Ensure that ExecPrepareExprList's result is all in one memory context.
Ensure that ExecPrepareExprList's result is all in one memory context.

Noted by Amit Langote.

Discussion: 0f6c3b6e01e7abe75a16f49385f20fa7db6fd922@lab.ntt.co.jp

lisakowen/gpdb,greenplum-db/gpdb,lisakowen/gpdb,adam8157/gpdb,adam8157/gpdb,50wu/gpdb,greenplum-db/gpdb,lisakowen/gpdb,xinzweb/gpdb,lisakowen/gpdb,xinzweb/gpdb,adam8157/gpdb,xinzweb/gpdb,lisakowen/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,xinzweb/gpdb,xinzweb/gpdb,50wu/gpdb,greenplum-db/gpdb,50wu/gpdb,adam8157/gpdb,greenplum-db/gpdb,adam8157/gpdb,xinzweb/gpdb,50wu/gpdb,50wu/gpdb,xinzweb/gpdb,50wu/gpdb,lisakowen/gpdb,50wu/gpdb,greenplum-db/gpdb,50wu/gpdb,lisakowen/gpdb,adam8157/gpdb,xinzweb/gpdb,greenplum-db/gpdb,lisakowen/gpdb,adam8157/gpdb,adam8157/gpdb