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| -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> |