Hi,
I'm having a problem with a query that performs a sequential scan on a
table when it should be performing an index scan. The interesting thing
is, when we dumped the database on another server, it performed an index
scan on that server. The systems are running the same versions of
postgres (7.4.8) and the problem persists after running an "ANALYZE
VERBOSE" and after a "REINDEX TABLE sq_ast FORCE". The only difference
that i can see is that the postgresql.conf files differ slightly, and
the hardware is different. Note that the system performing the
sequential scan is a Dual 2.8GHz Xeon, 4GB Ram, 300GB HDD. And the
system performing an index scan is not as powerful.
A copy of the postgresql.conf for the system performing the index scan
can be found at http://beta.squiz.net/~mmcintyre/postgresql_squiz_uk.conf
A copy of the postgresql.conf for the system performing the sequential
scan can be found at http://beta.squiz.net/~mmcintyre/postgresql_future.conf
The Query:
SELECT a.assetid, a.short_name, a.type_code, a.status, l.linkid,
l.link_type, l.sort_order, lt.num_kids, u.url, ap.path,
CASE u.http
WHEN '1' THEN 'http'
WHEN '0' THEN 'https'
END AS protocol
FROM ((sq_ast a LEFT JOIN sq_ast_url u ON a.assetid = u.assetid) LEFT
JOIN sq_ast_path ap ON a.assetid = ap.assetid),sq_ast_lnk l,
sq_ast_lnk_tree lt WHERE a.assetid = l.minorid AND
l.linkid = lt.linkid AND l.majorid = '2' AND
l.link_type <= 2 ORDER BY sort_order
The EXPLAIN ANALYZE from the system performing an sequential scan:
QUERY PLAN
Sort (cost=30079.79..30079.89 rows=42 width=113) (actual
time=39889.989..39890.346 rows=260 loops=1)
Sort Key: l.sort_order
-> Nested Loop (cost=25638.02..30078.65 rows=42 width=113) (actual
time=9056.336..39888.557 rows=260 loops=1)
-> Merge Join (cost=25638.02..29736.01 rows=25 width=109) (actual
time=9056.246..39389.359 rows=260 loops=1)
Merge Cond: (("outer".assetid)::text = "inner"."?column5?")
-> Merge Left Join (cost=25410.50..29132.82 rows=150816
width=97) (actual time=8378.176..38742.111 rows=150567 loops=1)
Merge Cond: (("outer".assetid)::text =
("inner".assetid)::text)
-> Merge Left Join (cost=25410.50..26165.14 rows=150816
width=83) (actual time=8378.130..9656.413 rows=150489 loops=1)
Merge Cond: ("outer"."?column5?" = "inner"."?column4?")
-> Sort (cost=25408.17..25785.21 rows=150816
width=48) (actual time=8377.733..8609.218 rows=150486 loops=1)
Sort Key: (a.assetid)::text
-> Seq Scan on sq_ast a (cost=0.00..12436.16
rows=150816 width=48) (actual time=0.011..5578.231 rows=151378 loops=1)
-> Sort (cost=2.33..2.43 rows=37 width=43) (actual
time=0.364..0.428 rows=37 loops=1)
Sort Key: (u.assetid)::text
-> Seq Scan on sq_ast_url u (cost=0.00..1.37
rows=37 width=43) (actual time=0.023..0.161 rows=37 loops=1)
-> Index Scan using sq_ast_path_ast on sq_ast_path ap
(cost=0.00..2016.98 rows=45893 width=23) (actual time=0.024..14041.571 rows=45812
loops=1)
-> Sort (cost=227.52..227.58 rows=25 width=21) (actual
time=131.838..132.314 rows=260 loops=1)
Sort Key: (l.minorid)::text
-> Index Scan using sq_ast_lnk_majorid on sq_ast_lnk l
(cost=0.00..226.94 rows=25 width=21) (actual time=0.169..126.201 rows=260 loops=1)
Index Cond: ((majorid)::text = '2'::text)
Filter: (link_type <= 2)
-> Index Scan using sq_ast_lnk_tree_linkid on sq_ast_lnk_tree lt
(cost=0.00..13.66 rows=3 width=8) (actual time=1.539..1.900 rows=1 loops=260)
Index Cond: ("outer".linkid = lt.linkid)
Total runtime: 39930.395 ms
The EXPLAIN ANALYZE from the system performing an index scan scan:
Sort (cost=16873.64..16873.74 rows=40 width=113) (actual
time=2169.905..2169.912 rows=13 loops=1)
Sort Key: l.sort_order
-> Nested Loop (cost=251.39..16872.58 rows=40 width=113) (actual
time=45.724..2169.780 rows=13 loops=1)
-> Merge Join (cost=251.39..16506.42 rows=32 width=109) (actual
time=45.561..2169.012 rows=13 loops=1)
Merge Cond: (("outer".assetid)::text = "inner"."?column5?")
