Skip allocating hash table in EXPLAIN-only mode.

This is a backpatch of commit 2cccb62, backpatched due to popular
demand. Backpatch to all supported versions.

Author: Alexey Bashtanov
Discussion: https://www.postgresql.org/message-id/36823f65-050d-ae24-aa4d-a37726998240%40imap.cc

Author: hlinnaka

src/backend/executor/nodeAgg.c

@@ -2488,7 +2488,11 @@ ExecInitAgg(Agg *node, EState *estate, int eflags)
 		aggstate->hash_pergroup = pergroups;
 
 		find_hash_columns(aggstate);
-		build_hash_table(aggstate);
+
+		/* Skip massive memory allocation if we are just doing EXPLAIN */
+		if (!(eflags & EXEC_FLAG_EXPLAIN_ONLY))
+			build_hash_table(aggstate);
+
 		aggstate->table_filled = false;
 	}