To: pgsql-sql@postgresql.org
From: Thomas Kellerer <spam_eater@gmx.net>
Subject: Re: Retrieve most recent 1 record from joined table
Date: Mon, 25 Aug 2014 07:59:18 +0200
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agharta schrieb am 22.08.2014 um 10:05:
> Joining the tables, how to get ONLY most recent record per table3(t3_date)??
> 
> Query example:
> 
> select * from table1 as t1
> inner join table2 t2 on (t1.t1_id = t2.t1_id and t2.t2_value like('%ab%') )
> inner join table3 t3 on (t2.t2_id = t3.t2_id and t3.t3_date <= timestamp '2014-08-20')
> order by t3.t2_id, t3.t3_date desc
> 

This seems to be slightly faster, especially with the following index:

  create index idx_t3_combined on table3 (t2_id, t3_date desc, t3_id);

select *
from table1 as t1
  join table2 t2 on t1.t1_id = t2.t1_id and t2.t2_value like '%ab%'
  join (
     select distinct on (t2_id) t3_id, 
            t3_date,
            t2_id
     from table3
     order by t2_id, t3_date desc
  ) t3 on t3.t2_id = t2.t2_id 
order by t3.t2_id, t3.t3_date desc
;

I also had to increase the work_mem in order to avoid disk based sorting for the joins


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