From 388d7f8c62b407005f56b6262a678d9fb96ea18c Mon Sep 17 00:00:00 2001 From: Bruce Momjian Date: Thu, 21 May 2020 19:49:30 -0400 Subject: [PATCH] doc: Simplify mention of unique indexes for NULL control Discussion: https://postgr.es/m/2304.1586532634@sss.pgh.pa.us Backpatch-through: 9.5 --- doc/src/sgml/indices.sgml | 22 +++------------------- 1 file changed, 3 insertions(+), 19 deletions(-) diff --git a/doc/src/sgml/indices.sgml b/doc/src/sgml/indices.sgml index 86539a781c..56fd0a3d3d 100644 --- a/doc/src/sgml/indices.sgml +++ b/doc/src/sgml/indices.sgml @@ -705,15 +705,6 @@ CREATE INDEX test1_lower_col1_idx ON test1 (lower(col1)); - - Expression indexes also allow control over the scope of unique indexes. - For example, this unique index prevents duplicate integer values from - being stored in a double precision-typed column: - -CREATE UNIQUE INDEX test1_uniq_int ON tests ((floor(double_col))) - - - If we were to declare this index UNIQUE, it would prevent creation of rows whose col1 values differ only in case, @@ -953,18 +944,11 @@ CREATE UNIQUE INDEX tests_success_constraint ON tests (subject, target) WHERE success; This is a particularly efficient approach when there are few - successful tests and many unsuccessful ones. + successful tests and many unsuccessful ones. It is also possible to + allow only one null in a column by creating a unique partial index + with an IS NULL restriction. - - This index allows only one null in the indexed column by using a - partial index clause to process only null column values, and using - an expression index clause to index true instead - of null: - -CREATE UNIQUE INDEX tests_target_one_null ON tests ((target IS NULL)) WHERE target IS NULL; - -