Re: bad plan - Mailing list pgsql-performance

Gaetano Mendola wrote:
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> Richard Huxton wrote:
>
>>Gaetano Mendola wrote:
>>
>>
>>>running a 7.4.5 engine, I'm facing this bad plan:
>>>
>>>empdb=# explain analyze SELECT
>>>name,url,descr,request_status,url_status,size_mb,estimated_start,request_time_stamp
>>>
>>>empdb-#                    FROM v_sc_user_request
>>>empdb-#                    WHERE
>>>empdb-#                         login = 'babinow1'
>>>empdb-#                    LIMIT 10 ;
>>
>>
>>>                     ->  Subquery Scan vsp  (cost=985.73..1016.53
>>>rows=1103 width=12) (actual time=25.328..1668.754 rows=493 loops=31)
>>>                           ->  Merge Join  (cost=985.73..1011.01
>>>rows=1103 width=130) (actual time=25.321..1666.666 rows=493 loops=31)
>>>                                 Merge Cond: ("outer".id_program =
>>>"inner".id_program)
>>
>>
>>The problem to address is in this subquery. That's a total of 31 x
>>(1668.754 - 25.328) = 50seconds (about).
>>
>>Since your query is so simple, I'm guessing v_sc_user_request is a view.
>>Can you provide the definition?
>
>
> Of course:
>
>
>
> CREATE OR REPLACE VIEW v_sc_user_request AS
>   SELECT
>       *
>   FROM
>       v_sat_request vsr LEFT OUTER JOIN v_sc_packages vsp USING ( id_package )
>   WHERE
>       vsr.request_time > now() - '1 month'::interval AND
>       vsr.expired = FALSE
>   ORDER BY id_sat_request DESC
> ;
>
>
> CREATE OR REPLACE VIEW v_sc_packages AS
>   SELECT
>      *
>   FROM
>       v_programs           vpr,
>       v_packages           vpk,
>       v_sequences          vs
>
>   WHERE
>      ------------ JOIN -------------
>       vpr.id_program = vs.id_program AND
>       vpk.id_package = vs.id_package AND
>      -------------------------------
>       vs.estimated_start IS NOT NULL
> ;
>
> CREATE OR REPLACE VIEW v_sat_request AS
>   SELECT
>      *
>   FROM
>      sat_request sr,
>      url         u,
>      user_login  ul
>   WHERE
>      ---------------- JOIN ---------------------
>      sr.id_url  = u.id_url AND
>      sr.id_user = ul.id_user
>      -------------------------------------------
> ;

OK, so looking at the original EXPLAIN the order of processing seems to be:
1. v_sat_request is evaluated and filtered on login='...' (lines 7..15)
This gives us 31 rows
2. The left-join from v_sat_request to v_sc_packages is processed (lines
5..6)
This involves the subquery scan on vsp (from line 16) where it seems to
think the best idea is a merge join of programs to sequences.

So - I think we need to look at the performance of your view
"v_sc_packages" and the views that it depends on. OK - can you reply to
this with just the definitions of v_sc_packages and what it depends on,
and we can have a look at that.

Do you need all these tables involved in this query? I don't think PG is
smart enough to completely discard a join if it's not needed by the
output. Thinking about it, I'm not sure you could safely.
--
   Richard Huxton
   Archonet Ltd