Here is a simple test case for this strange behaviour :
annonces=> CREATE TABLE test.current (id INTEGER PRIMARY KEY, description
TEXT);
INFO: CREATE TABLE / PRIMARY KEY creera un index implicite
<<current_pkey>> pour la table <<current>>
CREATE TABLE
annonces=> CREATE TABLE test.archive (id INTEGER PRIMARY KEY, description
TEXT);
INFO: CREATE TABLE / PRIMARY KEY creera un index implicite
<<archive_pkey>> pour la table <<archive>>
CREATE TABLE
annonces=> CREATE VIEW test.all AS SELECT * FROM test.archive UNION ALL
SELECT * FROM test.current ;
CREATE VIEW
let's populate...
annonces=> INSERT INTO test.current SELECT id, description FROM annonces;
INSERT 0 11524
annonces=> INSERT INTO test.archive SELECT id, description FROM
archive_ext;
INSERT 0 437351
annonces=> ANALYZE test.archive;
ANALYZE
annonces=> ANALYZE test.current;
ANALYZE
This is the bookmarks table...
SELECT count(*), list_id FROM bookmarks GROUP BY list_id;
count | list_id
-------+---------
15 | 7
5 | 5
150 | 4
3 | 3
15 | 2
2 | 1
6 | 8
I want to list the stuff I bookmarked :
annonces=> EXPLAIN ANALYZE SELECT * FROM test.current WHERE id IN (SELECT
annonce_id FROM bookmarks WHERE list_id IN ('4'));
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------
Hash Join (cost=6.58..532.84 rows=140 width=203) (actual
time=0.747..5.283 rows=150 loops=1)
Hash Cond: ("outer".id = "inner".annonce_id)
-> Seq Scan on current (cost=0.00..467.24 rows=11524 width=203)
(actual time=0.006..3.191 rows=11524 loops=1)
-> Hash (cost=6.23..6.23 rows=140 width=4) (actual time=0.244..0.244
rows=150 loops=1)
-> HashAggregate (cost=4.83..6.23 rows=140 width=4) (actual
time=0.155..0.184 rows=150 loops=1)
-> Seq Scan on bookmarks (cost=0.00..4.45 rows=150
width=4) (actual time=0.008..0.097 rows=150 loops=1)
Filter: (list_id = 4)
Total runtime: 5.343 ms
(8 lignes)
annonces=> set enable_hashjoin TO 0;
SET
annonces=> EXPLAIN ANALYZE SELECT * FROM test.current WHERE id IN (SELECT
annonce_id FROM bookmarks WHERE list_id IN ('4'));
QUERY PLAN
--------------------------------------------------------------------------------------------------------------------------------
Nested Loop (cost=4.83..824.22 rows=140 width=203) (actual
time=0.219..1.034 rows=150 loops=1)
-> HashAggregate (cost=4.83..6.23 rows=140 width=4) (actual
time=0.158..0.199 rows=150 loops=1)
-> Seq Scan on bookmarks (cost=0.00..4.45 rows=150 width=4)
(actual time=0.011..0.096 rows=150 loops=1)
Filter: (list_id = 4)
-> Index Scan using current_pkey on current (cost=0.00..5.83 rows=1
width=203) (actual time=0.005..0.005 rows=1 loops=150)
Index Cond: (current.id = "outer".annonce_id)
Total runtime: 1.108 ms
(7 lignes)
Hm, the row estimates on the "bookmarks" table are spot on ; why did it
choose the hash join ?
Now, if I want to access the "all" view which contains the union of the
"current" and "archive" table :
annonces=> set enable_hashjoin TO 1;
SET
annonces=> EXPLAIN ANALYZE SELECT * FROM test.all WHERE id IN (SELECT
annonce_id FROM bookmarks WHERE list_id IN ('4'));
QUERY PLAN
--------------------------------------------------------------------------------------------------------------------------------
Hash Join (cost=6.58..33484.41 rows=314397 width=36) (actual
time=8300.484..8311.784 rows=150 loops=1)
Hash Cond: ("outer"."?column1?" = "inner".annonce_id)
-> Append (cost=0.00..23596.78 rows=449139 width=219) (actual
time=6.390..8230.821 rows=448875 loops=1)
-> Seq Scan on archive (cost=0.00..18638.15 rows=437615
width=219) (actual time=6.389..8175.491 rows=437351 loops=1)
-> Seq Scan on current (cost=0.00..467.24 rows=11524 width=203)
(actual time=0.022..8.985 rows=11524 loops=1)
-> Hash (cost=6.23..6.23 rows=140 width=4) (actual time=0.255..0.255
rows=150 loops=1)
-> HashAggregate (cost=4.83..6.23 rows=140 width=4) (actual
time=0.168..0.197 rows=150 loops=1)
-> Seq Scan on bookmarks (cost=0.00..4.45 rows=150
width=4) (actual time=0.015..0.102 rows=150 loops=1)
Filter: (list_id = 4)
Total runtime: 8311.870 ms
(10 lignes)
annonces=> set enable_hashjoin TO 0;
SET
