diff --git a/src/backend/utils/cache/syscache.c b/src/backend/utils/cache/syscache.c
index 945935476c1..5d0cca6177c 100644
--- a/src/backend/utils/cache/syscache.c
+++ b/src/backend/utils/cache/syscache.c
@@ -39,6 +39,7 @@
 #include "catalog/pg_description.h"
 #include "catalog/pg_enum.h"
 #include "catalog/pg_event_trigger.h"
+#include "catalog/pg_extension.h"
 #include "catalog/pg_foreign_data_wrapper.h"
 #include "catalog/pg_foreign_server.h"
 #include "catalog/pg_foreign_table.h"
@@ -994,6 +995,18 @@ static const struct cachedesc cacheinfo[] = {
 			0
 		},
 		2
+	},
+	/* intentionally out of alphabetical order, to avoid an ABI break: */
+	{ExtensionRelationId,		/* EXTENSIONOID */
+		ExtensionOidIndexId,
+		1,
+		{
+			Anum_pg_extension_oid,
+			0,
+			0,
+			0
+		},
+		2
 	}
 };
 
diff --git a/src/include/utils/syscache.h b/src/include/utils/syscache.h
index 27c892d24c5..bffa8c45b5e 100644
--- a/src/include/utils/syscache.h
+++ b/src/include/utils/syscache.h
@@ -109,9 +109,11 @@ enum SysCacheIdentifier
 	TYPENAMENSP,
 	TYPEOID,
 	USERMAPPINGOID,
-	USERMAPPINGUSERSERVER
+	USERMAPPINGUSERSERVER,
+	/* intentionally out of alphabetical order, to avoid an ABI break: */
+	EXTENSIONOID
 
-#define SysCacheSize (USERMAPPINGUSERSERVER + 1)
+#define SysCacheSize (EXTENSIONOID + 1)
 };
 
 extern void InitCatalogCache(void);