pgsql: Avoid failure to open dropped detached partition

Avoid failure to open dropped detached partition

When a partition is detached and immediately dropped, a prepared
statement could try to compute a new partition descriptor that includes
it. This leads to this kind of error:
ERROR: could not open relation with OID 457639

Avoid this by skipping the partition in expand_partitioned_rtentry if it
doesn't exist.

Noted by me while investigating bug #18559. Kuntal Gosh helped to
identify the exact failure.

Backpatch to 14, where DETACH CONCURRENTLY was introduced.

Author: Álvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>
Reviewed-by: Kuntal Ghosh <kuntalghosh(dot)2007(at)gmail(dot)com>
Reviewed-by: Junwang Zhao <zhjwpku(at)gmail(dot)com>
Discussion: https://postgr.es/m/202408122233.bo4adt3vh5bi@alvherre.pgsql

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/52f3de874bbeaf16b3ae2efb505c1117be8355cc

Modified Files
--------------
src/backend/optimizer/util/inherit.c | 13 +++++++++++--
1 file changed, 11 insertions(+), 2 deletions(-)