Re: PoC/WIP: Extended statistics on expressions

On 3/24/21 2:36 PM, Dean Rasheed wrote:
> On Wed, 24 Mar 2021 at 10:22, Tomas Vondra
> <tomas(dot)vondra(at)enterprisedb(dot)com> wrote:
>>
>> Thanks, it seems to be some thinko in handling in PlaceHolderVars, which
>> seem to break the code's assumptions about varnos. This fixes it for me,
>> but I need to look at it more closely.
>>
>
> I think that makes sense.
>

AFAIK the primary issue here is that the two places disagree. While
estimate_num_groups does this

varnos = pull_varnos(root, (Node *) varshere);
if (bms_membership(varnos) == BMS_SINGLETON)
{ ... }

the add_unique_group_expr does this

varnos = pull_varnos(root, (Node *) groupexpr);

That is, one looks at the group expression, while the other look at vars
extracted from it by pull_var_clause(). Apparently for PlaceHolderVar
this can differ, causing the crash.

So we need to change one of those places - my fix tweaked the second
place to also look at the vars, but maybe we should change the other
place? Or maybe it's not the right fix for PlaceHolderVars ...

> Reviewing the docs, I noticed a couple of omissions, and had a few
> other suggestions (attached).
>

Thanks! I'll include that in the next version of the patch.

regards

--
Tomas Vondra
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