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Date: Mon, 20 Apr 2020 20:31:29 -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 Mon, Apr 20, 2020 at 04:21:32PM -0400, Tom Lane wrote:
> [ sorry, I'd lost track of this thread ]
> 
> Bruce Momjian <bruce@momjian.us> writes:
> > 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.
> 
> Well, that's not the case:
> 
> regression=# create index tenk1_null_index on tenk1((1)) where ten is null;
> CREATE INDEX
> regression=# explain select * from tenk1 where ten is null;
>                                    QUERY PLAN                                   
> --------------------------------------------------------------------------------
>  Index Scan using tenk1_null_index on tenk1  (cost=0.12..8.14 rows=1 width=244)
> (1 row)
> 
> (Maybe it was true at some time in the past, but not any more.)
> 
> Also, it complicates the example, and since you didn't explain the
> reason for the complication, I think it's pretty confusing.
> 
> But really I still don't see the need for these additional examples
> at all.  It's especially weird that what you want to do is have
> some examples on that page have <example> markup and others not.

OK, seems like only you and I care about this issue, which I take to
mean that we should minimize what we are adding here.  What the attached
patch does is to remove the previous commit, and just add a sentence to
the last example to mention the ability restrict a column to a single
NULL.

-- 
  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..13285bb0dd 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,
@@ -953,18 +944,11 @@ 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>

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