Re: Faster distinct query?

On Sep 23, 2021, at 10:36 AM, Geoff Winkless <pgsqladmin@geoff.dj> wrote:

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