Re: Unique index problem - Mailing list pgsql-general

Thank you.

I used the syntax with 2 indexes, it works for me.
But why does NULL != NULL?

------ Original Message ------
From: "Marc Mamin" <M.Mamin@intershop.de>
To: "Sterpu Victor" <victor@caido.ro>
Cc: "PostgreSQL General" <pgsql-general@postgresql.org>; "Andreas Kretschmer" <akretschmer@spamfence.net>; "Scott Marlowe" <scott.marlowe@gmail.com>
Sent: 12/20/2015 11:44:35 PM
Subject: AW: [GENERAL] Unique index problem

____________________________________
 pgsql-general-owner@postgresql.org [pgsql-general-owner@postgresql.org]&quot; im Auftrag von &quot;Scott Marlowe [scott.marlowe@gmail.com]
ndet: Sonntag, 20. Dezember 2015 17:02
Sterpu Victor
PostgreSQL General
eff: Re: [GENERAL] Unique index problem

un, Dec 20, 2015 at 9:00 AM, Scott Marlowe <scott.marlowe@gmail.com> wrote:
 Sun, Dec 20, 2015 at 8:50 AM, Sterpu Victor <victor@caido.ro> wrote:
ello

 I created a unique index that doesn't seem to work when one column is NULL.
 Index is created like this: CREATE UNIQUE INDEX lab_tests_groups_siui_uni ON
 lab_tests_groups_siui(id_lab_tests_siui, id_lab_tests_groups, valid_from,
 id_lab_sample_types);
 Now I can run this insert twice and I will have 2 records in the database
 that seem to violate this index:
 INSERT INTO lab_tests_groups_siui(id_lab_tests_siui, id_lab_tests_groups,
 valid_from) VALUES(463, 9183, '2014-06-01');

 When I create the index like this "CREATE UNIQUE INDEX
 lab_tests_groups_siui_uni ON lab_tests_groups_siui(id_lab_tests_siui,
 id_lab_tests_groups, valid_from);" index works fine.

 I tested this on postgres 9.1.4 and 9.1.9.

 This is normal operation, as one NULL is unique from other NULLS, as
 far as the db is concerned. If you want it to work some other way, you
 need to use a value other than null, or make an index that's something
 like un

Hello,

 CREATE UNIQUE INDEX lab_tests_groups_siui_uni ON lab_tests_groups_siui(id_lab_tests_siui, id_lab_tests_groups, valid_from, id_lab_sample_types);

assuming that only id_lab_sample_types can be null, you could cover this with 2 partial indexes:

CREATE UNIQUE INDEX lab_tests_groups_siui_uni_a ON lab_tests_groups_siui(id_lab_tests_siui, id_lab_tests_groups, valid_from) WHERE (id_lab_sample_types IS NULL);
and
CREATE UNIQUE INDEX lab_tests_groups_siui_uni_b ON lab_tests_groups_siui(id_lab_tests_siui, id_lab_tests_groups, valid_from, id_lab_sample_types) WHERE (id_lab_sample_types IS NOT NULL);

There is a serious caveat though: queries that don't contains a "id_lab_sample_types IS [NOT] NULL" condition will ignore the index.

Maybe there is also a way using DISTINCT(id_lab_tests_siui, id_lab_tests_groups, valid_from, id_lab_sample_types) in the index definition, but I've never tried that and suspect the planner will also have trouble to include such an index in the plan.

regards,

Marc Mamin

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