[Index]Index Scan

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2013年12月04日 ⁄ 综合 ⁄ 共 4633字 ⁄ 字号小中大 ⁄ 评论关闭

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Case 2

Index B-Tree

Index的叶节点指向RowID，然后根据RowID再找对应的Block

语法：CREATE INDEX NAME_SALARY_IDX ON PERSON(LAST_NAME ASC,SALARY DESC);

Index Unique Scan

对于unique index来说,如果where条件后面是=，那么就会执行index unique scan。

AskTom:well, the optimizer can look at an index that is unique and say "ah-hah, if you use "where x =:x and y = :y and ...." I'm going to get ONE row back, I can cost that much better" (refer)

Index Unique Scan的条件：

1.Index是唯一性索引;

2.where条件类似于 x = and y=: y ...;

Example:

CREATE TABLE PT_TEST AS SELECT * FROM DBA_OBJECTS;

CREATE UNIQUE INDEX UNIQUE_IDX ON PT_TEST(OBJECT_ID);    --UNIQUE_IDX是唯一索引

EXEC DBMS_STATS.GATHER_TABLE_STATS('APPS','PT_TEST', CASCADE=>TRUE ); --重做统计

SQL> set auto trace

SQL> select * from pt_test where object_id=10;

Elapsed: 00:00:00.47

Execution Plan
----------------------------------------------------------
Plan hash value: 2398730171

------------------------------------------------------------------------------------------
| Id  | Operation                   | Name       | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |            |     1 |    97 |     3   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| PT_TEST    |     1 |    97 |     3   (0)| 00:00:01 |
|*  2 |   INDEX UNIQUE SCAN         | UNIQUE_IDX |     1 |       |     2   (0)| 00:00:01 |
------------------------------------------------------------------------------------------

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

   2 - access("OBJECT_ID"=10)


Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
          4  consistent gets
          0  physical reads
          0  redo size
       1194  bytes sent via SQL*Net to client
        338  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

SQL>

Performance:index unique scan的性能非常好。

Index Range Scan

Case 1

基于上面的case，还是以object_id这个唯一性索引列为条件

SQL> select owner from pt_test where object_id<10;

8 rows selected.

Elapsed: 00:00:00.45

Execution Plan
----------------------------------------------------------
Plan hash value: 1470047708

------------------------------------------------------------------------------------------
| Id  | Operation                   | Name       | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |            |     1 |    11 |     4   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| PT_TEST    |     1 |    11 |     4   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN          | UNIQUE_IDX |     1 |       |     3   (0)| 00:00:01 |
------------------------------------------------------------------------------------------

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

   2 - access("OBJECT_ID"<10)


Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
          6  consistent gets
          0  physical reads
          0  redo size
        388  bytes sent via SQL*Net to client
        338  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          8  rows processed

SQL>

对于上边这个例子，Index Unique Scan和Index Range Scan在B Tree上的搜索路径是一样的，只是Index Unique Scan在找到应该含有要找的Index Key的block后便停止了搜索，因为该键是唯一的。而Index Range Scan还要循着指针继续找下去直到条件不满足时。

Case 2

这个时候给owner字段增加一个非唯一性索引

create index ind_owner on pt_test(owner);  --给owner字段增加非唯一性索引

SQL> select owner from pt_test where owner='SCOTT';

123 rows selected.

Elapsed: 00:00:00.80

Execution Plan
----------------------------------------------------------
Plan hash value: 2280863269

------------------------------------------------------------------------------
| Id  | Operation        | Name      | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------
|   0 | SELECT STATEMENT |           |  1472 |  7360 |     6   (0)| 00:00:01 |
|*  1 |  INDEX RANGE SCAN| IND_OWNER |  1472 |  7360 |     6   (0)| 00:00:01 |
------------------------------------------------------------------------------

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

   1 - access("OWNER"='SCOTT')


Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
         13  consistent gets
          0  physical reads
          0  redo size
       1653  bytes sent via SQL*Net to client
        426  bytes received via SQL*Net from client
         10  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
        123  rows processed

SQL>

对于none unique index来说，如果where 条件后面出现了=,>,<,betweed...and...的时候（返回多行值的时候），就有可能执行index range scan。

Performance:It depends ..How selective ? Throw-away later ? single block IO, CF

The problem is that it is impossible to tell how many rows the range scan is scanning. A range scan that scans 5 rows on average will probably be fairly efficient. However a range scan scanning 10000 rows on average will probably be causing grief.