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| author | Bruce Momjian <bruce@momjian.us> | 2019-12-27 14:49:08 -0500 |
|---|---|---|
| committer | Bruce Momjian <bruce@momjian.us> | 2019-12-27 14:49:08 -0500 |
| commit | a9760d0f3cb523336b5fdd9d6c5985e39a8588a1 (patch) | |
| tree | 65a338789f24bb5104da2b4c7ecfdcfc5e2334b1 /doc/src | |
| parent | 650692a18d29c44c8019545c02ba42c3efd2c964 (diff) | |
| download | postgresql-a9760d0f3cb523336b5fdd9d6c5985e39a8588a1.tar.gz postgresql-a9760d0f3cb523336b5fdd9d6c5985e39a8588a1.zip | |
doc: add examples of creative use of unique expression indexes
Unique expression indexes can constrain data in creative ways, so show
two examples.
Reported-by: Tuomas Leikola
Discussion: https://postgr.es/m/156760275564.1127.12321702656456074572@wrigleys.postgresql.org
Backpatch-through: 9.4
Diffstat (limited to 'doc/src')
| -rw-r--r-- | doc/src/sgml/indices.sgml | 19 |
1 files changed, 19 insertions, 0 deletions
diff --git a/doc/src/sgml/indices.sgml b/doc/src/sgml/indices.sgml index 95c0a1926c5..c54bf0dbbdb 100644 --- a/doc/src/sgml/indices.sgml +++ b/doc/src/sgml/indices.sgml @@ -706,6 +706,15 @@ CREATE INDEX test1_lower_col1_idx ON test1 (lower(col1)); </para> <para> + 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 <type>double precision</type>-typed column: +<programlisting> +CREATE UNIQUE INDEX test1_uniq_int ON tests ((floor(double_col))) +</programlisting> + </para> + + <para> If we were to declare this index <literal>UNIQUE</literal>, it would prevent creation of rows whose <literal>col1</literal> values differ only in case, as well as rows whose <literal>col1</literal> values are actually identical. @@ -946,6 +955,16 @@ CREATE UNIQUE INDEX tests_success_constraint ON tests (subject, target) This is a particularly efficient approach when there are few successful tests and many unsuccessful ones. </para> + + <para> + 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 <literal>true</literal> instead + of <literal>null</literal>: +<programlisting> +CREATE UNIQUE INDEX tests_target_one_null ON tests ((target IS NULL)) WHERE target IS NULL; +</programlisting> + </para> </example> <para> |