mysql - 在查询中添加第三个表会减慢结果时间

Question

我正在使用一个查询，该查询在一秒钟内给出结果。

查询是，

SELECT creator.first_name, image.id, image.image_code, image.project, image.location, image.image_date, image.image_view, image.copyright 
FROM img_images image, img_creator creator 
WHERE creator.image_id = image.id AND (
(image.image_code LIKE '%text%') 
OR (image.project LIKE '%text%') 
OR (image.location LIKE '%text%')
OR (creator.first_name LIKE '%text%')) 
ORDER BY creator.first_name

现在，如果我在此查询中添加第三个表，大约需要 20 到 30 秒才能给出结果。

查询，

SELECT creator.first_name, image.id, image.image_code, image.project, image.location, image.image_date, image.image_view, image.copyright, sources.author 
FROM img_images image, img_creator creator, img_source sources 
WHERE creator.image_id = image.id AND sources.image_id = image.id AND (
(image.image_code LIKE '%text%') 
OR (image.project LIKE '%text%') 
OR (image.location LIKE '%text%')
OR (creator.first_name LIKE '%text%')) 
ORDER BY creator.first_name

如何优化此查询以快速给出响应？ 所有表中都存在正确的索引。

Answer 1

有时，在没有过滤条件的表上强制左连接有助于 RDBMS 选择更好的计划:

SELECT creator.first_name, image.id, image.image_code, image.project, image.location,
       image.image_date, image.image_view, image.copyright, sources.author
FROM img_images image
JOIN img_creator creator ON (creator.image_id = image.id OR creator.first_name LIKE '%text%')
LEFT JOIN img_source sources ON (sources.image_id = image.id)
WHERE image.image_code LIKE '%text%' OR image.project LIKE '%text%' OR image.location LIKE '%text%'
ORDER BY creator.first_name;

此外，当结果集中的行数预计较小，但查询很复杂和/或涉及大型表时，我发现值得尝试单独查找单个值:

SELECT creator.first_name, image.id, image.image_code, image.project, image.location,
       image.image_date, image.image_view, image.copyright
       (SELECT author FROM img_source WHERE image_id = image.id) AS author
FROM img_images image
JOIN img_creator creator ON (creator.image_id = image.id OR creator.first_name LIKE '%text%')
WHERE image.image_code LIKE '%text%' OR image.project LIKE '%text%' OR image.location LIKE '%text%'
ORDER BY creator.first_name;