mysqlselect优化数据update select
请教下各位大神，mysq update数据优化
遇到的问题是这样的。
表a有2个字段，ready_count,download_count
表b是统计表，有type,id
两个表的储存引擎都是MYISAM
需要把表b的数据count(1) group by(type)之后update到表a的那两个字段中。
我现在的SQL是这样的：
update a _t1
set
_t1.ready_count = (
SELECT count(1) FROM b _t2 WHERE _t2.tid = _t1.id and _t2.type = 1
),
_t1.download_count = (
SELECT count(1) FROM b _t2 WHERE _t2.tid = _t1.id and _t2.type = 2
),
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