我有一張表,我們稱它爲「foos」,其中有近600萬條記錄。我正在運行以下查詢:使用ORDER和LIMIT子句的極慢PostgreSQL查詢
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;
此查詢需要很長時間才能運行(Rails在運行時超時)。有問題的所有ID的索引。好奇的是,如果我刪除ORDER BY
子句或LIMIT
子句,它幾乎是瞬間運行。
我假設ORDER BY
和LIMIT
的存在使得PostgreSQL在查詢計劃中做出了一些不好的選擇。任何人有任何想法如何解決這個問題?
萬一有幫助,這裏是EXPLAIN
所有三種情況:
//////// 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)
+1有趣的問題。 – 2011-05-17 22:25:43
您的查詢與您的查詢計劃不符。如果您需要幫助,至少要提供完整的相關詳細信息... – 2011-05-18 02:11:34
對不同的查詢/計劃感到抱歉;我試圖混淆一下,但回想起來,我不知道爲什麼。我將在明天更新實際的查詢和計劃。 – jakeboxer 2011-05-18 05:36:52