From: | Bruce Momjian <bruce(at)momjian(dot)us> |
---|---|
To: | Tuomas Leikola <tuomas(dot)leikola(at)gmail(dot)com> |
Cc: | pgsql-docs(at)lists(dot)postgresql(dot)org |
Subject: | Re: uniqueness and null could benefit from a hint for dba |
Date: | 2019-11-05 17:13:06 |
Message-ID: | 20191105171306.GA5839@momjian.us |
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Lists: | pgsql-docs |
On Tue, Oct 29, 2019 at 02:00:38PM +0200, Tuomas Leikola wrote:
> On Thu, Oct 24, 2019 at 5:31 PM Bruce Momjian <bruce(at)momjian(dot)us> 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 <bruce(at)momjian(dot)us> 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 +
Attachment | Content-Type | Size |
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unique.diff | text/x-diff | 1.5 KB |
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