Re: n_mod_since_analyze isn't reset at table truncation

On Fri, Mar 05, 2021 at 03:31:33PM +0900, Fujii Masao wrote:
>
> On 2021/03/04 12:40, Julien Rouhaud wrote:
> > > In that case, conversely, we want to trigger autoanalyze ASAP because the contents in the table was changed very much?
> >
> > We might want, but wouldn't keeping the current n_mod_since_analyze would make
> > things unpredictable?
>
> I don't think so. autoanalyze still works based on the number of
> modifications since last analyze, so ISTM that's predictable...

If we keep n_mod_since_analyze, autoanalyze might trigger after the first write
or might wait for a full cycle of writes, depending on the kept value. So yes
it can make autoanalyze triggers earlier in some cases, but that's not
predictable from the TRUNCATE even point of view.

> > Also the selectivity estimation functions already take into account the actual
> > relation size, so the estimates aren't completely off after a truncate.
>
> But the statistics is out of date and which may affect the planning badly?
> Also if generic plan is used, it's not updated until next analyze, i.e.,
> generic plan created based on old statistics is used for a while.
>
> So I'm still not sure why you want to defer autoanalyze in that case.

I don't especially want to defer autoanalyze in that case. But an autoanalyze
happening quickly after a TRUNCATE is critical for performance, I'd prefer to
find a way to trigger autoanalyze reliably.