数据库中plan_hash_value有什么用

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plan_hash_value
 
判断sql的访问路径是否改变的主要方法是:v$sql.plan_hash_value的值是否改变。如果不同的sql语句含有相同的

实验：
---创建表dept
SQL> create table dept as select * from scott.dept;

Table created.

---执行2条sql查询dept表
SQL> select deptno,dname from dept where deptno=10;

    DEPTNO DNAME
---------- --------------
        10 ACCOUNTING

SQL> select deptno,dname from dept;

    DEPTNO DNAME
---------- --------------
        10 ACCOUNTING
        20 RESEARCH
        30 SALES
        40 OPERATIONS

---通过v$sql查询关于dept的sql的address,hash_value,child_number,plan_hash_value,sql_text
col SQL_TEXT for a100  
select address,hash_value,child_number,plan_hash_value,sql_text from v$sql where sql_text like '%DEPT%';

ADDRESS          HASH_VALUE CHILD_NUMBER PLAN_HASH_VALUE SQL_TEXT
---------------- ---------- ------------ --------------- ----------------------------------------------------------------------------------------------------
000000008B589B40 3749466620            0       315352865 SELECT NVL(SUM(C1),0), NVL(SUM(C
                                                         2),0) FROM (SELECT 1 AS C1, 1 AS C
                                                         2 FROM "SYS"."DEPT" "DEPT") SAMPLESUB

0000000061211A40 2958346034            0      2657262937 select ADDRESS, HASH_VALUE, SQL_ID, PLAN_HASH_VALUE, CHILD_ADDRESS,          CHILD_NUMBER,  TIMESTAM
                                                         P, OPERATION,                                         OPTIONS, OBJECT_NODE, OBJECT#, OBJECT_OWNER, O
                                                         BJECT_NAME,                    OBJECT_ALIAS, OBJECT_TYPE, OPTIMIZER,
                                                                 ID, PARENT_ID, DEPTH, POSITION, SEARCH_COLUMNS, COST, CARDINALITY,           BYTES, OTHER_TA
                                                         G, PARTITION_START, PARTITION_STOP, PARTITION_ID,             OTHER, DISTRIBUTION, CPU_COST, IO_COST
                                                         , TEMP_SPACE,                          ACCESS_PREDICATES, FILTER_PREDICATES, PROJECTION, TIME, QBLOC
                                                         K_NAME,         REMARKS, OTHER_XML                                                           from GV
                                                         $SQL_PLAN                                                             where inst_id = USERENV('Insta
                                                         nce')

000000008B5D3908 3410315986            0       903671040 select address,hash_value,child_number,plan_hash_value,sql_text from v$sql where sql_text like '%DEP
                                                         T%'

000000008B626668 3145184715            0       315352865 SELECT NVL(SUM(C1),0), NVL(SUM(C
                                                         2),0) FROM (SELECT 1 AS C1, CASE WHEN "DEPT"."DEPTNO"=10 THEN 1 ELSE 0 END AS C2 FROM "SYS"."DEPT" "DEPT") SAMPLES
                                                         UB
                                                         
---通过v$sql_plan查询执行计划                                                         
SQL>  SELECT operation, options, object_name, cost  FROM v$sql_plan  WHERE address ='000000008B589B40' and HASH_VALUE='3749466620';

OPERATION                                                    OPTIONS                                                      OBJECT_NAME                          COST
------------------------------------------------------------ ------------------------------------------------------------
SELECT STATEMENT                                                                                                                                                  2
SORT                                                         AGGREGATE
TABLE ACCESS                                                 FULL                                                         DEPT                                    2

SQL>
SQL> SELECT operation, options, object_name, cost  FROM v$sql_plan  WHERE address ='000000008B626668' and HASH_VALUE='3145184715';

OPERATION                                                    OPTIONS                                                      OBJECT_NAME                          COST
------------------------------------------------------------ ------------------------------------------------------------ ------------------------------ ----------
SELECT STATEMENT                                                                                                                                                  2
SORT                                                         AGGREGATE
TABLE ACCESS                                                 FULL                                                         DEPT                                    2
查询的执行计划完全一样


--对表dept做修改，增加索引
SQL> create index idx_dept_deptno on dept(deptno);

Index created.

