我有一個查詢幾乎用了 22 秒來填充資料。
SELECT
TP.*,
GROUP_CONCAT(DISTINCT TS.`subject_name`) AS subjects,
GROUP_CONCAT(DISTINCT TC.`class_name`) AS classes,
GROUP_CONCAT(DISTINCT TT.`tution_name`) AS tution_type,
GROUP_CONCAT(DISTINCT TL.`name`) AS locations
FROM `tutor_profile` TP
LEFT JOIN `tutor_to_subject` TTS ON TP.`tutor_id`=TTS.`tutor`
LEFT JOIN `tutor_subjects` TS ON TS.`subject_id`=TTS.`subject`
LEFT JOIN `tutor_to_class` TTC ON TP.`tutor_id`=TTC.`tutor`
LEFT JOIN `tutor_classes` TC ON TC.`class_id`=TTC.`class`
LEFT JOIN `tutor_to_tution_type` TTTT ON TP.`tutor_id`=TTTT.`tutor`
LEFT JOIN `tution_types` TT ON TT.`tution_id`=TTTT.`tution_type`
LEFT JOIN `tutor_to_locality` TTL ON TP.`tutor_id`=TTL.`tutor`
LEFT JOIN `tutor_locality` TL ON TL.`id`=TTL.`locality`
WHERE 1=1 AND TP.`status` = 1
GROUP BY TP.`tutor_id`
ORDER BY TP.`date_added` DESC LIMIT 0 , 25
有沒有辦法提高它的性能？
uj5u.com熱心網友回復：
您可以嘗試在輸出串列中使用相關子查詢而不是 JOIN 和 GROUP BY：
SELECT
TP.*,
( SELECT GROUP_CONCAT(DISTINCT TS.`subject_name`)
FROM `tutor_to_subject` TTS
LEFT JOIN `tutor_subjects` TS ON TS.`subject_id`=TTS.`subject`
WHERE TP.`tutor_id`=TTS.`tutor` ) AS subjects,
-- the same for another output columns
FROM `tutor_profile` TP
WHERE 1=1 AND TP.`status` = 1
ORDER BY TP.`date_added` DESC LIMIT 0 , 25
uj5u.com熱心網友回復：
替換這些
GROUP_CONCAT(DISTINCT TS.`subject_name`) AS subjects,
LEFT JOIN `tutor_to_subject` TTS ON TP.`tutor_id`=TTS.`tutor`
LEFT JOIN `tutor_subjects` TS ON TS.`subject_id`=TTS.`subject`
和
( SELECT GROUP_CONCAT(DISTINCT TS.`subject_name`
FROM `tutor_to_subject` TTS
JOIN `tutor_subjects` TS ON TS.`subject_id` = TTS.`subject`
WHERE TP.`tutor_id` = TTS.`tutor`
) AS subjects,
并將 JOIN 用于TS. 對其他 3 個 group_concat Left join 執行同樣的操作。
對于多對多表，改進索引：
CREATE TABLE tutor_to_subject
tutor_id ...,
subject_id ...,
PRIMARY KEY(tutor_id, subject_id),
INDEX(subject_id)
) ENGINE=InnoDB;
更多討論：多 ：多映射
此外，擺脫 ，GROUP BY因為它不再需要。
并添加
INDEX(status, date_added)
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