Re: [SQL] Unique indexes not unique?

[Tomasz Myrta]

Jimmy Mäkelä wrote:

I found that Postgres isn't behaving like I thought when using a unique index in
combination with NULL-values...
Is this a bug or specified in the SQL-standard? If its a bug, is it fixed in a
recent version? We are using 7.2.3

This is the results I got:

intranet=# create table foo (a varchar(10), b varchar(10));
CREATE
intranet=# create unique index foo_idx on foo using btree(a, b);
CREATE
intranet=# insert into "foo" (a, b) values ('apa', 'banan');
INSERT 26229704 1
intranet=# insert into "foo" (a, b) values ('apa', 'banan');
ERROR: Cannot insert a duplicate key into unique index foo_idx
intranet=# insert into "foo" (a, b) values ('apa', null);
INSERT 26229706 1
intranet=# insert into "foo" (a, b) values ('apa', null);
INSERT 26229707 1

I'm not sure unique index works properly for null values. I can't explain, why. Maybe it comes from SQL standard - null i a special value and can't be compared using default operators to other non null values:
1>null =null
1<null =null
1=null =null

And another completely unrelated question... I have got a table with a composite
index on A andBb and an index on A
which I query with something like this:

SELECT * FROM "table"
WHERE (a = 1 OR a = 2 OR a = 3) AND b > 1232132 AND b < 123123123213123

Postgres then chooses to use the index for A three times, which is really slow
on my table...
Then I rewrote the query like:

SELECT * FROM "table"
WHERE a = 1 AND b > 1232132 AND b < 123123123213123
UNION SELECT * FROM "table"
WHERE a = 2 AND b > 1232132 AND b < 123123123213123
UNION SELECT * FROM "table"
WHERE a = 3 AND b > 1232132 AND b < 123123123213123

Try to rewrite your query to show postgres how to use index on AB:
SELECT * FROM "table"
WHERE
(a = 1 AND b > 1232132 AND b < 123123123213123) or
(a = 2 AND b > 1232132 AND b < 123123123213123) or
(a = 3 AND b > 1232132 AND b < 123123123213123);

Regards,
Tomasz Myrta




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