MySQL的EXISTS（）很慢

這個查詢MySQL的EXISTS（）很慢

SELECT 
    itemID, 
    locationParentID, 
    locationID, 
    categoryParentID, 
    categoryID, 
    itemTitle, 
    itemDetails, 
    itemAttributes, 
    itemPictures, 
    itemFeatured, 
    itemSpotlight, 
    itemAdded 
FROM items 
WHERE items.siteID IN('".$cfg['site']['siteShares']."') 
    AND EXISTS(SELECT 1 FROM sites_locations sl WHERE items.locationID = sl.locationID AND siteID = '".$cfg['site']['siteID']."' LIMIT 1) 
    AND EXISTS(SELECT 1 FROM sites_categories sc WHERE items.categoryID = sc.categoryID AND siteID = '".$cfg['site']['siteID']."' LIMIT 1) 
    AND itemStatus = '1' 
    AND itemAdded > '".$cfg['timestamp']."'

的作品，但它需要高達6秒

因爲我猜它會更快，我可以用JOIN做

這裏就是我嘗試過JOIN，但它會返回所有結果，而不僅僅是在sites_location表中具有位置的項目。

SELECT 
    itemID, 
    locationParentID, 
    categoryParentID, 
    categoryID, 
    itemTitle, 
    itemDetails, 
    itemAttributes, 
    itemPictures, 
    itemFeatured, 
    itemSpotlight, 
    itemAdded 
FROM items LEFT JOIN sites_locations 
    ON items.locationID = sites_locations.locationID 
    AND sites_locations.siteID = items.siteID 
WHERE items.siteID IN('1,2') AND itemStatus = '1' 
    AND itemAdded > '1356048000' ORDER BY itemID DESC LIMIT 15

來源

2013-02-22 Ian Wilkinson

爲什麼要使用在所有的存在？看起來像一個普通的交叉連接（即使是天生的）會在這裏排序 – Najzero 2013-02-22 10:28:10

你的索引是什麼？ – h2ooooooo 2013-02-22 10:28:10

items table primary = itemID; sites_locations表中的主要auto和locationID是關係; – 2013-02-22 10:32:53