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help / color / mirror / Atom feedFrom: Bruce Momjian <[email protected]>
To: Tuomas Leikola <[email protected]>
Cc: [email protected]
Subject: Re: uniqueness and null could benefit from a hint for dba
Date: Tue, 5 Nov 2019 12:13:06 -0500
Message-ID: <[email protected]> (raw)
In-Reply-To: <CALhc4GA=Bxs-w5Fo9x301-sWbBOcC7eQW=wTEEYUQhA1vA_Chg@mail.gmail.com>
References: <[email protected]>
<[email protected]>
<CALhc4GAq4H9DSeC7tU6Nkr34zykiA4g8WQ+suWmXSXTDApgHEA@mail.gmail.com>
<[email protected]>
<CALhc4GA=Bxs-w5Fo9x301-sWbBOcC7eQW=wTEEYUQhA1vA_Chg@mail.gmail.com>
On Tue, Oct 29, 2019 at 02:00:38PM +0200, Tuomas Leikola wrote:
> On Thu, Oct 24, 2019 at 5:31 PM Bruce Momjian <[email protected]> wrote:
>
> Uh, I am wondering if it is just too details for our docs. Can you
> think of some text and its location?
>
>
>
> "Unique indexes on functions can be used to create special types of
> constraints, like considering unique values equal (coalesce) or only allowing a
> single unique integer value of a float column (floor). A filtered unique index
> only enforces uniqueness on the subset of rows that match the filter."
>
> I guess this would append to the paragraph "When an index is declared unique,
> multiple table rows with equal indexed values are not allowed. Null values are
> not considered equal. A multicolumn unique index will only reject cases where
> all indexed columns are equal in multiple rows.". Links to the mentioned
> keywords and concepts would of course be helpful, but not strictly necessary.
>
> I also thought of mentioning the caveat of (un)scannability of function or
> filtered indexes but maybe that is a bit much.
I have reviewed our documentation and found approrpiate places to
mention your floor() example, and my IS NULL example. Patch attached.
--
Bruce Momjian <[email protected]> http://momjian.us
EnterpriseDB http://enterprisedb.com
+ As you are, so once was I. As I am, so you will be. +
+ Ancient Roman grave inscription +
Attachments:
[text/x-diff] unique.diff (1.5K, 2-unique.diff)
download | inline diff:
diff --git a/doc/src/sgml/indices.sgml b/doc/src/sgml/indices.sgml
new file mode 100644
index 95c0a19..c54bf0d
*** a/doc/src/sgml/indices.sgml
--- b/doc/src/sgml/indices.sgml
*************** CREATE INDEX test1_lower_col1_idx ON tes
*** 706,711 ****
--- 706,720 ----
</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,
as well as rows whose <literal>col1</literal> values are actually identical.
*************** CREATE UNIQUE INDEX tests_success_constr
*** 946,951 ****
--- 955,970 ----
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>
+ </para>
</example>
<para>
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