From: Bruce Momjian <bruce@momjian.us>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-committers@lists.postgresql.org
Subject: Re: pgsql: doc: add examples of creative use of unique expression indexes
Date: Fri, 10 Apr 2020 20:17:09 -0400
Message-ID: <20200411001709.GD24988@momjian.us> (raw)
In-Reply-To: <13031.1586560889@sss.pgh.pa.us>
References: <E1ikvbp-0005jW-E9@gemulon.postgresql.org>
<2304.1586532634@sss.pgh.pa.us>
<20200410214821.GC24988@momjian.us>
<13031.1586560889@sss.pgh.pa.us>
On Fri, Apr 10, 2020 at 07:21:29PM -0400, Tom Lane wrote:
> Bruce Momjian <bruce@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.
--
Bruce Momjian <bruce@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 +
Attachments:index.diff (text/x-diff, 1.8K) downloaddiff --git a/doc/src/sgml/indices.sgml b/doc/src/sgml/indices.sgmlindex 1be209a2fe..e8205a07b5 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 +947,17 @@ 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;+CREATE UNIQUE INDEX tests_target_one_null ON tests ((true)) WHERE target IS NULL;
</programlisting>
- </para>- </example>+ </para>
<para>
Finally, a partial index can also be used to override the system's
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To: pgsql-committers@postgresql.org
Cc: bruce@momjian.us, tgl@sss.pgh.pa.us, pgsql-committers@lists.postgresql.org
Subject: Re: pgsql: doc: add examples of creative use of unique expression indexes
In-Reply-To: <20200411001709.GD24988@momjian.us>
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