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doc: add examples of creative use of unique expression indexes

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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
This commit is contained in:
Bruce Momjian 2019-12-27 14:49:08 -05:00
parent 5d72f85d4c
commit 2034bcc845
1 changed files with 19 additions and 0 deletions
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19
doc/src/sgml/indices.sgml
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@ -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>