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doc: Simplify mention of unique indexes for NULL control 

Discussion: https://postgr.es/m/2304.1586532634@sss.pgh.pa.us

Backpatch-through: 9.5
This commit is contained in:
Bruce Momjian 2020-05-21 19:49:30 -04:00
parent 8de137017a
commit c7c0db90c2
1 changed files with 3 additions and 19 deletions
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22
doc/src/sgml/indices.sgml
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@ -702,15 +702,6 @@ 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</>, it would prevent
creation of rows whose <literal>col1</> values differ only in case,
@ -948,18 +939,11 @@ CREATE UNIQUE INDEX tests_success_constraint ON tests (subject, target)
WHERE success;
</programlisting>
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 <literal>IS NULL</literal> restriction.
</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>