Subject: Re: Huge generated UNION ALL faster than JOIN?
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-performance@lists.postgresql.org
References: <965737b8-0e86-9e61-e006-1f4d927e3c4f@googlemail.com>
 <23129.1557684481@sss.pgh.pa.us>
From: Ancoron Luciferis <ancoron.luciferis@googlemail.com>
Openpgp: preference=signencrypt
Autocrypt: addr=ancoron.luciferis@googlemail.com; prefer-encrypt=mutual;
 keydata=
 mQINBE3Z7bUBEAC3ZYDTnxOiu0JWJR+6XzFQmJEVncGMehyuE9GBfyaNYnsNKmFITkubNVB9
 tN/xgn5FJ9eg0WediTaWXigHNDDOPSNHZGaf1nHu0AQNOgpEyJETtX/A56PXQ82+VwoKCNUN
 v8+m5CJ8njWZlZMCVA3AcQ1vR2VRR6O0pzEVPDCDMutPytkRl+XrPoPhxMTVEmwM4jfWwmfI
 NgiOZnsueU78VW8Go1qTN20YdVOdVpbPWfyp8VIGYBNjEEefv5f/l+zt4Z+CQMmvunDUdaKY
 85opNIMLqC3dDY0AQG+FpcDLhHWLcuGLmE+Ghiix0G3kX4QX/fH0/2KtVNdfnZtcpq+5rim0
 E97PVQe4EixVD/DivpwvNMWqQ2GJlOCx52+J13ZSfe/63XpjL7TNqhR3wMnNuMidLMgLoH5z
 RtQ2FH9zeYlKgqMUkmwvEHVI+M4oQRE0rzgqx6xHbkPIFEov9ZziQVwCtct+Ja9ho/BXoe/T
 VIFVOGEp7DG12QSaiLILhzTRUkDXqa+BH9H4V2Hsv77nT2JQ7KO9pM5gb4wq4gtLFo7MMTUq
 jlqjD2vOm3NY3tg1s7HTggVNgGsWhh9JlNR3v9SRGckKVIIvwDxvzo+Z4iji2SLElWb9BWiH
 Xku5IEuWGTmC+nOzSvbSxTaC53JcppC5kKXj11N73uw5eR4s3wARAQABtDRBbmNvcm9uIEx1
 Y2lmZXJpcyA8YW5jb3Jvbi5sdWNpZmVyaXNAZ29vZ2xlbWFpbC5jb20+iQI4BBMBAgAiBQJN
 2e21AhsDBgsJCAcDAgYVCAIJCgsEFgIDAQIeAQIXgAAKCRBzrR1GWXG7SN0DD/9qJR1Bsm52
 W/T+XKQ0DApt3rCVgt9e5gvk4mUgtUIufIVOx3t/s2qw84VcSyNVclfoecLF0JdBWavobyCn
 JAIVkgGt0wGF6Kzsbiq7vlqDHDYiB7prvY4vj6bpmEQGYBBmWPG6/AVvBRJsvvaSGTqqdVth
 4MYl43VxDZD+aVq52o0ROQAWyZC28kevTvRxusLuwZc4Q7HvOuAGNeD4aGLIgbOpyiS4mkC3
 i2B++/cQ+zurzCWqCi1U/0sFXtwV2Qh6gjZK/TvZogTMUxjftZdvP49R5NrEB6AWx5ztp/RO
 7Jy2j+Xm4OAteb5uGegzyoPKzr7B9O76GtfXLgLyZjpBIpuudosx+vJ9g7wuDHWXj7f5QKZQ
 PLqDO5SxI/Yi5tM4byag9/KZ7aHmeCXuAOnRTvKh6xMyB0fq6aPtpRlek4NcHWdT2SaR1y5K
 XxqWM/x7wLUyQo1lvjPTWX2MS70AjPJ5BdZzEjoA3zucT/ibu+CRjbyAOx5XwIBTnutj/+oI
 osslCFbMMj6n9NRoJidmy8q2nMYH+lMP1nTbdLCLesURVlTVLcMxIQhsjRZC/E6tu86C6rHB
 VrNjlUHkXHwJ9qQ3FbFOudqPUatYcfJSI7PmPbxWqNhcnVXlA1yndT7kAfWsmIxaJvDw09p5
 dxkrNZi0LYoyLmMLImCb0yCHibkCDQRN2e21ARAAuQP7cntv+HFvl+3aUMZ68vTcV+0AfFfN
 DqZy6UYNbggIkuTtE5c5O88oBaJ/4idyAwzleb0Ess51xgYExv1zdxj61i+FfTOXEHhXp+uG
 Zk3mYVlp5juZwdEClrNkMTZehP6zvEvem5MkGGe3RBg7v4svg+xiaqaPMeN2Kj7KbObrlKcA
 F9A/4lRr5F0lH/RJaIaIUAgccVbRQdN1Z0JD6CU54sNlOCvkRqq1zNgmDSq/6WPs+WMkd0FT
 yMeyus9sPOmBrld4Wp6jHJSt7cYzq45F87b6oUPPVs4uSa1CN6IZZKk1rmQzuuFclTdw3Ari
 kjBKhFXe0gwsahPUYMOaICc62ivGKhHW/6HXh0JrPP9zqc/hMSY/fXffBXKAab4xnCA7/oUU
 4W8/wD4qvq0w77i5bUJrHg+AhFEGDXWPfMCYjIE0oLTCYK696yhhVpVSFD8c3Uj1tSQERbBN
 7qpDKGSOjRZyPLAH0yOAVgqteLkDhzwhqgwBhCXEOQAWqgvP2JI8cvbQYprG5uQnsS6TkYlq
 1/nmmV+6HfMmitEp7siGaq0UmVZHBrGD+BQjuLkKPZ95YCSwc8xM/Y6viOIxvgisVRNXqo0r
 QW/FYXKnz0i7ZQTYE4Rad487mtd+eAcW1LhwBlM5dJRxx6Ee8dKkhjvQjap4MmghpNltULUF
 yGkAEQEAAYkCHwQYAQIACQUCTdnttQIbDAAKCRBzrR1GWXG7SOmfEACisSp8ZEfX6zVvzLNu
 YVXlvnQcd+w7VtN9+MMB3P9xZUku82/flaZTVvCbQ/kDRT9BrLdNcxM9dgt6Fgih49qN6+TI
 2Atl+nBO9KtKaBc8wFs06VEEYkaIvtiS99KJO2wjQXKE1/Fj/HLKET62BctFFk5fgTdzHpxZ
 2yovkQ+cPhkkyKpW7/UjFYn2Phpvg3KU1kcH/5oyGjRzCFTT0ERlyKWA/X/XxzX7+k2LVEh0
 QzpuZbn/S0ldHDtP+fMoZZAHjVpWUu6A0zLYIHQkX/X4gkBvlWwvVuh27lDULJ8elrECyGOp
 46G14r1t6bSnESm9W9AcvtG1pQQMv7zoHXJNgYew2cbkSxgBSjNNxnkUPIJdvX8Bel/iQzLr
 EXCaZ/myDJ4RNrzkvRBWdOWMvBkIV3yXogp6CyYzq5IaQgSIiv5Hr17ssuNR+FS5aZBocJru
 pCqLD/AdS3MZLBeRGmpDWkuj4uc4/4tLDIHcmCYia47FD2OuGSbLDz4BBeEmR37CkFsxIq6f
 eszCf1awGfZv7i0ddVFT3gtJTBoRE6VPkpcMrYaEjhtbrGI9PwB+7rNd4V9O6qPwV450KldB
 FwlHzOjWNRkfDUc3sgRB3QXlH3bVsHqUVBBTXf5YwqxeQtIRe/A7ICU3kLx8BKiUZXhOoTur
 jG7LftY+nte9Tt8CTQ==
Message-ID: <a850f693-2167-8a2b-f720-b27852d905c4@googlemail.com>
Date: Mon, 13 May 2019 00:23:14 +0200
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101
 Thunderbird/60.6.1
MIME-Version: 1.0
In-Reply-To: <23129.1557684481@sss.pgh.pa.us>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 8bit
Precedence: bulk

