On Thu, Dec 03, 2009 at 03:56:05PM +0000, Thomas Hamilton wrote: > > The following bug has been logged online: > > Bug reference: 5231 > Logged by: Thomas Hamilton > Email address: thomashamilto...@yahoo.com > PostgreSQL version: 8.3.8 > Operating system: Ubuntu 4.2.4 > Description: SELECT DISTINCT poorly implemented vs SELECT ... GROUP > BY > Details: > > SELECT DISTINCT does a Sort followed by Unique. > > SELECT ... GROUP BY, which is logically equivalent, performs a > HashAggregate. > > When run against a large dataset with a small number of distinct results > HashAggregate is an order of magnitude more efficient! > > Since the spec does not require DISTINCT to return sorted results, I don't > believe Sort ... Unique will ever be more efficient than HashAggregate. > > Therefore, in order to maximize performance, DISTINCT should always be > implemented as HashAggregate.
In 8.4 and above, SELECT DISTINCT can use HashAggregates rather than Sort/Unique -- though I'm not sure it always does. Anyway, an upgrade should handle this for you. However, this isn't really a bug, it's a feature, so don't expect to see it in an 8.3.x version. -- Josh / eggyknap
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