From: | Claudio Freire <klaussfreire(at)gmail(dot)com> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | Teodor Sigaev <teodor(at)sigaev(dot)ru>, David Rowley <david(dot)rowley(at)2ndquadrant(dot)com>, Teodor Sigaev <teodor(at)postgrespro(dot)ru>, Pgsql Hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Perfomance bug in v10 |
Date: | 2017-06-02 14:20:47 |
Message-ID: | CAGTBQpbDcDfYWBpamXnL+uoPcNQv-gHTp5GP-J9P1Tb=14NHaA@mail.gmail.com |
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On Fri, Jun 2, 2017 at 10:27 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Teodor Sigaev <teodor(at)sigaev(dot)ru> writes:
>>> Teodor, could you check if this patch fixes your real-world problem?
>
>> It works fine with original query, thank you. But some other query slowdowns for
>> ~10% (9 secs vs 10 secs). Look at following part of plans of huge query:
>> ...
>> As you said, it removes Materialize node, although it's useful here.
>
> Meh. If it's expecting only one outer row, it shouldn't be using a
> Materialize on the inner side, period. That's not sane per the cost
> model. You haven't shown us enough to guess why the rowcount estimates
> are so far off reality in this query, but I don't think it's the fault
> of this patch if the result is slightly slower given that much error.
>
> Most likely, the answer for your real-world problem is "you need to
> ANALYZE before running the query".
>
> regards, tom lane
I don't know. Perhaps the risky part is assuming rows=1 for non-unique
inner scans. In fact a wrongly estimated rows=1 outer scan would be
just as risky.
There were old threads about considering a risk factor when estimating
plans, and I'm thinking this issue is the planner failing to do
exactly that.
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