---再次查看
SQL> select address,hash_value,child_number,plan_hash_value,sql_text from v$sql where sql_text like '%DEPT%';

ADDRESS          HASH_VALUE CHILD_NUMBER PLAN_HASH_VALUE SQL_TEXT
---------------- ---------- ------------ --------------- ----------------------------------------------------------------------------------------------------
000000008B589B40 3749466620            0       315352865 SELECT NVL(SUM(C1),0), NVL(SUM(C
                                                         2),0) FROM (SELECT 1 AS C1, 1 AS C
                                                         2 FROM "SYS"."DEPT" "DEPT") SAMPLESUB

0000000061211A40 2958346034            0      2657262937 select ADDRESS, HASH_VALUE, SQL_ID, PLAN_HASH_VALUE, CHILD_ADDRESS,          CHILD_NUMBER,  TIMESTAM
                                                         P, OPERATION,                                         OPTIONS, OBJECT_NODE, OBJECT#, OBJECT_OWNER, O
                                                         BJECT_NAME,                    OBJECT_ALIAS, OBJECT_TYPE, OPTIMIZER,
                                                                 ID, PARENT_ID, DEPTH, POSITION, SEARCH_COLUMNS, COST, CARDINALITY,           BYTES, OTHER_TA
                                                         G, PARTITION_START, PARTITION_STOP, PARTITION_ID,             OTHER, DISTRIBUTION, CPU_COST, IO_COST
                                                         , TEMP_SPACE,                          ACCESS_PREDICATES, FILTER_PREDICATES, PROJECTION, TIME, QBLOC
                                                         K_NAME,         REMARKS, OTHER_XML                                                           from GV
                                                         $SQL_PLAN                                                             where inst_id = USERENV('Insta
                                                         nce')

000000008B5D3908 3410315986            0       903671040 select address,hash_value,child_number,plan_hash_value,sql_text from v$sql where sql_text like '%DEP
                                                         T%'

000000008E0A74E0 1470990285            0               0 LOCK TABLE "DEPT" IN SHARE MODE  NOWAIT
000000008B626668 3145184715            0       315352865 SELECT NVL(SUM(C1),0), NVL(SUM(C
                                                         2),0) FROM (SELECT 1 AS C1, CASE WHEN "DEPT"."DEPTNO"=10 THEN 1 ELSE 0 END AS C2 FROM "SYS"."DEPT" "DEPT") SAMPLES
                                                         UB

----再次执行上述相同的2条sql
SQL> select deptno,dname from dept where deptno=10;

    DEPTNO DNAME
---------- --------------
        10 ACCOUNTING

SQL> select deptno,dname from dept;

    DEPTNO DNAME
---------- --------------
        10 ACCOUNTING
        20 RESEARCH
        30 SALES
        40 OPERATIONS
        
---查询v$sql下的address,hash_value,child_number,plan_hash_value,sql_text
SQL> select address,hash_value,child_number,plan_hash_value,sql_text from v$sql where sql_text like '%DEPT%';

ADDRESS          HASH_VALUE CHILD_NUMBER PLAN_HASH_VALUE SQL_TEXT
---------------- ---------- ------------ --------------- ----------------------------------------------------------------------------------------------------
000000008B589B40 3749466620            0       315352865 SELECT NVL(SUM(C1),0), NVL(SUM(C
                                                         2),0) FROM (SELECT 1 AS C1, 1 AS C
                                                         2 FROM "SYS"."DEPT" "DEPT") SAMPLESUB

