小编niz*_*zha的帖子

我有 2 种不同的方式来查询在执行时表现出性能差异的内容。第一种方法是

EXPLAIN select SOME_COLUMNS
from
( select *
  from A
  where CONDITION
) p
inner join
( select *
  from B
)st
on p.id = st.id;

此查询的输出返回：

"id"   "select_type"   "table"    "type"   "possible_keys"   "key"    "key_len"   "ref"   "rows"   "Extra"

 1       PRIMARY     derived3  ALL       NULL                 NULL    NULL    NULL      25607     "   "
 1        PRIMARY    derived2  ALL        NULL                NULL     NULL    NULL      21037   Using where; Using join buffer
 3        DERIVED             A    ALL          NULL               NULL      NULL    NULL      23202    "   "
 2        DERIVED             B    ref        IDX_A_TYPE_ID …