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以本人愚见，MySQL对语句的执行是有优化的。MySQL会对执行的语句进行处理，生成多种执行方案，然后选取成本最低的方案。下面是两条语句的执行计划：

mysql> explain SELECT * FROM user WHERE  id = 1088248166370832385 and deleted =0\G

*************************** 1. row ***************************

id: 1

select_type: SIMPLE

table: user

partitions: NULL

type: const

possible_keys: PRIMARY

key: PRIMARY

key_len: 8

ref: const

rows: 1

filtered: 100.00

Extra: NULL

mysql> explain SELECT * FROM user WHERE deleted=0 AND id = 1088248166370832385\G

*************************** 1. row ***************************

id: 1

select_type: SIMPLE

table: user

partitions: NULL

type: const

possible_keys: PRIMARY

key: PRIMARY

key_len: 8

ref: const

rows: 1

filtered: 100.00

Extra: NULL

两个的执行计划是一样的。它们的效率应该会相等吧。