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2025-04-06 Update From: SLTechnology News&Howtos shulou NAV: SLTechnology News&Howtos > Database >
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Simple query, return the same, use charge_id to associate, only 0.5s, but if you use order_id, it takes 18s! Why?
When using order_id, the execution plan uses Using join buffer (Block Nested Loop); the reason is ascertained: the order_id character set in order_forInit is utf8, and the order_id character set in order_item_forInit is utf8mb4, different character sets cause two join when you can't use the index, there will be "Using join buffer (Block Nested Loop)". Change the order_id character set in order_forInit to utf8mb4, and there will be no performance problem! Using join buffer (Block Nested Loop) will not appear
Explain
Select count (*) from
Order_forInit a
Order_item_forInit c
Product d
WHERE
-- a.order_id = c.order_id
A.charge_id = c.charge_id
AND c.product_id = d. Product_id
Appendix:
The difference between the mysql character set utf8 and utf8mb4: https://blog.csdn.net/qq_37054881/article/details/90023611
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