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.