Question

row_examined是否只检查PK索引（索引列）？或者所有行数据都要检查？

我有一张带有PK和UNIQUE KEY的表

   show index from  test_table;
+-----------------+------------+----------------+--------------+--------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+    
| Table           | Non_unique | Key_name       | Seq_in_index | Column_name        | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-----------------+------------+----------------+--------------+--------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| test_table      |          0 | PRIMARY        |            1 | test_table_PK      | A         |      105464 |     NULL | NULL   |      | BTREE      |         |               |
| test_table      |          0 | test_table_UNQ |            1 | test_table_UNQ     | A         |      105464 |     NULL | NULL   |      | BTREE      |         |               |
+-----------------+------------+----------------+--------------+--------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

通过test_table_PK限制70000,10从test_table组中选择*;

结果：

         id: 1
select_type: SIMPLE
      table: test_table
       type: index
possible_keys: NULL
      key: PRIMARY
  key_len: 8
      ref: NULL
     rows: 70010
    Extra:

我的问题是：

在上面的查询结果中为什么possible_keys为空？甚至表中都包含两个键。

为什么mysql检查了70010条记录？

如何减少rows_examined的数量？

下面是实际查询，大约需要13秒才能执行。

EXPLAIN EXTENDED select * from test_table

LEFT JOIN rel_test_table_1

ON 
     test_table.test_table_PK = rel_test_table_1.test_table_PK
LEFT JOIN test_table_2
ON 
    rel_test_table_1.test_table_2_PK= test_table_2.test_table_2_PK
LEFT JOIN rel_test_table_2
ON
     test_table_2.test_table_2_PK = rel_test_table_2.test_table_2_PK
LEFT JOIN test_table_3
ON 
    rel_test_table_2.test_table_3_PK = test_table_3.test_table_3_PK 
limit 70000,10\G

查询结果 *********************** 1.排 ************** *********

       id: 1
select_type: SIMPLE
    table: test_table
     type: ALL
possible_keys: NULL
      key: NULL
  key_len: NULL
      ref: NULL
     rows: 105578
  filtered: 100.00
    Extra:

*********************** 2.排 *********** ************

       id: 1
 select_type: SIMPLE
    table: rel_test_table_1
     type: eq_ref
 possible_keys: test_table_PK,rel_test_table_1_FK1_IDX
      key: test_table_PK
  key_len: 8
      ref: db2486db.test_table.test_table_PK
     rows: 1
 filtered: 100.00
    Extra:

*********************** 3.排 *********** ************

       id: 1
 select_type: SIMPLE
    table: test_table_2
     type: eq_ref
 possible_keys: PRIMARY
      key: PRIMARY
  key_len: 8
      ref: db2486db.rel_test_table_1.test_table_2_PK
     rows: 1
 filtered: 100.00
    Extra:

*********************** 4.排 *********** ************

       id: 1
 select_type: SIMPLE
    table: rel_test_table_2
     type: ref
 possible_keys: PRIMARY,test_table_PK,rel_test_table_2_FK1_IDX
      key: PRIMARY
  key_len: 8
      ref: db2486db.test_table.test_table_PK
     rows: 1
 filtered: 100.00
    Extra: Using index

*********************** 5.排 *********** ************

       id: 1
 select_type: SIMPLE
    table: test_table_3
     type: eq_ref
possible_keys: PRIMARY
      key: PRIMARY
  key_len: 8
      ref: db2486db.rel_test_table_2.test_table_PK
     rows: 1
 filtered: 100.00
    Extra:

Rows_examined在MySql中的意思是什么

0 个答案: