If we have a single hashclause, list_copy(hashclauses) is unnecessary. This adjustment can save the overhead of function calls and memory operation.

Author: tenderwg

src/backend/utils/adt/selfuncs.c

@@ -3798,7 +3798,7 @@ estimate_multivariate_bucketsize(PlannerInfo *root, RelOptInfo *inner,
 								 List *hashclauses,
 								 Selectivity *innerbucketsize)
 {
-	List	   *clauses = list_copy(hashclauses);
+	List	   *clauses;
 	List	   *otherclauses = NIL;
 	double		ndistinct = 1.0;
 
@@ -3810,6 +3810,7 @@ estimate_multivariate_bucketsize(PlannerInfo *root, RelOptInfo *inner,
 		 */
 		return hashclauses;
 
+	clauses = list_copy(hashclauses);
 	while (clauses != NIL)
 	{
 		ListCell   *lc;