Re: How slow is DISTINCT?

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 andthe first related other data it can find.  SELECT
DISTINCTlooks forthe unique combinaiton of all fields selected.  In a multi-table, manyfield query, the former is
fasterthan the latter bceause less data isbeing checked for uniqueness, and extra data is simply discarded.
 

Tom, please correct me if I'm totally out on a limb, here.

-Josh

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