Question

我们正在使用以下SQL删除表中的大量行，如here所述：

 DELETE FROM MYTABLE
               WHERE     UPDT_TIMESTMP < v_Cut_Off_Date
                     AND ROWNUM <= C_MAX_DELETE;

我注意到UPDT_TIMESTMP可以为NULL。初始创建后，此字段存储上次更新记录时的TIMESTAMP值。因此，如果更新时间为NULL，我希望修改我的SQL以考虑CREATE TIME。

 DELETE FROM MYTABLE
               WHERE     NVL(UPDT_TIMESTMP, CRET_TIMESTMP) < v_Cut_Off_Date
                     AND ROWNUM <= C_MAX_DELETE;

我的偏好是禁止NULL并更新将UPDT_TIMESTMP列的值设置为CRET_TIMESTMP值，但这不是一个选项。

由于表格很大，每个月大约有20M记录，而且每个月我都会删除一个月的旧数据，我想确保我可以快速找到要删除的记录。

使用此原始 SQL，

DELETE FROM COMMRCL_CORE_CLM_DTL
      WHERE UPDT_TIMESTMP < SYSDATE AND ROWNUM <= C_MAX_DELETE;

...没有索引，这是使用Toad for Oracle的查询计划：

Plan
DELETE STATEMENT  ALL_ROWSCost: 2  Bytes: 41  Cardinality: 1            
    3 DELETE FIN_IT_RPT.COMMRCL_CORE_CLM_DTL        
        2 COUNT STOPKEY     
            1 TABLE ACCESS FULL TABLE FIN_IT_RPT.COMMRCL_CORE_CLM_DTL Cost: 2  Bytes: 41  Cardinality: 1

在上次更新时间添加了索引：

CREATE INDEX FIN_IT_RPT.COMMRCL_CORE_CLM_DTL_UPDTM ON FIN_IT_RPT.COMMRCL_CORE_CLM_DTL
(UPDT_TIMESTMP)
LOGGING
TABLESPACE USERS
PCTFREE    10
INITRANS   2
MAXTRANS   255
STORAGE    (
            MAXSIZE          UNLIMITED
            PCTINCREASE      0
            BUFFER_POOL      DEFAULT
            FLASH_CACHE      DEFAULT
            CELL_FLASH_CACHE DEFAULT
           )
NOPARALLEL;

在最后更新时间（使用索引）上添加索引后的查询计划

Plan
DELETE STATEMENT  ALL_ROWSCost: 0  Bytes: 41  Cardinality: 1            
    3 DELETE FIN_IT_RPT.COMMRCL_CORE_CLM_DTL        
        2 COUNT STOPKEY     
            1 INDEX RANGE SCAN INDEX FIN_IT_RPT.COMMRCL_CORE_CLM_DTL_UPDTM Cost: 0  Bytes: 41  Cardinality: 1

如果更新时间为NULL，则修改查询以使用创建日期

DELETE FROM COMMRCL_CORE_CLM_DTL
      WHERE NVL(UPDT_TIMESTMP, CRET_TIMESTMP) < SYSDATE AND ROWNUM <= C_MAX_DELETE;

在创建时间上添加了单独的索引

CREATE INDEX FIN_IT_RPT.COMMRCL_CORE_CLM_DTL_CRET ON 
FIN_IT_RPT.COMMRCL_CORE_CLM_DTL
(CRET_TIMESTMP)
LOGGING
TABLESPACE USERS
PCTFREE    10
INITRANS   2
MAXTRANS   255
STORAGE    (
            MAXSIZE          UNLIMITED
            PCTINCREASE      0
            BUFFER_POOL      DEFAULT
            FLASH_CACHE      DEFAULT
            CELL_FLASH_CACHE DEFAULT
           )
NOPARALLEL;

添加2个单独的索引后，检查查询计划。

DELETE STATEMENT  ALL_ROWSCost: 2  Bytes: 54  Cardinality: 1            
    3 DELETE FIN_IT_RPT.COMMRCL_CORE_CLM_DTL        
        2 COUNT STOPKEY     
            1 TABLE ACCESS FULL TABLE FIN_IT_RPT.COMMRCL_CORE_CLM_DTL Cost: 2  Bytes: 54  Cardinality: 1

