Re: Death postgres

On Thu, May 11, 2023 at 1:56 AM Peter J. Holzer <hjp-pgsql(at)hjp(dot)at> wrote:

> On 2023-05-10 22:52:47 +0200, Marc Millas wrote:
> > On Wed, May 10, 2023 at 7:24 PM Peter J. Holzer <hjp-pgsql(at)hjp(dot)at>
> wrote:
> >
> > On 2023-05-10 16:35:04 +0200, Marc Millas wrote:
> > > Unique (cost=72377463163.02..201012533981.80 rows=1021522829864
> width=
> > 97)
> > > -> Gather Merge (cost=72377463163.02..195904919832.48 rows=
> > 1021522829864 width=97)
> > ...
> > > -> Parallel Hash Left Join (cost=
> > 604502.76..1276224253.51 rows=204304565973 width=97)
> > > Hash Cond: ((t1.col_ano)::text =
> (t2.col_ano)::text)
> > ...
> > >
> > > //so.. the planner guess that those 2 join will generate 1000
> billions
> > rows...
> >
> > Are some of the col_ano values very frequent? If say the value 42
> occurs
> > 1 million times in both table_a and table_b, the join will create 1
> > trillion rows for that value alone. That doesn't explain the crash
> or the
> > disk usage, but it would explain the crazy cost (and would probably
> be a
> > hint that this query is unlikely to finish in any reasonable time).
> >
> >
> > good guess, even if a bit surprising: there is one (and only one)
> "value" which
> > fit your supposition: NULL
>
> But NULL doesn't equal NULL, so that would result in only one row in the
> left join. So that's not it.
>

if so... how ???

>
> hp
>
> --
> _ | Peter J. Holzer | Story must make more sense than reality.
> |_|_) | |
> | | | hjp(at)hjp(dot)at | -- Charles Stross, "Creative writing
> __/ | http://www.hjp.at/ | challenge!"
>