我有一个跨数据库的查询...
SELECT
`DM_Server`.`Jobs`.*,
`DM_Server`.servers.Description AS server,
digital_inventory.params,
products.products_id,
products.products_pdfupload,
customers.customers_firstname,
customers.customers_lastname
FROM `DM_Server`.`Jobs`
INNER JOIN `DM_Server`.servers ON servers.ServerID = Jobs.Jobs_ServerID
JOIN `cpod_live`.`digital_inventory` ON digital_inventory.jobname = Jobs.Jobs_Name
JOIN `cpod_live`.`products` ON products.products_pdfupload = CONCAT(digital_inventory.jobname, ".pdf")
JOIN `cpod_live`.`customers` ON customers.customers_id = products.cID
ORDER BY `DM_Server`.`Jobs`.Jobs_StartTime DESC LIMIT 50
一切都很顺利,直到我将它们改为 LEFT JOIN。我认为如果不指定连接类型,就默认为 LEFT JOIN。难道这不是这样吗?
LEFT JOIN
比一个INNER JOIN
慢很多的原因是什么? - Ben