-> Merge Left Join (cost=2.33..15881.92 rows=149982 width=97)
(actual time=0.530..1948.718 rows=138569 loops=1)
Merge Cond: (("outer".assetid)::text =
("inner".assetid)::text)
-> Merge Left Join (cost=2.33..13056.04 rows=149982
width=83) (actual time=0.406..953.781 rows=138491 loops=1)
Merge Cond: (("outer".assetid)::text =
"inner"."?column4?")
-> Index Scan using sq_ast_pkey on sq_ast a
(cost=0.00..14952.78 rows=149982 width=48) (actual time=0.154..388.872 rows=138488
loops=1)
-> Sort (cost=2.33..2.43 rows=37 width=43) (actual
time=0.235..0.264 rows=37 loops=1)
Sort Key: (u.assetid)::text
-> Seq Scan on sq_ast_url u (cost=0.00..1.37
rows=37 width=43) (actual time=0.036..0.103 rows=37 loops=1)
-> Index Scan using sq_ast_path_ast on sq_ast_path ap
(cost=0.00..1926.18 rows=42071 width=23) (actual time=0.110..105.918 rows=42661
loops=1)
-> Sort (cost=249.05..249.14 rows=36 width=21) (actual
time=0.310..0.324 rows=13 loops=1)
Sort Key: (l.minorid)::text
-> Index Scan using sq_ast_lnk_majorid on sq_ast_lnk l
(cost=0.00..248.12 rows=36 width=21) (actual time=0.141..0.282 rows=13 loops=1)
Index Cond: ((majorid)::text = '2'::text)
Filter: (link_type <= 2)
-> Index Scan using sq_ast_lnk_tree_linkid on sq_ast_lnk_tree lt
(cost=0.00..11.41 rows=2 width=8) (actual time=0.043..0.045 rows=1 loops=13)
Index Cond: ("outer".linkid = lt.linkid)
Total runtime: 2170.165 ms
(22 rows)
THE DESC of the sq_ast table.
future_v3_schema=# \d sq_ast
Table "public.sq_ast"
Column | Type | Modifiers
-----------------------+-----------------------------+---------------------------------------------
assetid | character varying(15) | not null
type_code | character varying(100) | not null
version | character varying(20) | not null default '0.0.0'::character
varying
name | character varying(255) | not null default ''::character varying
short_name | character varying(255) | not null default ''::character
varying
status | integer | not null default 1
languages | character varying(50) | not null default ''::character varying
charset | character varying(50) | not null default ''::character varying
force_secure | character(1) | not null default '0'::bpchar
created | timestamp without time zone | not null
created_userid | character varying(255) | not null
updated | timestamp without time zone | not null
updated_userid | character varying(255) | not null
published | timestamp without time zone |
published_userid | character varying(255) |
status_changed | timestamp without time zone |
status_changed_userid | character varying(255) |
Indexes:
"sq_asset_pkey" primary key, btree (assetid)
"sq_ast_created" btree (created)
"sq_ast_name" btree (name)
"sq_ast_published" btree (published)
"sq_ast_type_code" btree (type_code)
"sq_ast_updated" btree (updated)
Any ideas?
--
Marc McIntyre
MySource Matrix Lead Developer
---------------------------(end of broadcast)---------------------------
TIP 9: In versions below 8.0, the planner will ignore your desire to
choose an index scan if your joining column's datatypes do not
match
