Thread: Many left outer joins with limit performance

From:
Gerhard Wiesinger
Date:

Hello,

I want to use postgresql for data entries (every minute) from a central heating
system where the timestamp is logged in a table log. For flexibility in the
future for future values and for implementing several high level types I've
modelled the values in a separate key/value table called log_details.

A Query for the last valid entry for today looks like (also defined as a view),
sometimes used without the limit:
SELECT
   l.id AS id,
   l.datetime AS datetime,
   l.tdate AS tdate,
   l.ttime AS ttime,
   d1.value  AS Raumsolltemperatur,
   d2.value  AS Raumtemperatur,
-- a lot more here, stripped for readibility, see link
FROM
   log l
-- Order is relevant here
LEFT OUTER JOIN key_description k1  ON k1.description = 'Raumsolltemperatur'
LEFT OUTER JOIN log_details d1      ON l.id = d1.fk_id AND d1.fk_keyid =
k1.keyid
-- Order is relevant here
LEFT OUTER JOIN key_description k2  ON k2.description = 'Raumtemperatur'
LEFT OUTER JOIN log_details d2      ON l.id = d2.fk_id AND d2.fk_keyid =
k2.keyid
-- a lot more here, stripped for readibility, see link
WHERE
   -- 86400 entries in that timeframe
   datetime    >= '1970-01-01 00:00:00+02'
   AND datetime < '1970-01-02 00:00:00+02'
ORDER BY
   datetime DESC
LIMIT 1;

For me a perfect query plan would look like:
1.) Fetch the one and only id from table log (or fetch even all necessary id
entries when no limit is specifie)
2.) Make the left outer joins

Details (machine details, table definition, query plans, etc.)
can be found to due size limitations at:
http://www.wiesinger.com/tmp/pg_perf.txt

Any ideas how to improve the performance on left outer joins only and how to
improve the planner to get better results?

For this special case a better solution exists but I thing the planner has to
do the work.
-- ...
WHERE
     -- Also slow: id IN
     -- OK: id =
     id = (
       SELECT
         id
       FROM
         log
       WHERE
         datetime    >= '1970-01-01 00:00:00+02'
         AND datetime < '1970-01-02 00:00:00+02'
       ORDER BY
         datetime DESC
       LIMIT 1
     )
ORDER BY
     datetime DESC LIMIT 1;

Any ideas?

Thnx.

Ciao,
Gerhard

--
http://www.wiesinger.com/

From:
Tom Lane
Date:

Gerhard Wiesinger <> writes:
> FROM
>    log l
> -- Order is relevant here
> LEFT OUTER JOIN key_description k1  ON k1.description = 'Raumsolltemperatur'
> LEFT OUTER JOIN log_details d1      ON l.id = d1.fk_id AND d1.fk_keyid = k1.keyid

Surely this query is just plain broken?  You're forming a cross product
of the relevant log lines with the k1 rows having description =
'Raumsolltemperatur' (I assume this isn't unique, else it's not clear
what the point is) and then the subsequent left join cannot get rid of
anything.  I think probably you meant something different, like

FROM
   log l
LEFT OUTER JOIN log_details d1      ON l.id = d1.fk_id
LEFT OUTER JOIN key_description k1  ON k1.description = 'Raumsolltemperatur' AND d1.fk_keyid = k1.keyid

            regards, tom lane

From:
Gerhard Wiesinger
Date:

Hello Tom,

The query was logically ok. The main problem was that the VIEW had an
ORDER BY clause where cost went up to very high. Indices and unique
constraints were minor optimizations.

Conclusio: Don't create ORDER BY in VIEW unless really necessary

Ciao,
Gerhard

--
http://www.wiesinger.com/


On Fri, 1 May 2009, Tom Lane wrote:

> Gerhard Wiesinger <> writes:
>> FROM
>>    log l
>> -- Order is relevant here
>> LEFT OUTER JOIN key_description k1  ON k1.description = 'Raumsolltemperatur'
>> LEFT OUTER JOIN log_details d1      ON l.id = d1.fk_id AND d1.fk_keyid = k1.keyid
>
> Surely this query is just plain broken?  You're forming a cross product
> of the relevant log lines with the k1 rows having description =
> 'Raumsolltemperatur' (I assume this isn't unique, else it's not clear
> what the point is) and then the subsequent left join cannot get rid of
> anything.  I think probably you meant something different, like
>
> FROM
>   log l
> LEFT OUTER JOIN log_details d1      ON l.id = d1.fk_id
> LEFT OUTER JOIN key_description k1  ON k1.description = 'Raumsolltemperatur' AND d1.fk_keyid = k1.keyid
>
>             regards, tom lane
>
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