diff --git a/src/backend/utils/cache/catcache.c b/src/backend/utils/cache/catcache.c
index 6fd672435f..97de82e4e5 100644
--- a/src/backend/utils/cache/catcache.c
+++ b/src/backend/utils/cache/catcache.c
@@ -26,6 +26,7 @@
 #ifdef CATCACHE_STATS
 #include "storage/ipc.h"		/* for on_proc_exit */
 #endif
+#include "storage/lmgr.h"
 #include "utils/builtins.h"
 #include "utils/fmgroids.h"
 #include "utils/memutils.h"
@@ -1032,8 +1033,16 @@ InitCatCachePhase2(CatCache *cache, bool touch_index)
 	{
 		Relation	idesc;
 
+		/*
+		 * We must lock the underlying catalog before opening the index to
+		 * avoid deadlock, since index_open could possibly result in reading
+		 * this same catalog, and if anyone else is exclusive-locking this
+		 * catalog and index they'll be doing it in that order.
+		 */
+		LockRelationOid(cache->cc_reloid, AccessShareLock);
 		idesc = index_open(cache->cc_indexoid, AccessShareLock);
 		index_close(idesc, AccessShareLock);
+		UnlockRelationOid(cache->cc_reloid, AccessShareLock);
 	}
 }
 
diff --git a/src/backend/utils/cache/relcache.c b/src/backend/utils/cache/relcache.c
index a2037789f1..ea8d458547 100644
--- a/src/backend/utils/cache/relcache.c
+++ b/src/backend/utils/cache/relcache.c
@@ -1629,6 +1629,12 @@ RelationClose(Relation relation)
  *	We assume that at the time we are called, we have at least AccessShareLock
  *	on the target index.  (Note: in the calls from RelationClearRelation,
  *	this is legitimate because we know the rel has positive refcount.)
+ *
+ *	If the target index is an index on pg_class or pg_index, we'd better have
+ *	previously gotten at least AccessShareLock on its underlying catalog,
+ *	else we are at risk of deadlock against someone trying to exclusive-lock
+ *	the heap and index in that order.  This is ensured in current usage by
+ *	only applying this to indexes being opened or having positive refcount.
  */
 static void
 RelationReloadIndexInfo(Relation relation)
@@ -3352,6 +3358,10 @@ RelationGetIndexPredicate(Relation relation)
  * Attribute numbers are offset by FirstLowInvalidHeapAttributeNumber so that
  * we can include system attributes (e.g., OID) in the bitmap representation.
  *
+ * Caller had better hold at least RowExclusiveLock on the target relation
+ * to ensure that it has a stable set of indexes.  This also makes it safe
+ * (deadlock-free) for us to take locks on the relation's indexes.
+ *
  * The returned result is palloc'd in the caller's memory context and should
  * be bms_free'd when not needed anymore.
  */