pgsql: Allow ALTER TABLE .. SET NOT NULL to skip provably unnecessarys

Allow ALTER TABLE .. SET NOT NULL to skip provably unnecessary scans.

If existing CHECK or NOT NULL constraints preclude the presence
of nulls, we need not look to see whether any are present.

Sergei Kornilov, reviewed by Stephen Frost, Ildar Musin, David Rowley,
and by me.

Discussion: http://postgr.es/m/81911511895540@web58j.yandex.ru

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/bbb96c3704c041d139181c6601e5bc770e045d26

Modified Files
--------------
doc/src/sgml/ref/alter_table.sgml         |  13 +++-
src/backend/commands/tablecmds.c          | 111 ++++++++++++++++++++++++------
src/test/regress/expected/alter_table.out |  54 +++++++++++++++
src/test/regress/sql/alter_table.sql      |  40 +++++++++++
4 files changed, 195 insertions(+), 23 deletions(-)