Re: Propose a new hook for mutating the query bounds

Hi,

Thanks for the previous feedbacks!

> The way the hook is used seems pretty inconvenient, though.

I see the problem, and I agree.

I looked into how other hooks work, and I am wondering if it looks ok if
we: pass a pointer to the hook, and let the hook check if there is any
information applicable. If there is none, the hook just returns False and
we let the rest of the code handle. If it is true, we get the selectivity
from the hook and return it. So something like

```
if (clauselist_selectivity_hook &&
(*clauselist_selectivity_hook) (root, clauses, varRelid, jointype, sjinfo,
use_extended_stats, &s1))
{
return s1;
}
```

What I am trying to mock is the get_index_stats_hook (
https://github.com/taminomara/psql-hooks/blob/master/Detailed.md#get_index_stats_hook).

Am I understanding your idea correctly and does this look somehow better?

Best regards,
Xiaozhe

On Wed, Nov 17, 2021 at 7:47 PM Tomas Vondra <tomas(dot)vondra(at)enterprisedb(dot)com>
wrote:

> On 11/17/21 16:39, Xiaozhe Yao wrote:
> > Hi Tom,
> >
> > Thanks for your feedback. I completely agree with you that a
> > higher-level hook is better suited for this case. I have adjusted the
> > PoC patch to this email.
> >
> > Now it is located in the clauselist_selectivity_ext function, where we
> > first check if the hook is defined. If so, we let the hook estimate the
> > selectivity and return the result. With this one, I can also develop
> > extensions to better estimate the selectivity.
> >
>
> I think clauselist_selectivity is the right level, because this is
> pretty similar to what extended statistics are doing. I'm not sure if
> the hook should be called in clauselist_selectivity_ext or in the plain
> clauselist_selectivity. But it should be in clauselist_selectivity_or
> too, probably.
>
> The way the hook is used seems pretty inconvenient, though. I mean, if
> you do this
>
> if (clauselist_selectivity_hook)
> return clauselist_selectivity_hook(...);
>
> then what will happen when the ML model has no information applicable to
> a query? This is called for all relations, all conditions, etc. and
> you've short-circuited all the regular code, so the hook will have to
> copy all of that. Seems pretty silly and fragile.
>
> IMO the right approach is what statext_clauselist_selectivity is doing,
> i.e. estimate clauses, mark them as estimated in a bitmap, and let the
> rest of the existing code take care of the remaining clauses. So more
> something like
>
> if (clauselist_selectivity_hook)
> s1 *= clauselist_selectivity_hook(..., &estimatedclauses);
>
>
> regards
>
> --
> Tomas Vondra
> EnterpriseDB: http://www.enterprisedb.com
> The Enterprise PostgreSQL Company
>