On Fri, Apr 10, 2020 at 08:17:09PM -0400, Bruce Momjian wrote:
> On Fri, Apr 10, 2020 at 07:21:29PM -0400, Tom Lane wrote:
> > Bruce Momjian <br...@momjian.us> writes:
> > > I agree with your analysis. I still want to have some mention that
> > > partial indexes can be used to create single-NULL columns, which might
> > > be required for compatibility with other databases. Attached is an
> > > updated patch which removes the previous commit but adds a mention of
> > > this.
> >
> > The single-null thing is probably a useful example, but please make
> > it an actual separate example, or at least its own para outside the
> > existing <example> sections.
> >
> > Also, the existing example demonstrating that seems overcomplicated;
> > why not just
> >
> > create unique index ... (1) where (foo is null);
>
> I ended up using "true" since that is ony one byte; patch attached.
[ thread moved to docs]
I now remember that I wrote the first IS NULL in:
CREATE UNIQUE INDEX tests_target_one_null ON tests ((target IS NULL)) WHERE
target IS NULL;
in hope that if someone is looking for the null value in the column, the
IS NULL would allow the index to be used to find it, while 1 or true
would not.
Also, I think the most popular use for this ability would be for
multi-column indexes where you want only one NULL value for a
combination of columns, e.g.:
CREATE UNIQUE INDEX tests_target_one_null ON test (x, (y IS NULL)) WHERE y IS
NULL;
I have added that. It also hows the use of columns and expressions in
the same index. Proposed patch attached.
--
Bruce Momjian <br...@momjian.us> https://momjian.us
EnterpriseDB https://enterprisedb.com
+ As you are, so once was I. As I am, so you will be. +
+ Ancient Roman grave inscription +
diff --git a/doc/src/sgml/indices.sgml b/doc/src/sgml/indices.sgml
index 1be209a2fe..2790fb2a89 100644
--- a/doc/src/sgml/indices.sgml
+++ b/doc/src/sgml/indices.sgml
@@ -705,15 +705,6 @@ 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</literal>, it would prevent
creation of rows whose <literal>col1</literal> values differ only in case,
@@ -956,16 +947,22 @@ CREATE UNIQUE INDEX tests_success_constraint ON tests (subject, target)
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>:
+ </example>
+
+ <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>
+ This index allows only one null value for each value in the first
+ referenced column:
+<programlisting>
+CREATE UNIQUE INDEX tests_combo_one_null ON tests (col1, (target IS NULL)) WHERE target IS NULL;
+</programlisting>
+ </para>
<para>
Finally, a partial index can also be used to override the system's
