pgsql: On attach, consider skipping validation of subpartitions individ

On attach, consider skipping validation of subpartitions individually.

If the table attached as a partition is itself partitioned, individual
partitions might have constraints strong enough to skip scanning the
table even if the table actually attached does not. This is pretty
cheap to check, and possibly a big win if it works out.

Amit Langote, with test case changes by me.

Discussion: http://postgr.es/m/1f08b844-0078-aa8d-452e-7af3bf77d05f@lab.ntt.co.jp

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/14f67a8ee282ebc0de78e773fbd597f460ab4a54

Modified Files
--------------
src/backend/commands/tablecmds.c | 15 +++++++++++++++
src/test/regress/expected/alter_table.out | 14 ++++++++++++++
src/test/regress/sql/alter_table.sql | 14 ++++++++++++++
3 files changed, 43 insertions(+)