我有一个表，让我们称之为“foos”，有近600万条记录。我运行以下查询：
SELECT "foos".*
FROM "foos"
INNER JOIN "bars" ON "foos".bar_id = "bars".id
WHERE (("bars".baz_id = 13266))
ORDER BY "foos"."id" DESC
LIMIT 5 OFFSET 0;
此查询需要很长时间才能运行(Rails在运行时超时)。有问题的所有ID都有索引。好奇的部分是，如果我删除ORDER BY子句或LIMIT子句，它几乎立即运行。
我假设，ORDER BY和LIMIT的存在使得Postgresql在查询计划中做出一些不好的选择。任何人都有任何想法如何解决这个问题？
如果它有帮助，这里是所有3种情况的EXPLAIN：
//////// Both ORDER and LIMIT
SELECT "foos".*
FROM "foos"
INNER JOIN "bars" ON "foos".bar_id = "bars".id
WHERE (("bars".baz_id = 13266))
ORDER BY "foos"."id" DESC
LIMIT 5 OFFSET 0;
QUERY PLAN
--------------------------------------------------------------------------------------------------------------------
Limit (cost=0.00..16663.44 rows=5 width=663)
-> Nested Loop (cost=0.00..25355084.05 rows=7608 width=663)
Join Filter: (foos.bar_id = bars.id)
-> Index Scan Backward using foos_pkey on foos (cost=0.00..11804133.33 rows=4963477 width=663)
Filter: (((NOT privacy_protected) OR (user_id = 67962)) AND ((status)::text = 'DONE'::text))
-> Materialize (cost=0.00..658.96 rows=182 width=4)
-> Index Scan using index_bars_on_baz_id on bars (cost=0.00..658.05 rows=182 width=4)
Index Cond: (baz_id = 13266)
(8 rows)
//////// Just LIMIT
SELECT "foos".*
FROM "foos"
INNER JOIN "bars" ON "foos".bar_id = "bars".id
WHERE (("bars".baz_id = 13266))
LIMIT 5 OFFSET 0;
QUERY PLAN
---------------------------------------------------------------------------------------------------------------------------------------
Limit (cost=0.00..22.21 rows=5 width=663)
-> Nested Loop (cost=0.00..33788.21 rows=7608 width=663)
-> Index Scan using index_bars_on_baz_id on bars (cost=0.00..658.05 rows=182 width=4)
Index Cond: (baz_id = 13266)
-> Index Scan using index_foos_on_bar_id on foos (cost=0.00..181.51 rows=42 width=663)
Index Cond: (foos.bar_id = bars.id)
Filter: (((NOT foos.privacy_protected) OR (foos.user_id = 67962)) AND ((foos.status)::text = 'DONE'::text))
(7 rows)
//////// Just ORDER
SELECT "foos".*
FROM "foos"
INNER JOIN "bars" ON "foos".bar_id = "bars".id
WHERE (("bars".baz_id = 13266))
ORDER BY "foos"."id" DESC;
QUERY PLAN
---------------------------------------------------------------------------------------------------------------------------------------
Sort (cost=36515.17..36534.19 rows=7608 width=663)
Sort Key: foos.id
-> Nested Loop (cost=0.00..33788.21 rows=7608 width=663)
-> Index Scan using index_bars_on_baz_id on bars (cost=0.00..658.05 rows=182 width=4)
Index Cond: (baz_id = 13266)
-> Index Scan using index_foos_on_bar_id on foos (cost=0.00..181.51 rows=42 width=663)
Index Cond: (foos.bar_id = bars.id)
Filter: (((NOT foos.privacy_protected) OR (foos.user_id = 67962)) AND ((foos.status)::text = 'DONE'::text))
(8 rows)
