pgsql: Have CLUSTER ignore partitions not owned by caller

Have CLUSTER ignore partitions not owned by caller

If a partitioned table has partitions owned by roles other than the
owner of the partitioned table, don't include them in the to-be-
clustered list. This is similar to what VACUUM FULL does (except we do
it sooner, because there is no reason to postpone it). Add a simple
test to verify that only owned partitions are clustered.

While at it, change memory context switch-and-back to occur once per
partition instead of outside of the loop.

Author: Justin Pryzby <pryzby(at)telsasoft(dot)com>
Reviewed-by: Zhihong Yu <zyu(at)yugabyte(dot)com>
Reviewed-by: Michael Paquier <michael(at)paquier(dot)xyz>
Discussion: https://postgr.es/m/20220411140609.GF26620@telsasoft.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/3f19e176ae0f55a653d62e1504dbe5ad8c1006a0

Modified Files
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src/backend/commands/cluster.c | 19 ++++++++++++-------
src/test/regress/expected/cluster.out | 25 +++++++++++++++++++++++++
src/test/regress/sql/cluster.sql | 19 +++++++++++++++++++
3 files changed, 56 insertions(+), 7 deletions(-)