mysql之 rand 隨機查詢記錄效率
SELECT * FROM `table` ORDER BY RAND() LIMIT 5 不推薦
一個15萬餘條的庫,查詢5條數據,居然要8秒以上
官方手冊:You cannot use a column with RAND() values in an ORDER BY clause, because ORDER BY would evaluate the column multiple times.
搜索Google:
SELECT *
FROM `table` AS t1 JOIN (SELECT ROUND(RAND() * (SELECT MAX(id) FROM `table`)) AS id) AS t2
WHERE t1.id >= t2.id
ORDER BY t1.id ASC LIMIT 5;
產生連續的5條記錄。解決辦法只能是每次查詢一條,查詢5次;15萬條的表,查詢只需要0.01秒不到
MySQL的論壇:
SELECT *
FROM `table`
WHERE id >= (SELECT FLOOR( MAX(id) * RAND()) FROM `table` )
ORDER BY id LIMIT 1;
需要0.5秒,速度不錯
改寫:
SELECT * FROM `table`
WHERE id >= (SELECT floor(RAND() * (SELECT MAX(id) FROM `table`)))
ORDER BY id LIMIT 1;
效率提高,只有0.01秒
加上MIN(id)的判斷:
SELECT * FROM `table`
WHERE id >= (SELECT floor( RAND() * ((SELECT MAX(id) FROM `table`)-(SELECT MIN(id) FROM `table`)) + (SELECT MIN(id) FROM `table`)))
ORDER BY id LIMIT 1;
SELECT *
FROM `table` AS t1 JOIN (SELECT ROUND(RAND() * ((SELECT MAX(id) FROM `table`)-(SELECT MIN(id) FROM `table`))+(SELECT MIN(id) FROM `table`)) AS id) AS t2
WHERE t1.id >= t2.id
ORDER BY t1.id LIMIT 1;
最後在php中對這兩個語句進行分別查詢10次,
前者花費時間 0.147433 秒
後者花費時間 0.015130 秒
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