| From: | Mithun Cy <mithun(dot)cy(at)enterprisedb(dot)com> |
|---|---|
| To: | Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> |
| Cc: | Douglas Doole <dougdoole(at)gmail(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: possible optimizations - pushing filter before aggregation |
| Date: | 2016-11-19 14:11:59 |
| Message-ID: | CAD__OuhP04+=mn0DGOytpVM_g_qWWGwJCG8DXnoJSAkhMCVk0w@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Sat, Nov 19, 2016 at 8:59 AM, Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>
wrote:
>
>
>
>> SELECT MIN(a) m FROM
>> (SELECT a FROM t WHERE a=2) AS v(a)
>>
>> The subquery is going to return an intermediate result of:
>>
>> V:A
>> ---
>> 2
>>
>> And the minimum of that is 2, which is the wrong answer.
>>
>
> yes, you have true,
>
> In above case wondering if we could do this
Min(a) = 2 is the condition, generate condition *"a <= 2"* and push it down
as scan key. Since pushed down condition is lossy one for us ( it gives
values < 2), finally do a recheck of *"Min(a) = 2"*.
For Max(a) = 2 we can have *"a >=2"*,
If both are given we can combine them appropriately.
--
Thanks and Regards
Mithun C Y
EnterpriseDB: http://www.enterprisedb.com
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