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关于 MySQL 函数效率的疑问
source link: https://www.v2ex.com/t/816330
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在清理公司旧代码时顺便做点性能优化，发现公司里有很多业务在用
select date_format(create_time, '%Y-%m-%d') dateDay, count(*) count
from table
GROUP BY date_format(create_time, '%Y-%m-%d')
ORDER BY date_format(create_time, '%Y-%m-%d')
这样的语句全表扫描+日期转换。一个 47 万行的表这么跑一遍差不多 550-600ms 。而当我用 date(create_time) 换掉之后，耗时就会降到 230-300ms 。类似的还有用 CONCAT(YEAR(create_time), '-', MONTH(update_time)) dateMonth 替换 date_format(create_time, '%Y-%m') 之后，耗时也能减少 30%以上。对于 mysql 来说， date 这样的函数是比 date_format 的效率高出非常多的么？
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