diff --git a/src/backend/executor/nodeAgg.c b/src/backend/executor/nodeAgg.c
index d87677d659..196fe09c52 100644
--- a/src/backend/executor/nodeAgg.c
+++ b/src/backend/executor/nodeAgg.c
@@ -3665,7 +3665,11 @@ ExecInitAgg(Agg *node, EState *estate, int eflags)
 							&aggstate->hash_ngroups_limit,
 							&aggstate->hash_planned_partitions);
 		find_hash_columns(aggstate);
-		build_hash_tables(aggstate);
+
+		/* Skip massive memory allocation when running only explain */
+		if (!(eflags & EXEC_FLAG_EXPLAIN_ONLY))
+			build_hash_tables(aggstate);
+
 		aggstate->table_filled = false;
 
 		/* Initialize this to 1, meaning nothing spilled, yet */