public inbox for [email protected]  

help / color / mirror / Atom feed

From: Albe Laurenz <[email protected]>
To: 'Akihiko Odaki  *EXTERN*' <[email protected]>
Cc: [email protected] <[email protected]>
Subject: Re: Inappropriate inner table for nested loop join
Date: Fri, 23 Jun 2017 13:05:49 +0000
Message-ID: <A737B7A37273E048B164557ADEF4A58B53A5D97E@ntex2010i.host.magwien.gv.at> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
	<A737B7A37273E048B164557ADEF4A58B53A5D88F@ntex2010i.host.magwien.gv.at>
	<[email protected]>
	<[email protected]>
List-Unsubscribe:  <mailto:[email protected]?body=unsub%20pgsql-performance>

Akihiko Odaki wrote:
> On 2017-06-23 20:20, Albe Laurenz wrote:
>> You could either try to do something like
>>
>> SELECT *
>> FROM (SELECT "posts".*
>>        FROM "posts"
>>           JOIN "follows" ON "follows"."target_account" = "posts"."account"
>>        WHERE "follows"."owner_account" = $1
>>        OFFSET 0) q
>> ORDER BY "posts"."timestamp"
>> LIMIT 100;
> 
> Now I wonder whether it actually sorted or not. As you said, I want to
> "find rows with the greatest 'timestamp', match with rows from 'posts'
> in a nested loop and stop as soon as it has found 100 matches".
> 
> However, it seems to query 100 records without any consideration for
> "timestamp", and then sorts them. That is not expected. Here is a
> abstract query plan:
> 
>   Limit
>     ->  Sort
>           Sort Key: posts.id DESC
>           ->  Nested Loop
>                 ->  Seq Scan on follows
>                       Filter: (owner_account = $1)
>                 ->  Index Scan using index_posts_on_account on posts
>                       Index Cond: (account_id = follows.target_account)
> 
> index_posts_on_account is an obsolete index on "posts" and only for
> "account". So it does nothing for sorting "timestamp".

Yes, if you replace posts.timestamp with q.timestamp, it should
sort by that.

Could you send CREATE TABLE and CREATE INDEX statements so I can try it?

Yours,
Laurenz Albe

-- 
Sent via pgsql-performance mailing list ([email protected])
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-performance

reply

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Reply to all the recipients using the --to and --cc options:
  reply via email

  To: [email protected]
  Cc: [email protected], [email protected]
  Subject: Re: Inappropriate inner table for nested loop join
  In-Reply-To: <A737B7A37273E048B164557ADEF4A58B53A5D97E@ntex2010i.host.magwien.gv.at>

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

This inbox is served by agora; see mirroring instructions
for how to clone and mirror all data and code used for this inbox