annonces=> EXPLAIN ANALYZE SELECT * FROM test.all WHERE id IN (SELECT
annonce_id FROM bookmarks WHERE list_id IN ('4'));
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------------
Merge Join (cost=79604.61..84994.98 rows=314397 width=36) (actual
time=6944.229..7109.371 rows=150 loops=1)
Merge Cond: ("outer".annonce_id = "inner".id)
-> Sort (cost=11.22..11.57 rows=140 width=4) (actual
time=0.326..0.355 rows=150 loops=1)
Sort Key: bookmarks.annonce_id
-> HashAggregate (cost=4.83..6.23 rows=140 width=4) (actual
time=0.187..0.218 rows=150 loops=1)
-> Seq Scan on bookmarks (cost=0.00..4.45 rows=150
width=4) (actual time=0.028..0.126 rows=150 loops=1)
Filter: (list_id = 4)
-> Sort (cost=79593.40..80716.25 rows=449139 width=36) (actual
time=6789.786..7014.815 rows=448625 loops=1)
Sort Key: "all".id
-> Append (cost=0.00..23596.78 rows=449139 width=219) (actual
time=0.013..391.447 rows=448875 loops=1)
-> Seq Scan on archive (cost=0.00..18638.15 rows=437615
width=219) (actual time=0.013..332.353 rows=437351 loops=1)
-> Seq Scan on current (cost=0.00..467.24 rows=11524
width=203) (actual time=0.013..8.396 rows=11524 loops=1)
Total runtime: 37226.846 ms
The IN() is quite small (150 values), but the two large tables are
seq-scanned... is there a way to avoid this ?
-------------------------------------------------------------------------------------------------------------------------------------
Another nitpick : let's redo the first query differently.
annonces=> EXPLAIN ANALYZE SELECT * FROM test.current WHERE id IN (SELECT
annonce_id FROM bookmarks WHERE list_id IN ('4'));
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------
Hash Join (cost=6.58..532.84 rows=140 width=203) (actual
time=0.794..5.791 rows=150 loops=1)
Hash Cond: ("outer".id = "inner".annonce_id)
-> Seq Scan on current (cost=0.00..467.24 rows=11524 width=203)
(actual time=0.003..3.554 rows=11524 loops=1)
-> Hash (cost=6.23..6.23 rows=140 width=4) (actual time=0.265..0.265
rows=150 loops=1)
-> HashAggregate (cost=4.83..6.23 rows=140 width=4) (actual
time=0.179..0.210 rows=150 loops=1)
-> Seq Scan on bookmarks (cost=0.00..4.45 rows=150
width=4) (actual time=0.021..0.102 rows=150 loops=1)
Filter: (list_id = 4)
Total runtime: 5.853 ms
annonces=> EXPLAIN ANALYZE SELECT a.* FROM test.current a, (SELECT
DISTINCT annonce_id FROM bookmarks WHERE list_id IN ('4')) AS b WHERE
a.id=b.annonce_id;
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------
Hash Join (cost=12.37..538.63 rows=140 width=203) (actual
time=0.812..5.362 rows=150 loops=1)
Hash Cond: ("outer".id = "inner".annonce_id)
-> Seq Scan on current a (cost=0.00..467.24 rows=11524 width=203)
(actual time=0.005..3.227 rows=11524 loops=1)
-> Hash (cost=12.02..12.02 rows=140 width=4) (actual
time=0.296..0.296 rows=150 loops=1)
-> Unique (cost=9.87..10.62 rows=140 width=4) (actual
time=0.215..0.265 rows=150 loops=1)
-> Sort (cost=9.87..10.25 rows=150 width=4) (actual
time=0.215..0.226 rows=150 loops=1)
Sort Key: bookmarks.annonce_id
-> Seq Scan on bookmarks (cost=0.00..4.45 rows=150
width=4) (actual time=0.007..0.104 rows=150 loops=1)
Filter: (list_id = 4)
Total runtime: 5.429 ms
Hm, it does Sort + Unique + Hash ; the Hash alone would have been better.
Replacing DISTINCT with GROUP BY removes the sort.
annonces=> EXPLAIN ANALYZE SELECT a.* FROM test.current a, (SELECT
annonce_id FROM bookmarks WHERE list_id IN ('4') GROUP BY annonce_id) AS b
WHERE a.id=b.annonce_id;
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------
Hash Join (cost=7.98..534.24 rows=140 width=203) (actual
time=0.811..5.557 rows=150 loops=1)
Hash Cond: ("outer".id = "inner".annonce_id)
-> Seq Scan on current a (cost=0.00..467.24 rows=11524 width=203)
(actual time=0.006..3.434 rows=11524 loops=1)
-> Hash (cost=7.63..7.63 rows=140 width=4) (actual time=0.242..0.242
rows=150 loops=1)
-> HashAggregate (cost=4.83..6.23 rows=140 width=4) (actual
time=0.156..0.186 rows=150 loops=1)
-> Seq Scan on bookmarks (cost=0.00..4.45 rows=150
width=4) (actual time=0.008..0.097 rows=150 loops=1)
Filter: (list_id = 4)
Total runtime: 5.647 ms
