doc: mention unusability of dropped CHECK to verify NOT NULL
It's possible to use a CHECK (col IS NOT NULL) constraint to skip
scanning a table for nulls when adding a NOT NULL constraint on the same
column. However, if the CHECK constraint is dropped on the same command
that the NOT NULL is added, this fails, i.e., makes the NOT NULL addition
slow. The best we can do about it at this stage is to document this so
that users aren't taken by surprise.
(In Postgres 18 you can directly add the NOT NULL constraint as NOT
VALID instead, so there's no longer much use for the CHECK constraint,
therefore no point in building mechanism to support the case better.)
Reported-by: Andrew <psy2000usa@yahoo.com>
Reviewed-by: David G. Johnston <david.g.johnston@gmail.com>
Discussion: https://postgr.es/m/175385113607.786.16774570234342968908@wrigleys.postgresql.org
Branch
------
REL_15_STABLE
Details
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https://git.postgresql.org/pg/commitdiff/2f600115a5a95e94f4085a5114ace0116314c4a4
Modified Files
--------------
doc/src/sgml/ref/alter_table.sgml | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)
