pgsql: Don't add a redundant constraint when detaching a partition

Don't add a redundant constraint when detaching a partition

On ALTER TABLE .. DETACH CONCURRENTLY, we add a new table constraint
that duplicates the partition constraint. But if the partition already
has another constraint that implies that one, then that's unnecessary.
We were already avoiding the addition of a duplicate constraint if there
was an exact 'equal' match -- this just improves the quality of the check.

Author: Justin Pryzby <pryzby(at)telsasoft(dot)com>
Reviewed-by: Álvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>
Discussion: https://postgr.es/m/20210410184226.GY6592@telsasoft.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/7b357cc6ae553c0ecacdc11b2e5278b7bf477dba

Modified Files
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src/backend/commands/tablecmds.c | 61 ++++++++++++++-----------------
src/test/regress/expected/alter_table.out | 20 ++++++++++
src/test/regress/sql/alter_table.sql | 6 +++
3 files changed, 54 insertions(+), 33 deletions(-)