[CF 5704] v3 - adjust the place of list_copy() call to reduce the overhead of function call and memory copies.

This branch was automatically generated by a robot using patches from an
email thread registered at:

https://commitfest.postgresql.org/patch/5704

The branch will be overwritten each time a new patch version is posted to
the thread, and also periodically to check for bitrot caused by changes
on the master branch.

Patch(es): https://www.postgresql.org/message-id/CAHewXNnHBOO9NEa=NBDYOrwZL4oHu2NOcTYvqyNyWEswo8f5OQ@mail.gmail.com
Author(s): Tender Wang

Author: Commitfest Bot

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;