Re: View with an outer join - is there any way to optimise

First of all, check out this thread:

http://archives.postgresql.org/pgsql-general/2005-11/msg00734.php

I had a similar performance issue with a view.  Look at my solution and
it might help you out.

Second, you might want to change your token history status from a string
to an integer that references a status table.  If your view is causing a
sequential scan, you're going to end up will bazillions of string
comparisons.  I don't know if Postgres has some form of black magic
string comparison optimization, but I generally avoid string comparisons
when I am dealing with a few known values, as would be the case in a
status table.


Rich Doughty wrote:

>
> I have a view vw_tokens defined as
>
>   CREATE VIEW tokens.vw_tokens AS SELECT
>       -- too many columns to mention
>   FROM
>       tokens.ta_tokens      t LEFT JOIN
>       tokens.ta_tokenhist   i ON t.token_id = i.token_id AND
>                                      i.status   = 'issued'   LEFT JOIN
>       tokens.ta_tokenhist   s ON t.token_id = s.token_id AND
>                                      s.status   = 'sold'     LEFT JOIN
>       tokens.ta_tokenhist   r ON t.token_id = r.token_id AND
>                                    r.status   = 'redeemed'
>   ;
>
>
> the ta_tokens table contains approx 4 million records, and ta_tokenhist
> approx 10 millions. queries against the view itself on the primary key
> execute with no issues at all.
>
> I cannot however perform a meaningful join against this view. when i
> execute
>
>   SELECT *
>   FROM
>       tokens.ta_tokenhist h INNER JOIN
>       tokens.vw_tokens    t ON h.token_id = t.token_id
>   WHERE
>       h.sarreport_id = 9
>   ;
>
> PG forms the full output of the view. the query plan is
>
>
>  Hash Join  (cost=1638048.47..3032073.73 rows=1 width=702)
>    Hash Cond: (("outer".token_id)::integer = ("inner".token_id)::integer)
>    ->  Hash Left Join  (cost=1638042.45..3011803.15 rows=4052907
> width=470)
>          Hash Cond: (("outer".token_id)::integer =
> ("inner".token_id)::integer)
>          ->  Hash Left Join  (cost=1114741.93..2011923.86 rows=4052907
> width=322)
>                Hash Cond: (("outer".token_id)::integer =
> ("inner".token_id)::integer)
>                ->  Hash Left Join  (cost=559931.55..1093783.71
> rows=4052907 width=174)
>                      Hash Cond: (("outer".token_id)::integer =
> ("inner".token_id)::integer)
>                      ->  Seq Scan on ta_tokens t  (cost=0.00..73250.07
> rows=4052907 width=26)
>                      ->  Hash  (cost=459239.41..459239.41 rows=4114456
> width=152)
>                            ->  Seq Scan on ta_tokenhist i
> (cost=0.00..459239.41 rows=4114456 width=152)
>                                  Filter: ((status)::text =
> 'issued'::text)
>                ->  Hash  (cost=459239.41..459239.41 rows=3905186
> width=152)
>                      ->  Seq Scan on ta_tokenhist s
> (cost=0.00..459239.41 rows=3905186 width=152)
>                            Filter: ((status)::text = 'sold'::text)
>          ->  Hash  (cost=459239.41..459239.41 rows=2617645 width=152)
>                ->  Seq Scan on ta_tokenhist r  (cost=0.00..459239.41
> rows=2617645 width=152)
>                      Filter: ((status)::text = 'redeemed'::text)
>    ->  Hash  (cost=6.01..6.01 rows=1 width=236)
>          ->  Index Scan using fkx_tokenhist__sarreports on
> ta_tokenhist h  (cost=0.00..6.01 rows=1 width=236)
>                Index Cond: ((sarreport_id)::integer = 9)
>
>
> I have also tried explicitly querying token_id in the view, hoping
> to force a nested loop:
>
>
>   EXPLAIN
>   SELECT *
>   FROM
>       tokens.vw_tokens__user
>   WHERE
>     token_id IN (SELECT token_id FROM tokens.ta_tokenhist WHERE
> sarreport_id = 9);
>
>
>                                                 QUERY PLAN
> ----------------------------------------------------------------------------------------------------------
>
>  Hash IN Join  (cost=1638048.47..3032073.73 rows=1 width=470)
>    Hash Cond: (("outer".token_id)::integer = ("inner".token_id)::integer)
>    ->  Hash Left Join  (cost=1638042.45..3011803.15 rows=4052907
> width=470)
>          Hash Cond: (("outer".token_id)::integer =
> ("inner".token_id)::integer)
>          ->  Hash Left Join  (cost=1114741.93..2011923.86 rows=4052907
> width=322)
>                Hash Cond: (("outer".token_id)::integer =
> ("inner".token_id)::integer)
>                ->  Hash Left Join  (cost=559931.55..1093783.71
> rows=4052907 width=174)
>                      Hash Cond: (("outer".token_id)::integer =
> ("inner".token_id)::integer)
>                      ->  Seq Scan on ta_tokens t  (cost=0.00..73250.07
> rows=4052907 width=26)
>                      ->  Hash  (cost=459239.41..459239.41 rows=4114456
> width=152)
>                            ->  Seq Scan on ta_tokenhist i
> (cost=0.00..459239.41 rows=4114456 width=152)
>                                  Filter: ((status)::text =
> 'issued'::text)
>                ->  Hash  (cost=459239.41..459239.41 rows=3905186
> width=152)
>                      ->  Seq Scan on ta_tokenhist s
> (cost=0.00..459239.41 rows=3905186 width=152)
>                            Filter: ((status)::text = 'sold'::text)
>          ->  Hash  (cost=459239.41..459239.41 rows=2617645 width=152)
>                ->  Seq Scan on ta_tokenhist r  (cost=0.00..459239.41
> rows=2617645 width=152)
>                      Filter: ((status)::text = 'redeemed'::text)
>    ->  Hash  (cost=6.01..6.01 rows=1 width=4)
>          ->  Index Scan using fkx_tokenhist__sarreports on
> ta_tokenhist  (cost=0.00..6.01 rows=1 width=4)
>                Index Cond: ((sarreport_id)::integer = 9)
>
>
> Setting enable_mergejoin and enable_hashjoin to off results in a nested
> but still forms the view output.
>
>
> I can achieve the results i need be eliminating the view and writing
> the query manually but for various reasons i'd prefer to query a
> view.
>
> Any advice is greatly appreciated. i'm starting to wonder if the using
> a view in this instance is futile.
>
> Many thanks
>