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

John McCawley wrote:
> 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.

i'm not sure that'll help in my case as ta_tokens has a 1-to-many
relationship with ta_tokenhist.

there are various indexed tokenhist columns that i want to query on (in
addition to salesorder_id). none of them will return more than 100 or so
rows. it'd probably be easier to abandon the view altogether (which isn't
something i'd really like to do)

> 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.

interesting thought. of course, i'd rather postgres didn't do a full
sequential scan ;-)

>
>
> 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
>>

--

- Rich Doughty