diff --git a/src/backend/catalog/index.c b/src/backend/catalog/index.c
index 53b4c3c59b..1b39e1683c 100644
--- a/src/backend/catalog/index.c
+++ b/src/backend/catalog/index.c
@@ -115,6 +115,7 @@ static void validate_index_heapscan(Relation heapRelation,
 						Snapshot snapshot,
 						v_i_state *state);
 static Oid	IndexGetRelation(Oid indexId);
+static bool ReindexIsCurrentlyProcessingIndex(Oid indexOid);
 static void SetReindexProcessing(Oid heapOid, Oid indexOid);
 static void ResetReindexProcessing(void);
 static void SetReindexPending(List *indexes);
@@ -1747,8 +1748,8 @@ index_build(Relation heapRelation,
 	 * created it, or truncated twice in a subsequent transaction, the
 	 * relfilenode won't change, and nothing needs to be done here.
 	 */
-	if (heapRelation->rd_rel->relpersistence == RELPERSISTENCE_UNLOGGED
-	    && !smgrexists(indexRelation->rd_smgr, INIT_FORKNUM))
+	if (heapRelation->rd_rel->relpersistence == RELPERSISTENCE_UNLOGGED &&
+		!smgrexists(indexRelation->rd_smgr, INIT_FORKNUM))
 	{
 		RegProcedure ambuildempty = indexRelation->rd_am->ambuildempty;
 
@@ -1757,19 +1758,6 @@ index_build(Relation heapRelation,
 		OidFunctionCall1(ambuildempty, PointerGetDatum(indexRelation));
 	}
 
-	/*
-	 * If it's for an exclusion constraint, make a second pass over the heap
-	 * to verify that the constraint is satisfied.
-	 */
-	if (indexInfo->ii_ExclusionOps != NULL)
-		IndexCheckExclusion(heapRelation, indexRelation, indexInfo);
-
-	/* Roll back any GUC changes executed by index functions */
-	AtEOXact_GUC(false, save_nestlevel);
-
-	/* Restore userid and security context */
-	SetUserIdAndSecContext(save_userid, save_sec_context);
-
 	/*
 	 * If we found any potentially broken HOT chains, mark the index as not
 	 * being usable until the current transaction is below the event horizon.
@@ -1824,8 +1812,23 @@ index_build(Relation heapRelation,
 					   InvalidOid,
 					   stats->index_tuples);
 
-	/* Make the updated versions visible */
+	/* Make the updated catalog row versions visible */
 	CommandCounterIncrement();
+
+	/*
+	 * If it's for an exclusion constraint, make a second pass over the heap
+	 * to verify that the constraint is satisfied.  We must not do this until
+	 * the index is fully valid.  (Broken HOT chains shouldn't matter, though;
+	 * see comments for IndexCheckExclusion.)
+	 */
+	if (indexInfo->ii_ExclusionOps != NULL)
+		IndexCheckExclusion(heapRelation, indexRelation, indexInfo);
+
+	/* Roll back any GUC changes executed by index functions */
+	AtEOXact_GUC(false, save_nestlevel);
+
+	/* Restore userid and security context */
+	SetUserIdAndSecContext(save_userid, save_sec_context);
 }
 
 
@@ -2269,6 +2272,15 @@ IndexCheckExclusion(Relation heapRelation,
 	EState	   *estate;
 	ExprContext *econtext;
 
+	/*
+	 * If we are reindexing the target index, mark it as no longer being
+	 * reindexed, to forestall an Assert in index_beginscan when we try to
+	 * use the index for probes.  This is OK because the index is now
+	 * fully valid.
+	 */
+	if (ReindexIsCurrentlyProcessingIndex(RelationGetRelid(indexRelation)))
+		ResetReindexProcessing();
+
 	/*
 	 * Need an EState for evaluation of index expressions and partial-index
 	 * predicates.	Also a slot to hold the current tuple.
@@ -2989,8 +3001,8 @@ reindex_relation(Oid relid, int flags)
 
 			CommandCounterIncrement();
 
-			if (flags & REINDEX_REL_SUPPRESS_INDEX_USE)
-				RemoveReindexPending(indexOid);
+			/* Index should no longer be in the pending list */
+			Assert(!ReindexIsProcessingIndex(indexOid));
 
 			if (is_pg_class)
 				doneIndexes = lappend_oid(doneIndexes, indexOid);
@@ -3030,7 +3042,9 @@ reindex_relation(Oid relid, int flags)
  *		System index reindexing support
  *
  * When we are busy reindexing a system index, this code provides support
- * for preventing catalog lookups from using that index.
+ * for preventing catalog lookups from using that index.  We also make use
+ * of this to catch attempted uses of user indexes during reindexing of
+ * those indexes.
  * ----------------------------------------------------------------
  */
 
@@ -3048,6 +3062,16 @@ ReindexIsProcessingHeap(Oid heapOid)
 	return heapOid == currentlyReindexedHeap;
 }
 
+/*
+ * ReindexIsCurrentlyProcessingIndex
+ *		True if index specified by OID is currently being reindexed.
+ */
+static bool
+ReindexIsCurrentlyProcessingIndex(Oid indexOid)
+{
+	return indexOid == currentlyReindexedIndex;
+}
+
 /*
  * ReindexIsProcessingIndex
  *		True if index specified by OID is currently being reindexed,
@@ -3075,6 +3099,8 @@ SetReindexProcessing(Oid heapOid, Oid indexOid)
 		elog(ERROR, "cannot reindex while reindexing");
 	currentlyReindexedHeap = heapOid;
 	currentlyReindexedIndex = indexOid;
+	/* Index is no longer "pending" reindex. */
+	RemoveReindexPending(indexOid);
 }
 
 /*
diff --git a/src/test/regress/input/constraints.source b/src/test/regress/input/constraints.source
index 0d278212c0..b84d51e9e5 100644
--- a/src/test/regress/input/constraints.source
+++ b/src/test/regress/input/constraints.source
@@ -397,6 +397,9 @@ INSERT INTO circles VALUES('<(20,20), 10>', '<(10,10), 5>');
 ALTER TABLE circles ADD EXCLUDE USING gist
   (c1 WITH &&, (c2::circle) WITH &&);
 
+-- try reindexing an existing constraint
+REINDEX INDEX circles_c1_c2_excl;
+
 DROP TABLE circles;
 
 -- Check deferred exclusion constraint
diff --git a/src/test/regress/output/constraints.source b/src/test/regress/output/constraints.source
index d164b90af7..e2f2939931 100644
--- a/src/test/regress/output/constraints.source
+++ b/src/test/regress/output/constraints.source
@@ -543,6 +543,8 @@ ALTER TABLE circles ADD EXCLUDE USING gist
 NOTICE:  ALTER TABLE / ADD EXCLUDE will create implicit index "circles_c1_c2_excl1" for table "circles"
 ERROR:  could not create exclusion constraint "circles_c1_c2_excl1"
 DETAIL:  Key (c1, (c2::circle))=(<(0,0),5>, <(0,0),5>) conflicts with key (c1, (c2::circle))=(<(0,0),5>, <(0,0),4>).
+-- try reindexing an existing constraint
+REINDEX INDEX circles_c1_c2_excl;
 DROP TABLE circles;
 -- Check deferred exclusion constraint
 CREATE TABLE deferred_excl (