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
