Unique index problem

On Sun, Dec 20, 2015 at 8:50 AM, Sterpu Victor <victor@caido.ro> wrote:

Hello

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

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On Sun, Dec 20, 2015 at 9:00 AM, Scott Marlowe <scott.marlowe@gmail.com> wrote:

On Sun, Dec 20, 2015 at 8:50 AM, Sterpu Victor <victor@caido.ro> wrote:

Hello

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

dangit, stupid gmail sent early. anyway.

you'd have to make an index like unique index on (x,y,z) where field
is [not] null or something like that.

Basically NULL <> NULL <> a particular value.

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Sterpu Victor <victor@caido.ro> wrote:

Hello

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.

Thank you.

NULL isn't a compareable value, because of that you can't compare NULL
with NULL and so you can insert more than one NULL into the table.

You can set the column as NOT NULL or you can do something like this:

-- that's your problem
test=# create table foo (a int, b int, c int);
CREATE TABLE
test=*# create unique index on foo(a,b,c);
CREATE INDEX
test=*# insert into foo (a,b) values (1,2);
INSERT 0 1
test=*# insert into foo (a,b) values (1,2);
INSERT 0 1
test=*# rollback;
ROLLBACK

-- that's a solution
test=# create table foo (a int, b int, c int);
CREATE TABLE
test=*# create unique index on foo(coalesce(a::text,'NULL'),
coalesce(b::text,'NULL'), coalesce(c::text,'NULL'));
CREATE INDEX
test=*# insert into foo (a,b) values (1,2);
INSERT 0 1
test=*# insert into foo (a,b) values (1,2);
ERROR: duplicate key value violates unique constraint
"foo_coalesce_coalesce1_coalesce2_idx"
DETAIL: Key ((COALESCE(a::text, 'NULL'::text)), (COALESCE(b::text,
'NULL'::text)), (COALESCE(c::text, 'NULL'::text)))=(1, 2, NULL) already
exists.
test=*#

Maybe there are better solutions, it's a quick hack ;-)

DISCLAIMER:

That's a public mailing list ...

Andreas
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Andreas Kretschmer <akretschmer@spamfence.net> wrote:

Maybe there are better solutions, it's a quick hack ;-)

better solution:

test=*# create unique index on foo(a,b,c) where a is not null and b is
not null and c is not null;
CREATE INDEX

(partial index)

Andreas
--
Really, I'm not out to destroy Microsoft. That will just be a completely
unintentional side effect. (Linus Torvalds)
"If I was god, I would recompile penguin with --enable-fly." (unknown)
Kaufbach, Saxony, Germany, Europe. N 51.05082�, E 13.56889�

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Andreas Kretschmer <akretschmer@spamfence.net> wrote:

Andreas Kretschmer <akretschmer@spamfence.net> wrote:

Maybe there are better solutions, it's a quick hack ;-)

better solution:

sorry, doesn't work =:(

Andreas
--
Really, I'm not out to destroy Microsoft. That will just be a completely
unintentional side effect. (Linus Torvalds)
"If I was god, I would recompile penguin with --enable-fly." (unknown)
Kaufbach, Saxony, Germany, Europe. N 51.05082�, E 13.56889�

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____________________________________
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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On 12/20/15 10:18 AM, Andreas Kretschmer wrote:

test=*# create unique index on foo(a,b,c) where a is not null and b is
not null and c is not null;
CREATE INDEX

As you discovered, you'd have to build separate indexes for each of the
nullable fields:

UNIQUE ON (a,b) WHERE c IS NULL
a,c WHERE b IS NULL
b,c WHERE a IS NULL
a WHERE b IS NULL AND c IS NULL
....

That's doable for 3 fields, but obviously gets out of hand really quickly.

I wonder if it would be possible to construct an operator family
(class?) that would accept 2 rows( ie: ROW(a,b,c) ) and treat NULLs as
single values...
--
Jim Nasby, Data Architect, Blue Treble Consulting, Austin TX
Experts in Analytics, Data Architecture and PostgreSQL
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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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2015-12-21 7:39 GMT+01:00 Sterpu Victor <victor@caido.ro>:

Thank you.

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

because it was designed

http://www.w3schools.com/sql/sql_null_values.asp

Pavel

Seems to be working also with a single index, like this:
CREATE UNIQUE INDEX lab_tests_groups_siui_uni_c ON
lab_tests_groups_siui(id_lab_tests_siui, id_lab_tests_groups,
valid_from, coalesce(id_lab_sample_types, 0));

------ Original Message ------
From: "Pavel Stehule" <pavel.stehule@gmail.com>
To: "Sterpu Victor" <victor@caido.ro>
Cc: "Marc Mamin" <M.Mamin@intershop.de>; "PostgreSQL General"
<pgsql-general@postgresql.org>; "Andreas Kretschmer"
<akretschmer@spamfence.net>; "Scott Marlowe" <scott.marlowe@gmail.com>
Sent: 12/21/2015 8:44:14 AM
Subject: Re: [GENERAL] Unique index problem

On Sun, Dec 20, 2015 at 11:39 PM, Sterpu Victor <victor@caido.ro> wrote:

Thank you.

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

Because NULL literally means "an unknown, possibly unknowable value."

You need to stop thinking of NULL as A value. It is not.

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On Mon, Dec 21, 2015 at 1:00 PM, Scott Marlowe <scott.marlowe@gmail.com>
wrote:

On Sun, Dec 20, 2015 at 11:39 PM, Sterpu Victor <victor@caido.ro> wrote:

Thank you.

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

Because NULL literally means "an unknown, possibly unknowable value."

You need to stop thinking of NULL as A value. It is not.

​Right. It would be sort of like asking: "is an object which I can't see
the same as (==) or different from (!=) some other object which I can't
see?" Answer: "How would I know???"​

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