Re: explain analyze output: 0 rows, 1M loops

On Wed, Nov 1, 2017 at 12:25 PM, Justin Pryzby <pryzby(at)telsasoft(dot)com> wrote:

> On Wed, Nov 01, 2017 at 12:19:21PM -0700, David G. Johnston wrote:
> > On Wed, Nov 1, 2017 at 11:59 AM, Scott Marlowe <scott(dot)marlowe(at)gmail(dot)com>
> > wrote:
> >
> > > So some of my output from an explain analyze here has a line that says
> > > this:
> > >
> > > ex Scan using warranty_order_item_warranty_order_id_idx on
> > > warranty_order_item woi_1 (cost=0.57..277.53 rows=6 width=137) (actual
> > > time=0.110..0.111 rows=0 loops=1,010,844)
> > >
> >
> > Not my strong suit but, I'm pretty sure that reads: "The index was
> queried
> > 1M+ times and none of those inqueries resulted in a record being found".
> > IIUC I'd be wondering why some form of hash join wasn't used...
>
> Except that:
>
> https://www.postgresql.org/docs/current/static/using-explain.html
> "... the loops value reports the total number of executions of the node,
> and
> the actual time and ROWS VALUES SHOWN ARE AVERAGES PER-EXECUTION."
>

I seem to recall a somewhat recent commit that dealt with this. The
problem is that with 1M loops a small number of rows returned will be
indistinguishable from zero when computed as an average within finite
precision.

Seeing entire plans, and not just a single line of one, tends to help too.

David J.