Re: LIMIT 1 poor query plan

Yes, the composite index nailed it.

Thanks mate :)

On Sat, Oct 10, 2015 at 12:45 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Marcio Ribeiro <mribeiro(at)gmail(dot)com> writes:
> > Short question:
> > Why would pg optimizer choose a worst (slower) query plan for a
> > query with 'LIMIT 1' instead of, say, 'LIMIT 3'?
>
> > Complete scenario:
> > Query: 'SELECT * FROM a WHERE a.b_id = 42 ORDER BY created LIMIT 1'
> > - b_id is a FK to b;
> > - created is a datetime with the time of the creation of the row;
> > - both 'b' and 'created' are indexed separately
>
> > This query, with the LIMIT 1, uses the index on created, which is much
> > slower (10x) than if it used the index on b_id
>
> It's trying to avoid a sort; or to be less anthropomorphic, the estimated
> cost of scanning the "created" index until it hits the first row with
> b_id=42 is less than the estimated cost of collecting all the rows with
> b_id=42 and then sorting them by "created". The estimates unfortunately
> are kind of shaky because it's hard to predict how many rows will get
> skipped before finding one with b_id=42.
>
> If you do this type of query often enough to care about its performance,
> you could consider creating a two-column index on (b_id, created)
> (in that order).
>
> regards, tom lane
>

--
Marcio Ribeiro