MySQL 执行计划里的rows

<pre name="code" class="html">SQL> alter session set statistics_level=all;

Session altered.

SQL> select e.ename,e.job,d.dname from emp e,dept d  where e.deptno=d.deptno and e.sal<2000;
-------------------省略输出结果----------------------
8 rows selected.

SQL> select * from table(dbms_xplan.display_cursor(null,null,'ALLSTATS LAST'));

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------
SQL_ID  70xt266nm9y32, child number 0
-------------------------------------
select e.ename,e.job,d.dname from emp e,dept d  where e.deptno=d.deptno and e.sal<2000

Plan hash value: 351108634
--------------------------------------------------------------------------------------------------
| Id  | Operation                    | Name    | Starts | E-Rows | A-Rows |   A-Time   | Buffers |
--------------------------------------------------------------------------------------------------
|   1 |  NESTED LOOPS                |         |      1 |      4 |      8 |00:00:00.01 |      18 |
|*  2 |   TABLE ACCESS FULL          | EMP     |      1 |      4 |      8 |00:00:00.01 |       8 |
|   3 |   TABLE ACCESS BY INDEX ROWID| DEPT    |      8 |      1 |      8 |00:00:00.01 |      10 |
|*  4 |    INDEX UNIQUE SCAN         | PK_DEPT |      8 |      1 |      8 |00:00:00.01 |       2 |
--------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("E"."SAL"<2000)
4 - access("E"."DEPTNO"="D"."DEPTNO")
21 rows selected.

NESTED LOOPS表示嵌套循环，Starts表示某个操作执行的次数，那么这里嵌套循环操作一共执行了1次，EMP表离NESTED LOOPS最近，表示EMP表作为驱动表，
它执行了1次(全表扫描)返回了8条记录。Oracle这时候就会从这8条记录中读取一行，然后去匹配内部表，
因为驱动表返回了8条记录，所以内部循环会执行8次。

技巧1：查看真实基数

比如下面这个SQL

SQL> explain plan for SELECT UOM.PROD_SKID,
2         MAX (UOM.RELTV_CURR_QTY) RELTV_CURR_QTY,
3         MAX (UOM.STAT_CURR_VAL) STAT_CURR_VAL,
4         MAX (UOM.BAR_CURR_CODE) BAR_CURR_CODE
5  --modified by Daniel on 10/26/2010 for CR 326285 of QMR10.1 hypercare retrofit begin
6  FROM OPT_REF_UOM_TEMP_SDIM UOM,
7       REF_PROD_DIM PROD
8  WHERE UOM.RELTV_CURR_QTY = 1
9        AND PROD.CURR_IND = 'Y'
10        AND PROD.PROD_END_DATE = TO_DATE ('31-12-9999', 'dd-mm-yyyy')
11        AND PROD.PROD_SKID = UOM.PROD_SKID
12        AND PROD.BUOM_CURR_SKID = UOM.UOM_SKID
13  --modified by Daniel on 10/26/2010 for CR 326285 of QMR10.1 hypercare retrofit end
14  GROUP BY UOM.PROD_SKID;

Explained.

SQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT
-------------------------------------------------------------------------------------------------------

Plan hash value: 3215660883

-------------------------------------------------------------------------------------------------------
| Id  | Operation                     | Name                  | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT              |                       |    78 |  4212 | 15507   (1)| 00:01:47 |
|   1 |  HASH GROUP BY                |                       |    78 |  4212 | 15507   (1)| 00:01:47 |
|   2 |   NESTED LOOPS                |                       |       |       |            |          |
|   3 |    NESTED LOOPS               |                       |  3034 |   159K| 15506   (1)| 00:01:47 |
|*  4 |     TABLE ACCESS FULL         | OPT_REF_UOM_TEMP_SDIM |  2967 |   101K|   650  (14)| 00:00:05 |
|*  5 |     INDEX RANGE SCAN          | PROD_DIM_PK           |     3 |       |     2   (0)| 00:00:01 |
|*  6 |    TABLE ACCESS BY INDEX ROWID| PROD_DIM              |     1 |    19 |     5   (0)| 00:00:01 |
-------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

4 - filter("UOM"."RELTV_CURR_QTY"=1)
5 - access("PROD"."PROD_SKID"="UOM"."PROD_SKID")
6 - filter("PROD"."BUOM_CURR_SKID" IS NOT NULL AND "PROD"."PROD_END_DATE"=TO_DATE('
9999-12-31 00:00:00', 'syyyy-mm-dd hh24:mi:ss') AND "PROD"."CURR_IND"='Y' AND
"PROD"."BUOM_CURR_SKID"="UOM"."UOM_SKID")

22 rows selected.

请看ID=4这步，CBO估算它只返回2967行数据。ID=4这步前面有*，表示它有谓词过滤。
4 - filter("UOM"."RELTV_CURR_QTY"=1)

UOM是表OPT_REF_UOM_TEMP_SDIM 的别名

那么我们手工计算它应该返回的真实基数：

SQL> select count(*) from OPT_REF_UOM_TEMP_SDIM where "RELTV_CURR_QTY"=1;

COUNT(*)
----------
946432

CBO估算它只返回2967条数据，但是它应该返回94W条数据，这里相差太大了，说明执行计划走错了，它后面的索引会被扫描94W次。

Oracle里的Rows 表示CBO估算它返回的行数
Rows(E-Rows)     预估操作返回的记录条数

在MySQL里呢？

mysql> explain select t1.* from t2 ,t1 where t2.id=t1.id;
+----+-------------+-------+------+---------------+------+---------+------+------+----------------------------------------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra                                              |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------------------------------------------+
|  1 | SIMPLE      | t2    | ALL  | NULL          | NULL | NULL    | NULL |    2 | NULL                                               |
|  1 | SIMPLE      | t1    | ALL  | NULL          | NULL | NULL    | NULL |    3 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------------------------------------------+
2 rows in set (0.00 sec)

rows	rows	Estimate of rows to be examined   评估要检查的行

ows
表示MySQL根据表统计信息及索引选用情况，估算的找到所需的记录所需要读取的行数