Re: [PERFORM] bad plan

[Richard Huxton]

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

---------------------------(end of broadcast)---------------------------
TIP 9: the planner will ignore your desire to choose an index scan if your
     joining column's datatypes do not match