问题：为什么没有使用索引？

在同一索引

中添加了包含LAST UPDATE和CREATE TIME列的新索引

CREATE INDEX FIN_IT_RPT.COMMRCL_CORE_CLM_DTL_UPDCRT ON FIN_IT_RPT.COMMRCL_CORE_CLM_DTL
(UPDT_TIMESTMP, CRET_TIMESTMP)
LOGGING
TABLESPACE USERS
PCTFREE    10
INITRANS   2
MAXTRANS   255
STORAGE    (
            MAXSIZE          UNLIMITED
            PCTINCREASE      0
            BUFFER_POOL      DEFAULT
            FLASH_CACHE      DEFAULT
            CELL_FLASH_CACHE DEFAULT
           )
NOPARALLEL;

仍然没有使用索引。为什么？

Plan
DELETE STATEMENT  ALL_ROWSCost: 2  Bytes: 54  Cardinality: 1            
    3 DELETE FIN_IT_RPT.COMMRCL_CORE_CLM_DTL        
        2 COUNT STOPKEY     
            1 TABLE ACCESS FULL TABLE FIN_IT_RPT.COMMRCL_CORE_CLM_DTL Cost: 2  Bytes: 54  Cardinality: 1

我意识到表中没有太多数据会影响解释计划（我很少。）我必须生成数百万行才能真正了解期望的内容，或者我可以在不做的情况下获得一般性的想法是什么？

为什么索引没有在上面的示例中使用，或者我误读了计划？

更新

当我利用Mat的sugegstion将DELETE分解为两个更新时，第一个是CREATION日期：

DELETE FROM COMMRCL_CORE_CLM_DTL
      WHERE UPDT_TIMESTMP  < SYSDATE AND ROWNUM <= variable;

... UPDATE DATE上的索引用于第一个

Plan
DELETE STATEMENT  ALL_ROWSCost: 0  Bytes: 54  Cardinality: 1            
    3 DELETE FIN_IT_RPT.COMMRCL_CORE_CLM_DTL        
        2 COUNT STOPKEY     
            1 INDEX RANGE SCAN INDEX FIN_IT_RPT.COMMRCL_CORE_CLM_DTL_UPDCRT Cost: 0  Bytes: 54  Cardinality: 1

对于第二个SQL ......

DELETE FROM COMMRCL_CORE_CLM_DTL
      WHERE UPDT_TIMESTMP IS NULL AND  CRET_TIMESTMP < SYSDATE AND ROWNUM <= Variable;

使用包含两列的索引：

Plan

DELETE STATEMENT  ALL_ROWSCost: 0  Bytes: 54  Cardinality: 1            
    3 DELETE FIN_IT_RPT.COMMRCL_CORE_CLM_DTL        
        2 COUNT STOPKEY     
            1 INDEX RANGE SCAN INDEX FIN_IT_RPT.COMMRCL_CORE_CLM_DTL_UPDCRT Cost: 0  Bytes: 54  Cardinality: 1

Answer 1

在第二种情况下只使用不带NVL的单独DELETE语句：

DELETE FROM MYTABLE
           WHERE     UPDT_TIMESTMP IS NULL AND CRET_TIMESTMP < v_Cut_Off_Date
                 AND ROWNUM <= C_MAX_DELETE;

您可以将两个语句合并到... WHERE UPDT_TIMESTMP < v_Cut_Off_Date OR (UPDT_TIMESTMP IS NULL AND CRET_TIMESTMP < v_Cut_Off_Date) ...

的语句中

如果您只有少量UPDT_TIMESTMP IS NULL的记录，请使用MY_NVL(UPDT_TIMESTMP,CRET_TIMESTMP)创建基于函数的索引，其中函数MY_NVL返回CRET_TIMESTMP以获取UPDT_TIMESTMP IS NULL且NULL为UPDT_TIMESTMP IS NOT NULL，则where条件看起来像... WHERE UPDT_TIMESTMP < v_Cut_Off_Date OR MY_NVL(UPDT_TIMESTMP,CRET_TIMESTMP) < v_Cut_Off_Date ...

你也可以尝试使用NVL(UPDT_TIMESTMP, CRET_TIMESTMP)的基于函数的索引（正如David最初提出的那样 - 抱歉David，我还没看过你的评论）

简单删除查询。什么是最好的指数？

1 个答案: