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Re: Faster distinct query? - Mailing list pgsql-general

From Geoff Winkless
Subject Re: Faster distinct query?
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Msg-id CAEzk6fdsP_CfOe3bD3otw+J0rXBVQ2-Z+z_rKwnwbtpo9Fu_aQ@mail.gmail.com
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In response to Faster distinct query?  (Israel Brewster <ijbrewster@alaska.edu>)
Responses Re: Faster distinct query?  (Israel Brewster <ijbrewster@alaska.edu>)
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On Wed, 22 Sept 2021 at 21:05, Israel Brewster <ijbrewster@alaska.edu> wrote:
I was wondering if there was any way to improve the performance of this query:

SELECT station,array_agg(distinct(channel)) as channels FROM data GROUP BY station;

If you have tables of possible stations and channels (and if not, why not?), then an EXISTS query, something like

SELECT stations.name, ARRAY_AGG(channels.name)
  FROM stations, channels
  WHERE EXISTS
   (SELECT FROM data WHERE data.channels=channels.name AND data.station=stations.name)
GROUP BY stations.name

will usually be much faster, because it can stop scanning after the first match in the index.

Geoff

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