From 7b1053a577494871acb5e91c63abb2bade5b8dea Mon Sep 17 00:00:00 2001 From: =?utf8?q?=C3=81lvaro=20Herrera?= Date: Mon, 4 Aug 2025 13:26:45 +0200 Subject: [PATCH] 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 Reviewed-by: David G. Johnston Discussion: https://postgr.es/m/175385113607.786.16774570234342968908@wrigleys.postgresql.org --- doc/src/sgml/ref/alter_table.sgml | 7 ++++--- 1 file changed, 4 insertions(+), 3 deletions(-) diff --git a/doc/src/sgml/ref/alter_table.sgml b/doc/src/sgml/ref/alter_table.sgml index d1696991683..3bac4db7422 100644 --- a/doc/src/sgml/ref/alter_table.sgml +++ b/doc/src/sgml/ref/alter_table.sgml @@ -240,9 +240,10 @@ WITH ( MODULUS numeric_literal, REM provided none of the records in the table contain a NULL value for the column. Ordinarily this is checked during the ALTER TABLE by scanning the - entire table; however, if a valid CHECK constraint is - found which proves no NULL can exist, then the - table scan is skipped. + entire table, unless NOT VALID is specified; + however, if a valid CHECK constraint exists + (and is not dropped in the same command) which proves no + NULL can exist, then the table scan is skipped. If a column has an invalid not-null constraint, SET NOT NULL validates it. -- 2.47.2