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Date: Sat, 11 Apr 2020 08:25:32 -0400
From: Bruce Momjian <bruce@momjian.us>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: PostgreSQL-documentation <pgsql-docs@postgresql.org>
Subject: Re: pgsql: doc: add examples of creative use of unique expression
 indexes
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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 <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.

[ 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  <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 +

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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

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