Re: How slow is DISTINCT?

Josh Berkus wrote:
> Wei,
>
>
>>SELECT DISTINCT table1.tid, table1.name, table1.description FROM ...
>>
>>Does it equal to the scenario 2 or 3?
>>
>
> Three.
>
>
>>I am thinking SELECT DISTINCT table1.tid is just a variation (or the
>> other way around) of SELECT DISTINCT ON (table1.tid), is that right?
>>
>
> Wrong. SELECT DISTINCT ON table1.tid takes the table1.tid field and
> the first related other data it can find. SELECT DISTINCT looks for
> the unique combinaiton of all fields selected. In a multi-table, many
> field query, the former is faster than the latter bceause less data is
> being checked for uniqueness, and extra data is simply discarded.
>
> Tom, please correct me if I'm totally out on a limb, here.

Is that(DISTINCT ON) SQL standard compliant or a Postgresql extension?

--
Wei Weng
Network Software Engineer
KenCast Inc.