index 95c0a1926c5a6dd47581470f18d579608190254a..c54bf0dbbdb8c472aa266387c5939ef77ee44da7 100644 (file)
@@ -705,6 +705,15 @@ CREATE INDEX test1_lower_col1_idx ON test1 (lower(col1));
</programlisting>
</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,
@@ -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>