On 15/2/2024 18:10, Tomas Vondra wrote:
>
>
> On 2/15/24 07:50, Andrei Lepikhov wrote:
>> On 18/12/2023 19:53, Tomas Vondra wrote:
>>> On 12/18/23 11:40, Richard Guo wrote:
>>> The challenge is where to get usable information about correlation
>>> between columns. I only have a couple very rought ideas of what might
>>> try. For example, if we have multi-column ndistinct statistics, we might
>>> look at ndistinct(b,c) and ndistinct(b,c,d) and deduce something from
>>>
>>> ndistinct(b,c,d) / ndistinct(b,c)
>>>
>>> If we know how many distinct values we have for the predicate column, we
>>> could then estimate the number of groups. I mean, we know that for the
>>> restriction "WHERE b = 3" we only have 1 distinct value, so we could
>>> estimate the number of groups as
>>>
>>> 1 * ndistinct(b,c)
>> Did you mean here ndistinct(c,d) and the formula:
>> ndistinct(b,c,d) / ndistinct(c,d) ?
>
> Yes, I think that's probably a more correct ... Essentially, the idea is
> to estimate the change in number of distinct groups after adding a
> column (or restricting it in some way).
Thanks, I got it. I just think how to implement such techniques with
extensions just to test the idea in action. In the case of GROUP-BY we
can use path hook, of course. But what if to invent a hook on clauselist
estimation?
>> Do you implicitly bear in mind here the necessity of tracking clauses
>> that were applied to the data up to the moment of grouping?
>>
>
> I don't recall what exactly I considered two months ago when writing the
> message, but I don't see why we would need to track that beyond what we
> already have. Shouldn't it be enough for the grouping to simply inspect
> the conditions on the lower levels?
Yes, exactly. I've thought about looking into baserestrictinfos and, if
group-by references a subquery targetlist, into subqueries too.
--
regards,
Andrei Lepikhov
Postgres Professional
