Re: bad estimation together with large work_mem generates terrible slow hash joins

From: Tomas Vondra <tv(at)fuzzy(dot)cz>
To: pgsql-hackers(at)postgresql(dot)org
Subject: Re: bad estimation together with large work_mem generates terrible slow hash joins
Date: 2014-09-11 21:57:33
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On 11.9.2014 16:33, Tomas Vondra wrote:
> On 11 Září 2014, 15:31, Robert Haas wrote:
>> On Wed, Sep 10, 2014 at 5:09 PM, Tomas Vondra <tv(at)fuzzy(dot)cz> wrote:
>>> OK. So here's v13 of the patch, reflecting this change.
>> [...] It does three things:
>> (1) It changes NTUP_PER_BUCKET to 1. Although this increases memory
>> utilization, it also improves hash table performance, and the
>> additional memory we use is less than what we saved as a result of
>> commit 45f6240a8fa9d35548eb2ef23dba2c11540aa02a.
>> (2) It changes ExecChooseHashTableSize() to include the effect of the
>> memory consumed by the bucket array. If we care about properly
>> respecting work_mem, this is clearly right for any NTUP_PER_BUCKET
>> setting, but more important for a small setting like 1 than for the
>> current value of 10. I'm not sure the logic in this function is as
>> clean and simple as it can be just yet.
>> (3) It allows the number of batches to increase on the fly while the
>> hash join is in process. This case arises when we initially estimate
>> that we only need a small hash table, and then it turns out that there
>> are more tuples than we expect. Without this code, the hash table's
>> load factor gets too high and things start to suck.
>> I recommend separating this patch in two patches, the first covering
>> items (1) and (2) and the second covering item (3), which really seems
>> like a separate improvement.
> That probably makes sense.

Attached is the patch split as suggested:

(a) hashjoin-nbuckets-v14a-size.patch

* counting buckets towards work_mem
* changes in ExecChooseHashTableSize (due to the other changes)

(b) hashjoin-nbuckets-v14a-resize.patch

* rest of the patch, that is ...
* tracking optimal number of buckets
* dynamic resize of the hash table
* adding info the the EXPLAIN ANALYZE output


Attachment Content-Type Size
hashjoin-nbuckets-v14b-resize.patch text/x-diff 10.7 KB
hashjoin-nbuckets-v14a-size.patch text/x-diff 4.7 KB

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