On 12/05/2019 20:08, Tom Lane wrote:
> Ancoron Luciferis <ancoron.luciferis@googlemail.com> writes:
>> One of the things consuming most of the time was an Index Only Scan
>> executed millions of times. And on top came the Nested Loop which
>> finally reduced the rows but also took a lot of time to do so.
> 
>> Explain plan: https://explain.depesz.com/s/4GYT
> 
> The core problem you've got there is the misestimation of the join size:
> 
> Nested Loop (cost=0.71..0.30 rows=72,839,557 width=33) (actual time=19,504.096..315,933.158 rows=274 loops=1)
> 
> Anytime the planner is off by a factor of 250000x, it's not going to end
> well.  In this case, it's imagining that the LIMIT will kick in after just
> a very small part of the join is executed --- but in reality, the LIMIT
> is larger than the join output, so that we have to execute the whole join.
> With a more accurate idea of the join result size, it would have chosen
> a different plan.
> 
> What you ought to look into is why is that estimate so badly off.
> Maybe out-of-date stats, or you need to raise the stats target for
> one or both tables?
> 

I thought so as well and that's why I started investigating, but after
creating my own data set and a final analyze of both tables I ended up
with similar difference in estimation vs. actual:

https://explain.depesz.com/s/R7jp

Nested Loop (cost=25.17..514,965,251.12 rows=27,021,979 width=56)
(actual time=0.568..5.686 rows=274 loops=1)

...but this was fast due to the Merge Append being used and pushed-down
LIMIT.

>> My question now is why can't the optimizer generate a plan that in this
>> case does 114 loops of "events" scans instead of a million loops on the
>> "subscription_signal"?
> 
> I don't see any "events" table in that query, so this question isn't
> making a lot of sense to me.  But in any case, the answer probably boils
> down to "it's guessing that a plan like this will stop early without
> having to scan all of the large table".
> 
> 			regards, tom lane
> 

Yes, Timescale extension is mangling the partition names quite a lot. I
wonder if it would be possible to hold the result of the estimated
smaller reference data (114 subscription_signal.signal_id entries in
this case) in a VALUES list and then use that to filter the table with
the larger estimate instead of looping over.

Cheers,

	Ancoron