than my previous patch for the problem
found a few days ago that we mustn't try to update pg_index while we are
reindexing it.  System catalog indexes will always be created with
indcheckxmin = false during initdb, and with this modified code we should
never try to change their pg_index entries.  This avoids special-casing
system catalogs as the former patch did, and should provide a performance
benefit for many cases where REINDEX formerly caused an index to be
considered unusable for a short time.

Back-patch to 8.3 to cover all versions containing HOT.  Note that this
patch changes the API for index_build(), but I believe it is unlikely that
any add-on code is calling that directly.

src/backend/bootstrap/bootstrap.c

@@ -1134,7 +1134,7 @@ build_indices(void)
 		heap = heap_open(ILHead->il_heap, NoLock);
 		ind = index_open(ILHead->il_ind, NoLock);
 
-		index_build(heap, ind, ILHead->il_info, false);
+		index_build(heap, ind, ILHead->il_info, false, false);
 
 		index_close(ind, NoLock);
 		heap_close(heap, NoLock);

src/backend/catalog/heap.c

@@ -2569,7 +2569,7 @@ RelationTruncateIndexes(Relation heapRelation)
 
 		/* Initialize the index and rebuild */
 		/* Note: we do not need to re-establish pkey setting */
-		index_build(heapRelation, currentIndex, indexInfo, false);
+		index_build(heapRelation, currentIndex, indexInfo, false, true);
 
 		/* We're done with this index */
 		index_close(currentIndex, NoLock);

src/backend/catalog/index.c

@@ -1061,7 +1061,7 @@ index_create(Relation heapRelation,
 	}
 	else
 	{
-		index_build(heapRelation, indexRelation, indexInfo, isprimary);
+		index_build(heapRelation, indexRelation, indexInfo, isprimary, false);
 	}
 
 	/*
@@ -1680,8 +1680,11 @@ index_update_stats(Relation rel,
  * entries of the index and heap relation as needed, using statistics
  * returned by ambuild as well as data passed by the caller.
  *
- * Note: when reindexing an existing index, isprimary can be false;
+ * isprimary tells whether to mark the index as a primary-key index.
- * the index is already properly marked and need not be re-marked.
+ * isreindex indicates we are recreating a previously-existing index.
+ *
+ * Note: when reindexing an existing index, isprimary can be false even if
+ * the index is a PK; it's already properly marked and need not be re-marked.
  *
  * Note: before Postgres 8.2, the passed-in heap and index Relations
  * were automatically closed by this routine.  This is no longer the case.
@@ -1691,7 +1694,8 @@ void
 index_build(Relation heapRelation,
 			Relation indexRelation,
 			IndexInfo *indexInfo,
-			bool isprimary)
+			bool isprimary,
+			bool isreindex)
 {
 	RegProcedure procedure;
 	IndexBuildResult *stats;
@@ -1762,8 +1766,15 @@ index_build(Relation heapRelation,
 	 * If we found any potentially broken HOT chains, mark the index as not
 	 * being usable until the current transaction is below the event horizon.
 	 * See src/backend/access/heap/README.HOT for discussion.
-	 */
+	 *
-	if (indexInfo->ii_BrokenHotChain)
+	 * However, when reindexing an existing index, we should do nothing here.
+	 * Any HOT chains that are broken with respect to the index must predate
+	 * the index's original creation, so there is no need to change the
+	 * index's usability horizon.  Moreover, we *must not* try to change
+	 * the index's pg_index entry while reindexing pg_index itself, and this
+	 * optimization nicely prevents that.
+	 */
+	if (indexInfo->ii_BrokenHotChain && !isreindex)
 	{
 		Oid			indexId = RelationGetRelid(indexRelation);
 		Relation	pg_index;
@@ -1778,6 +1789,9 @@ index_build(Relation heapRelation,
 			elog(ERROR, "cache lookup failed for index %u", indexId);
 		indexForm = (Form_pg_index) GETSTRUCT(indexTuple);
 
+		/* If it's a new index, indcheckxmin shouldn't be set ... */
+		Assert(!indexForm->indcheckxmin);
+
 		indexForm->indcheckxmin = true;
 		simple_heap_update(pg_index, &indexTuple->t_self, indexTuple);
 		CatalogUpdateIndexes(pg_index, indexTuple);
@@ -2767,7 +2781,7 @@ reindex_index(Oid indexId, bool skip_constraint_checks)
 
 		/* Initialize the index and rebuild */
 		/* Note: we do not need to re-establish pkey setting */
-		index_build(heapRelation, iRel, indexInfo, false);
+		index_build(heapRelation, iRel, indexInfo, false, true);
 	}
 	PG_CATCH();
 	{
@@ -2787,6 +2801,16 @@ reindex_index(Oid indexId, bool skip_constraint_checks)
 	 * We can also reset indcheckxmin, because we have now done a
 	 * non-concurrent index build, *except* in the case where index_build
 	 * found some still-broken HOT chains.
+	 *
+	 * Note that it is important to not update the pg_index entry if we don't
+	 * have to, because updating it will move the index's usability horizon
+	 * (recorded as the tuple's xmin value) if indcheckxmin is true.  We don't
+	 * really want REINDEX to move the usability horizon forward ever, but we
+	 * have no choice if we are to fix indisvalid or indisready.  Of course,
+	 * clearing indcheckxmin eliminates the issue, so we're happy to do that
+	 * if we can.  Another reason for caution here is that while reindexing
+	 * pg_index itself, we must not try to update it.  We assume that
+	 * pg_index's indexes will always have these flags in their clean state.
 	 */
 	if (!skipped_constraint)
 	{

src/backend/commands/cluster.c

@@ -1398,6 +1398,12 @@ finish_heap_swap(Oid OIDOldHeap, Oid OIDNewHeap,
 	 * advantage to the other order anyway because this is all transactional,
 	 * so no chance to reclaim disk space before commit.  We do not need a
 	 * final CommandCounterIncrement() because reindex_relation does it.
+	 *
+	 * Note: because index_build is called via reindex_relation, it will never
+	 * set indcheckxmin true for the indexes.  This is OK even though in some
+	 * sense we are building new indexes rather than rebuilding existing ones,
+	 * because the new heap won't contain any HOT chains at all, let alone
+	 * broken ones, so it can't be necessary to set indcheckxmin.
 	 */
 	reindex_flags = REINDEX_REL_SUPPRESS_INDEX_USE;
 	if (check_constraints)

src/backend/commands/indexcmds.c

@@ -505,7 +505,7 @@ DefineIndex(RangeVar *heapRelation,
 	indexInfo->ii_BrokenHotChain = false;
 
 	/* Now build the index */
-	index_build(rel, indexRelation, indexInfo, primary);
+	index_build(rel, indexRelation, indexInfo, primary, false);
 
 	/* Close both the relations, but keep the locks */
 	heap_close(rel, NoLock);

src/include/catalog/index.h

@@ -75,7 +75,8 @@ extern void FormIndexDatum(IndexInfo *indexInfo,
 extern void index_build(Relation heapRelation,
 			Relation indexRelation,
 			IndexInfo *indexInfo,
-			bool isprimary);
+			bool isprimary,
+			bool isreindex);
 
 extern double IndexBuildHeapScan(Relation heapRelation,
 				   Relation indexRelation,