Question

这是查询

UPDATE hrs.rns_recon_ho c SET c.refr_numb = 
(
SELECT seqn_numb FROM hrs.rns_recon_ho p WHERE p.narr_1 = c.narr_1 
AND p.seqn_numb = p.refr_numb AND p.prod_code = 0
)
Where c.prod_code = 0 And c.refr_numb = 0 
AND c.narr_1 = '3/13/201211013198693442091'

这是执行计划

enter image description here 查询的哪一部分导致TABLE ACCESS(FULL)？

修改

我的错误，我应该更清楚。现在让我清楚一点，我需要知道如何将执行计划的各个部分与查询的各个部分联系起来。例如

查询的哪一部分导致TABLE ACCESS(FULL)
查询的哪一部分导致TABLE ACCESS(BY GLOBAL INDEX ROWID)

但是，显然INDEX(...)...PROD_CODE是由PROD_CODE字段引起的。

是否有任何经验法则，指南或技巧可以识别？或者，我们无法知道，因为优化器不会告诉我们为什么选择某条路径。

Answer 1

也许可以通过提示在一张桌子上强制进行全表扫描，例如

UPDATE hrs.rns_recon_ho c SET c.refr_numb =
(
SELECT /*+ FULL(p) */ seqn_numb FROM hrs.rns_recon_ho p WHERE p.narr_1 = c.narr_1
AND p.seqn_numb = p.refr_numb AND p.prod_code = 0
)
Where c.prod_code = 0 And c.refr_numb = 0
AND c.narr_1 = '3/13/201211013198693442091'

或

UPDATE /*+ FULL(c) */ hrs.rns_recon_ho c SET c.refr_numb =
(
SELECT seqn_numb FROM hrs.rns_recon_ho p WHERE p.narr_1 = c.narr_1
AND p.seqn_numb = p.refr_numb AND p.prod_code = 0
)
Where c.prod_code = 0 And c.refr_numb = 0
AND c.narr_1 = '3/13/201211013198693442091'

如果您的执行计划显示两个“全扫描”，您知道它最初是由另一个引起的。

Answer 2

让我重新创作：

CREATE TABLE RNS_RECON_HO ( PROD_CODE NUMBER,
            REFR_NUMB NUMBER,
            NARR_1 VARCHAR2 ( 25 ),
            SEQN_NUMB NUMBER primary key    );

CREATE INDEX TESTTT ON RNS_RECON_HO (PROD_CODE);
CREATE INDEX TESTTT1 ON RNS_RECON_HO (REFR_NUMB);
CREATE INDEX TESTTT2 ON RNS_RECON_HO (NARR_1);

现在复制你的计划：

SET AUTOTRACE ON

UPDATE
      RNS_RECON_HO C
SET
      C.REFR_NUMB    =
          ( SELECT
                 SEQN_NUMB
           FROM
                RNS_RECON_HO P
           WHERE
                    P.NARR_1 = C.NARR_1
                AND P.SEQN_NUMB = P.REFR_NUMB
                AND P.PROD_CODE = PROD_CODE )
WHERE
         C.PROD_CODE = 0
      AND C.REFR_NUMB = 0
      AND C.NARR_1 = '3/13/201211013198693442091';


0 rows updated.

Execution Plan
----------------------------------------------------------
   0       UPDATE STATEMENT Optimizer Mode=ALL_ROWS (Cost=2 Card=1 Bytes=78)
   1    0    UPDATE RNS_RECON_HO
   2    1      TABLE ACCESS FULL RNS_RECON_HO (Cost=2 Card=1 Bytes=78)
   3    1      TABLE ACCESS BY INDEX ROWID RNS_RECON_HO (Cost=5 Card=1 Bytes=91)
   4    3        INDEX RANGE SCAN TESTTT (Cost=1 Card=1)

Statistics
----------------------------------------------------------
        190  recursive calls
          0  spare statistic 3
          0  gcs messages sent
         25  db block gets from cache
          0  physical reads direct (lob)
          0  queue position update
          0  queue single row
          0  queue ocp pages
          0  HSC OLTP Compressed Blocks
          0  HSC IDL Compressed Blocks
          0  rows processed

<强>原因：

如果索引比表扫描更便宜（读取次数更少），优化器将只选择使用索引。这通常意味着WHERE子句标准需要映射到索引的前导（即最左边）列。因此，在where子句列上添加索引（PROD_CODE，REFR_NUMB，NARR_1）将避免这种情况。

确认检查此

CREATE INDEX TESTTT3 ON RNS_RECON_HO (PROD_CODE, REFR_NUMB, NARR_1);

SET AUTOTRACE ON

UPDATE
      RNS_RECON_HO C
SET
      C.REFR_NUMB    =
          ( SELECT
                 SEQN_NUMB
           FROM
                RNS_RECON_HO P
           WHERE
                    P.NARR_1 = C.NARR_1
                AND P.SEQN_NUMB = P.REFR_NUMB
                AND P.PROD_CODE = PROD_CODE )
WHERE
         C.PROD_CODE = 0
      AND C.REFR_NUMB = 0
      AND C.NARR_1 = '3/13/201211013198693442091';

0 rows updated.

Execution Plan
----------------------------------------------------------
   0       UPDATE STATEMENT Optimizer Mode=ALL_ROWS (Cost=1 Card=1 Bytes=78)
   1    0    UPDATE RNS_RECON_HO
   2    1      INDEX RANGE SCAN TESTTT3 (Cost=1 Card=1 Bytes=78)
   3    1      TABLE ACCESS BY INDEX ROWID RNS_RECON_HO (Cost=27 Card=1 Bytes=91)
   4    3        INDEX FULL SCAN TESTTT3 (Cost=26 Card=1)

Statistics
----------------------------------------------------------
          1  recursive calls
          0  spare statistic 3
          0  gcs messages sent
          0  db block gets from cache
          0  physical reads direct (lob)
          0  queue position update
          0  queue single row
          0  queue ocp pages
          0  HSC OLTP Compressed Blocks
          0  HSC IDL Compressed Blocks
          0  rows processed

但这只是为了改变计划，但从未承诺任何性能改进，因为您提供的输入有限。

关注：

外部查询正在进行FTS，因为它忽略了索引。因此，当我们放置一个复合键时，索引对于优化器来说是直截了当的，并且使用相同的索引。子查询使用索引，因为它与索引列上的外部查询连接。

如果您可以通过遵循核心规则接近可能的执行计划，那么如何确定准确的计划无法100％实现。

如何关联执行计划和SQL查询

2 个答案: