SQL优化

LIMIT语句优化

原语句：SELECT * FROM table ORDER BY id DESC LIMIT 190000,10000

优化后：SELECT * FROM table WHERE id < (SELECT MIN(alias.id) FROM (SELECT id FROM table ORDER BY id DESC LIMIT 190000) AS alias) ORDER BY id DESC LIMIT 10000

      或：SELECT * FROM table WHERE id <= (SELECT id FROM djqk ORDER BY id DESC LIMIT 190000,1 ) ORDER BY id DESC LIMIT 10000

COUNT函数优化

SELECT COUNT(*) 语法在没有where条件的语句中执行效率没有SELECT COUNT(col_name)快，但是在有where条件的语句中执行效率要快。