0000000061211A40 2958346034            0      2657262937 select ADDRESS, HASH_VALUE, SQL_ID, PLAN_HASH_VALUE, CHILD_ADDRESS,          CHILD_NUMBER,  TIMESTAM
                                                         P, OPERATION,                                         OPTIONS, OBJECT_NODE, OBJECT#, OBJECT_OWNER, O
                                                         BJECT_NAME,                    OBJECT_ALIAS, OBJECT_TYPE, OPTIMIZER,
                                                                 ID, PARENT_ID, DEPTH, POSITION, SEARCH_COLUMNS, COST, CARDINALITY,           BYTES, OTHER_TA
                                                         G, PARTITION_START, PARTITION_STOP, PARTITION_ID,             OTHER, DISTRIBUTION, CPU_COST, IO_COST
                                                         , TEMP_SPACE,                          ACCESS_PREDICATES, FILTER_PREDICATES, PROJECTION, TIME, QBLOC
                                                         K_NAME,         REMARKS, OTHER_XML                                                           from GV
                                                         $SQL_PLAN                                                             where inst_id = USERENV('Insta
                                                         nce')

000000008B5D3908 3410315986            0       903671040 select address,hash_value,child_number,plan_hash_value,sql_text from v$sql where sql_text like '%DEP
                                                         T%'

000000008E0A74E0 1470990285            0               0 LOCK TABLE "DEPT" IN SHARE MODE  NOWAIT
000000008B626668 3145184715            0       315352865 SELECT NVL(SUM(C1),0), NVL(SUM(C
                                                         2),0) FROM (SELECT 1 AS C1, CASE WHEN "DEPT"."DEPTNO"=10 THEN 1 ELSE 0 END AS C2 FROM "SYS"."DEPT" "DEPT") SAMPLES
                                                         UB

00000000613ACE30 1756886759            0      2570986044 SELECT NVL(SUM(C1),0), NVL(SUM(C2),0), NVL(SUM(C3),0
                                                         ) FROM (SELECT 1
                                                         AS C1, 1 AS C2, 1 AS C3  FROM "SYS"."DEPT" "DEPT" WHERE "DEPT"."DEPTNO"=10 AND ROWNUM <= 2500) SAMPL
                                                         ESUB


6 rows selected.

---再次查看执行计划
SQL>   SELECT operation, options, object_name, cost  FROM v$sql_plan  WHERE address ='000000008B589B40' and HASH_VALUE='3749466620';

OPERATION                                                    OPTIONS                                                      OBJECT_NAME                          COST
------------------------------------------------------------ ------------------------------------------------------------ ------------------------------ ----------
SELECT STATEMENT                                                                                                                                                  2
SORT                                                         AGGREGATE
TABLE ACCESS                                                 FULL                                                         DEPT                                    2

SQL> SELECT operation, options, object_name, cost  FROM v$sql_plan  WHERE address ='000000008B626668' and HASH_VALUE='3145184715';

OPERATION                                                    OPTIONS                                                      OBJECT_NAME                          COST
------------------------------------------------------------ ------------------------------------------------------------ ------------------------------ ----------
SELECT STATEMENT                                                                                                                                                  2
SORT                                                         AGGREGATE
TABLE ACCESS                                                 FULL                                                         DEPT                                    2

SQL> SELECT operation, options, object_name, cost  FROM v$sql_plan  WHERE address ='00000000613ACE30' and HASH_VALUE='1756886759';

OPERATION                                                    OPTIONS                                                      OBJECT_NAME                          COST
------------------------------------------------------------ ------------------------------------------------------------ ------------------------------ ----------
SELECT STATEMENT                                                                                                                                                  1
SORT                                                         AGGREGATE
VIEW                                                                                                                                                              1
COUNT                                                        STOPKEY
INDEX                                                        RANGE SCAN                                                   IDX_DEPT_DEPTNO                         1
最终发现执行计划已经改变，所以可以根据plan_hash_value值是否变化可知该sql语句的执行计划是否改变


通过v$sql视图的plan_hash_value值可以很方便的知道，该sql语句的执行计划是否改变，
通常我们的做法是分别将v$sql,v$sql_hash做两份快照，隔段时间后再做段快照，然后将其进行比较，查看是否有sql的执行计划已经改变。                                                       

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