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Date: Fri, 10 Apr 2020 17:48:21 -0400
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
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On Fri, Apr 10, 2020 at 11:30:34AM -0400, Tom Lane wrote:
> Bruce Momjian <bruce@momjian.us> writes:
> > doc:  add examples of creative use of unique expression indexes
> > https://git.postgresql.org/pg/commitdiff/a9760d0f3cb523336b5fdd9d6c5985e39a8588a1
> 
> We had a complaint [1] that this dropped an example into the middle of
> two related paragraphs.  I agree with that objection, and also notice
> that the extra example broke subsequent references to the "first example"
> and "second example".  I'm also unhappy that the other addition that this
> commit made was dropped inside Example 11.3; if we're going to use
> <example> markup at all, each one ought to be a coherent entity.
> 
> On top of that, I don't find that either example actually adds anything
> to the discussion, as the same points are being made in the existing
> text.  Therefore, I don't think it's worth trying to fix these problems,
> and propose just reverting this patch.
> 
> 			regards, tom lane
> 
> [1] https://www.postgresql.org/message-id/158648685043.655.3074746555320970574%40wrigleys.postgresql.org

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.

-- 
  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..3a8f93bac7 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,
@@ -953,17 +944,9 @@ 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.
-   </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>
+    successful tests and many unsuccessful ones.  Creating a unique
+    index with an <literal>IS NULL</literal> qualification can restrict
+    a column to a single <literal>NULL</literal> value.
    </para>
   </example>
 

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