11

我有一张有 150 万行的表。我运行一个查询来获取列中具有非重复值的记录。我观察到创建索引后查询性能下降的行为。我还使用具有 100% 估计百分比（计算模式）的 dbms_stats 来收集统计信息，以便 oracle 11g CBO 对查询计划做出更明智的决定，但它并没有提高查询执行时间。

 SQL> desc tab3;
 Name                 Null?    Type
 ----------------------------------------------
 COL1                          NUMBER(38)
 COL2                          VARCHAR2(100)
 COL3                          VARCHAR2(36)
 COL4                          VARCHAR2(36)
 COL5                          VARCHAR2(4000)
 COL6                          VARCHAR2(4000)
 MEASURE_0                     VARCHAR2(4000)
 MEASURE_1                     VARCHAR2(4000)
 MEASURE_2                     VARCHAR2(4000)
 MEASURE_3                     VARCHAR2(4000)
 MEASURE_4                     VARCHAR2(4000)
 MEASURE_5                     VARCHAR2(4000)
 MEASURE_6                     VARCHAR2(4000)
 MEASURE_7                     VARCHAR2(4000)
 MEASURE_8                     VARCHAR2(4000)
 MEASURE_9                     VARCHAR2(4000)

该列measure_0有 40 万个唯一值。

SQL> select count(*) from (select measure_0 from tab3 group by measure_0 having count(*) = 1) abc;

  COUNT(*)
----------
    403664

以下是带有执行计划的查询，请注意表上没有索引。

SQL> set autotrace traceonly;

SQL> SELECT * FROM (
  2     SELECT
  3             (ROWNUM -1) AS COL1,
  4             ft.COL1         AS OLD_COL1,
  5             ft.COL2,
  6             ft.COL3,
  7             ft.COL4,
  8             ft.COL5,
  9             ft.COL6,
 10             ft.MEASURE_0,
 11             ft.MEASURE_1,
 12             ft.MEASURE_2,
 13             ft.MEASURE_3,
 14             ft.MEASURE_4,
 15             ft.MEASURE_5,
 16             ft.MEASURE_6,
 17             ft.MEASURE_7,
 18             ft.MEASURE_8,
 19             ft.MEASURE_9
 20     FROM tab3 ft
 21     WHERE MEASURE_0 IN
 22      (
 23             SELECT MEASURE_0
 24             FROM tab3
 25             GROUP BY MEASURE_0
 26             HAVING COUNT(*) = 1
 27      )
 28  ) ABC WHERE COL1 >= 0 AND COL1 <=449;

450 rows selected.

Elapsed: 00:00:01.90

Execution Plan
----------------------------------------------------------
Plan hash value: 3115757351

------------------------------------------------------------------------------------
| Id  | Operation               | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT        |          |  1243 |    28M|   717K  (1)| 02:23:29 |
|*  1 |  VIEW                   |          |  1243 |    28M|   717K  (1)| 02:23:29 |
|   2 |   COUNT                 |          |       |       |            |          |
|*  3 |    HASH JOIN            |          |  1243 |    30M|   717K  (1)| 02:23:29 |
|   4 |     VIEW                | VW_NSO_1 |  1686K|  3219M|  6274   (2)| 00:01:16 |
|*  5 |      FILTER             |          |       |       |            |          |
|   6 |       HASH GROUP BY     |          |     1 |  3219M|  6274   (2)| 00:01:16 |
|   7 |        TABLE ACCESS FULL| TAB3     |  1686K|  3219M|  6196   (1)| 00:01:15 |
|   8 |     TABLE ACCESS FULL   | TAB3     |  1686K|    37G|  6211   (1)| 00:01:15 |
------------------------------------------------------------------------------------

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

   1 - filter("COL1">=0 AND "COL1"<=449)
   3 - access("MEASURE_0"="MEASURE_0")
   5 - filter(COUNT(*)=1)

Note
-----
   - dynamic sampling used for this statement (level=2)


Statistics
----------------------------------------------------------
        354  recursive calls
          0  db block gets
      46518  consistent gets
      45122  physical reads
          0  redo size
      43972  bytes sent via SQL*Net to client
        715  bytes received via SQL*Net from client
         31  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
        450  rows processed

查询占用1.90秒。如果我再次运行查询，则需要1.66秒。为什么第一次运行需要更多时间？

为了加快速度，我在查询中使用的两列上创建了索引。

SQL> create index ind_tab3_orgid on tab3(COL1);

Index created.

Elapsed: 00:00:01.68
SQL> create index ind_tab3_msr_0 on tab3(measure_0);

Index created.

