Re: How slow is DISTINCT? - Mailing list pgsql-sql

Josh Berkus wrote:
> Wei Wang,
> 
> 
>>How exactly slow is DISTINCT being processed in SQL engines? (not
>>limited to postgresql, though comments on postgresql would be most
>> relevant)
>>
> 
> I can only give you a relative result, based exlusively on my anecdotal
>  experience with 7.1:
> 
> Fast:  SELECT ...
> Slower: SELECT ... GROUP BY x,y,z
>      or:  SELECT DISCTINCT ON (x) ... (Postgres non-standard extension)
> SLowest: SELECT DISTINCT ...
> 
> The reason for this is that SELECT DISTINCT is effectively a GROUP BY
>  on all result fields of the query, and if a few of the aren't indexed
>  that requires a seq scan.
What if I do thing like

SELECT DISTINCT table1.tid, table1.name, table1.description FROM ...

Does it equal to the scenario 2 or 3?

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?

Thanks

-- 
Wei Weng
Network Software Engineer
KenCast Inc.