Re: Faster distinct query?

On Wed, Sep 22, 2021 at 1:21 PM Michael Lewis <mlewis(at)entrata(dot)com> wrote:

> In the future, please share the plan returned by explain analyze, and some
> data about how many rows in the involved tables,
>

I believe we consider it acceptable to link to an explain viewer, which is
what the OP did. Reading explain output in email has its own challenges,
and I'd rather have the website than a text attachment.

> How does the below work? It should do a very simple index scan only, then
> aggregate the relative few rows after the fact.
>
> select station, array_agg(distinct(channel)) as channels
> FROM(
> SELECT station,channel FROM data GROUP BY station,channel
> ) AS sub
> group by station;
>

Yeah, am pondering this too, though seems like the queries should be
identical so the plan/execution should be the same either way.

> If there is correlation between station & channel, then you might look at
> creating a multivariate statistics object and analyzing the table so the
> planner can make better choices
>

There is no where clause so I'm doubtful there is much to be gained going
down this path. The Index-Only scan seems like an optimal way to obtain
this data and the existing query already does that. The aggregation path
might vary though it seems like that shouldn't be the case here.

David J.