Elapsed: 00:00:01.83

当我在此之后第一次触发查询时，花了21秒才回来。而随后的运行将其提高到2.9秒。为什么甲骨文在第一次运行时要花这么多时间，是在预热还是什么......让我感到困惑！

这是需要2.9秒时的计划-

450 rows selected.

Elapsed: 00:00:02.92

Execution Plan
----------------------------------------------------------
Plan hash value: 240271480

-------------------------------------------------------------------------------------------------
| Id  | Operation                      | Name           | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT               |                |  1243 |    28M|   711K  (1)| 02:22:15 |
|*  1 |  VIEW                          |                |  1243 |    28M|   711K  (1)| 02:22:15 |
|   2 |   COUNT                        |                |       |       |            |          |
|   3 |    NESTED LOOPS                |                |       |       |            |          |
|   4 |     NESTED LOOPS               |                |  1243 |    30M|   711K  (1)| 02:22:15 |
|   5 |      VIEW                      | VW_NSO_1       |  1686K|  3219M|  6274   (2)| 00:01:16 |
|*  6 |       FILTER                   |                |       |       |            |          |
|   7 |        HASH GROUP BY           |                |     1 |  3219M|  6274   (2)| 00:01:16 |
|   8 |         TABLE ACCESS FULL      | TAB3           |  1686K|  3219M|  6196   (1)| 00:01:15 |
|*  9 |      INDEX RANGE SCAN          | IND_TAB3_MSR_0 |  1243 |       |     2   (0)| 00:00:01 |
|  10 |     TABLE ACCESS BY INDEX ROWID| TAB3           |  1243 |    28M|    44   (0)| 00:00:01 |
-------------------------------------------------------------------------------------------------

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

   1 - filter("COL1">=0 AND "COL1"<=449)
   6 - filter(COUNT(*)=1)
   9 - access("MEASURE_0"="MEASURE_0")

Note
-----
   - dynamic sampling used for this statement (level=2)


Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
     660054  consistent gets
      22561  physical reads
          0  redo size
      44358  bytes sent via SQL*Net to client
        715  bytes received via SQL*Net from client
         31  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
        450  rows processed

我预计时间会低于表未编入索引时的时间。为什么表的索引版本比非索引版本需要更多时间来获取结果？如果我没记错的话，就是 TABLE ACCESS BY INDEX ROWID 占用了时间。我可以强制 oracle 使用 TABLE ACCESS FULL 吗？

然后我收集了表格上的统计数据，以便 CBO 使用计算选项改进计划。所以现在统计数据是准确的。

SQL> EXECUTE dbms_stats.gather_table_stats (ownname=>'EQUBE67DP', tabname=>'TAB3',estimate_percent=>null,cascade=>true);

PL/SQL procedure successfully completed.

Elapsed: 00:01:02.47
SQL> set autotrace off;
SQL> select COLUMN_NAME,NUM_DISTINCT,SAMPLE_SIZE,HISTOGRAM,LAST_ANALYZED from dba_tab_cols where table_name = 'TAB3' ;

COLUMN_NAME                    NUM_DISTINCT SAMPLE_SIZE HISTOGRAM       LAST_ANALYZED
------------------------------ ------------ ----------- --------------- ---------
COL1                                1502257     1502257 NONE            27-JUN-12
COL2                                      0             NONE            27-JUN-12
COL3                                      1     1502257 NONE            27-JUN-12
COL4                                      0             NONE            27-JUN-12
COL5                                1502257     1502257 NONE            27-JUN-12
COL6                                1502257     1502257 NONE            27-JUN-12
MEASURE_0                            405609     1502257 HEIGHT BALANCED 27-JUN-12
MEASURE_1                            128570     1502257 NONE            27-JUN-12
MEASURE_2                           1502257     1502257 NONE            27-JUN-12
MEASURE_3                            185657     1502257 NONE            27-JUN-12
MEASURE_4                               901     1502257 NONE            27-JUN-12
MEASURE_5                                17     1502257 NONE            27-JUN-12
MEASURE_6                              2202     1502257 NONE            27-JUN-12
MEASURE_7                              2193     1502257 NONE            27-JUN-12
MEASURE_8                                21     1502257 NONE            27-JUN-12
MEASURE_9                             27263     1502257 NONE            27-JUN-12

我再次运行查询

450 rows selected.

Elapsed: 00:00:02.95

Execution Plan
----------------------------------------------------------
Plan hash value: 240271480

-------------------------------------------------------------------------------------------------
| Id  | Operation                      | Name           | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT               |                |    31M|   718G|  8046   (2)| 00:01:37 |
|*  1 |  VIEW                          |                |    31M|   718G|  8046   (2)| 00:01:37 |
|   2 |   COUNT                        |                |       |       |            |          |
|   3 |    NESTED LOOPS                |                |       |       |            |          |
|   4 |     NESTED LOOPS               |                |    31M|    62G|  8046   (2)| 00:01:37 |
|   5 |      VIEW                      | VW_NSO_1       |  4057 |  7931K|  6263   (2)| 00:01:16 |
|*  6 |       FILTER                   |                |       |       |            |          |
|   7 |        HASH GROUP BY           |                |     1 | 20285 |  6263   (2)| 00:01:16 |
|   8 |         TABLE ACCESS FULL      | TAB3           |  1502K|  7335K|  6193   (1)| 00:01:15 |
|*  9 |      INDEX RANGE SCAN          | IND_TAB3_MSR_0 |     4 |       |     2   (0)| 00:00:01 |
|  10 |     TABLE ACCESS BY INDEX ROWID| TAB3           |   779K|    75M|     3   (0)| 00:00:01 |
-------------------------------------------------------------------------------------------------

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

   1 - filter("COL1">=0 AND "COL1"<=449)
   6 - filter(COUNT(*)=1)
   9 - access("MEASURE_0"="MEASURE_0")


Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
     660054  consistent gets
      22561  physical reads
          0  redo size
      44358  bytes sent via SQL*Net to client
        715  bytes received via SQL*Net from client
         31  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
        450  rows processed

这次查询在2.9秒内返回（有时也需要3.9秒）。

我的目标是尽可能减少查询执行时间。但是在添加索引或计算统计信息之后，查询时间一直在增加。为什么会发生这种情况？即使保留索引，我该如何改进？

4

2 回答 2

13

首先，让我引用Tom Kyte的话：

一遍又一遍地对自己说

“全扫描不坏，索引不好”
“全扫描不坏，索引不好”
“全扫描不坏，索引不好”
“全扫描不坏，索引不好”
“全扫描不是邪恶的，索引是不好的”
“完全扫描不是邪恶的，索引是不好的”

索引并不总能提高性能，它们不是神奇的银弹（好像曾经存在过这样的东西：）

现在您要问为什么索引需要更长的时间。答案很简单：

  • 使用全表扫描： 46518 一致获取
  • 与您的索引：660054 一致

换句话说：Oracle 对索引执行比全表扫描更多的读取操作。发生这种情况是因为：

  1. FULL TABLE SCAN 读取是批量操作（一次很多块），因此是读取大量数据的有效方式
  2. 有时，当您从索引中读取时，您最终会多次读取完全相同的数据块。

至于为什么优化器选择使用这个明显效率不高的索引，这是因为即使有esimate_percent=100完整的直方图（你已经收集在MEASURE_0列上），通过优化器的简单分析仍然不能可靠地表达一些数据分布。特别是，分析器无法很好地理解跨列和跨表的依赖关系。这会导致错误的估计，从而导致糟糕的计划选择。

编辑： CBO 的工作假设似乎根本不适用于这种自联接（您的最后一个查询预计有 3100 万行，而只选择了 450 行！）。这很令人费解，因为该表只有 150 万行。您使用的是什么版本的 Oracle？

我想您会发现您可以删除自联接，从而通过分析提高查询性能：

SELECT * FROM (
   SELECT (ROWNUM -1) AS COL1, ABC.*
     FROM (
      SELECT
              ft.COL1 AS OLD_COL1,
              [...],
              COUNT(*) OVER (PARTITION BY MEASURE_O) nb_0
      FROM tab3 ft
      ) ABC 
    WHERE nb_0 = 1
      AND ROWNUM - 1 <= 449
   ) v
 WHERE COL1 >= 0;

您还问为什么第一次运行查询通常需要更多时间。这是因为有缓存在工作。在数据库级别有 SGA，所有块首先从磁盘复制，然后可以多次读取（第一次查询块总是物理读取）。然后一些系统还有一个独立的系统缓存，如果最近读取过数据，它会更快地返回数据。

进一步阅读：

于 2012-06-27T15:55:42.853 回答
3

这段代码如何执行？

SELECT ROWNUM - 1      AS col1
,      ft.col1         AS old_col1
,      ft.col2
,      ft.col3
,      ft.col4
,      ft.col5
,      ft.col6
,      ft.measure_0
,      ft.measure_1
,      ft.measure_2
,      ft.measure_3
,      ft.measure_4
,      ft.measure_5
,      ft.measure_6
,      ft.measure_7
,      ft.measure_8
,      ft.measure_9
FROM   tab3 ft
WHERE  NOT EXISTS (SELECT NULL
                   FROM   tab3   ft_prime
                   WHERE  ft_prime.measure_0 = ft.measure_0
                   AND    ft_prime.ROWID <> ft.ROWID)
AND    ROWNUM <= 450;
于 2012-06-27T19:46:18.530 回答