diff --git a/src/backend/utils/cache/catcache.c b/src/backend/utils/cache/catcache.c
index a8658116283..0b27bc034c0 100644
--- a/src/backend/utils/cache/catcache.c
+++ b/src/backend/utils/cache/catcache.c
@@ -24,6 +24,7 @@
 #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"
@@ -1009,8 +1010,16 @@ 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 f6853899b3c..59648a5ddec 100644
--- a/src/backend/utils/cache/relcache.c
+++ b/src/backend/utils/cache/relcache.c
@@ -1541,6 +1541,12 @@ 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
 RelationReloadClassinfo(Relation relation)