Re: uniqueness and null could benefit from a hint for dba

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@momjian.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@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

+ As you are, so once was I.  As I am, so you will be. +
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