Re: Speeding up select distinct

>>>>> "Merlin" == Merlin Moncure <merlin(dot)moncure(at)rcsonline(dot)com> writes:

>> Consider this query:
>>
>> SELECT distinct owner from pictures;

Merlin> [...]
>> Any ideas, apart from more or less manually maintaining a list of
>> distinct owners in another table ?

Merlin> you answered your own question. With a 20 row owners table,
Merlin> you should be directing your efforts there group by is
Merlin> faster than distinct, but both are very wasteful and
Merlin> essentially require s full seqscan of the detail table.

Merlin> With a little hacking, you can change 'manual maintenance'
Merlin> to 'automatic maintenance'.

Merlin> 1. create table owner as select distinct owner from
Merlin> pictures; 2. alter table owner add constraint
Merlin> owner_pkey(owner); 3. alter table pictures add constraint
Merlin> ri_picture_owner(owner) references owner; 4. make a little
Merlin> append_ownder function which adds an owner to the owner
Merlin> table if there is not already one there. Inline this to your
Merlin> insert statement on pictures.

I just wished there was a means to fully automate all this and render
it transparent to the user, just like an index.

Merlin> Voila! Merlin p.s. normalize your data always!

I have this:

pictures(
PictureID serial PRIMARY KEY,
Owner integer NOT NULL REFERENCES users,
[...]);
CREATE TABLE users (
UserID serial PRIMARY KEY,
Name character varying(255),
[...]);

Isn't it normalized ?

--
Laurent Martelli
laurent(at)aopsys(dot)com Java Aspect Components
http://www.aopsys.com/ http://jac.objectweb.org