On Mon, Apr 20, 2020 at 04:21:32PM -0400, Tom Lane wrote:
> [ sorry, I'd lost track of this thread ]
>
> Bruce Momjian <[email protected]> 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 <[email protected]> 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..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>
