【读书笔记】【收获，不止Oracle】位图索引（2）

本次试验讨论位图索引在即席查询中发挥的巨大作用。

即席查询（多维度报表查询：select * from t where col1=XXX and col2=XXX and col3=XXX...)

1.构造表，该表有性别，年龄范围，出生地等字段。投入约10万条的数据，为即席查询做准备。

SYS@ orcl>drop table t purge;

Table dropped.

SYS@ orcl>create table t (
2  name_id,
3  gender not null,
4  location not null,
5  age_group not null,
6  data
7  )
8  as
9  select rownum,
10  decode(ceil(dbms_random.value(0,2)),
11  1,'M',
12  2,'F')gender,
13  ceil(dbms_random.value(1,50)) location,
14  decode(ceil(dbms_random.value(0,3)),
15  1,'child',
16  2,'young',
17  3,'middle_age',
18  4,'old'),
19  rpad('*',20,'*')
20  from dual
21  connect by rownum<=100000;

Table created.

2.全表扫描的情况下的查询

SYS@ orcl>set linesize 1000
SYS@ orcl>set autotrace traceonly
SYS@ orcl>select *
2  from t
3  where gender='M'
4  and location in (1,10,30)
5  and age_group='child';

663 rows selected.

Execution Plan
----------------------------------------------------------
Plan hash value: 1601196873

--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |   575 | 27025 |   138   (3)| 00:00:02 |
|*  1 |  TABLE ACCESS FULL| T    |   575 | 27025 |   138   (3)| 00:00:02 |
--------------------------------------------------------------------------

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

1 - filter("GENDER"='M' AND ("LOCATION"=1 OR "LOCATION"=10 OR
"LOCATION"=30) AND "AGE_GROUP"='child')

Note
-----
- dynamic sampling used for this statement

Statistics
----------------------------------------------------------
0  recursive calls
0  db block gets
653  consistent gets
0  physical reads
0  redo size
13755  bytes sent via SQL*Net to client
865  bytes received via SQL*Net from client
46  SQL*Net roundtrips to/from client
0  sorts (memory)
0  sorts (disk)
663  rows processed

3.建立三个列的联合索引，三个列都为高度重复的列。虽然建立了联合索引，但仍然走的是全表扫描。

SYS@ orcl>create index idx_union on t(gender,location,age_group);

Index created.

SYS@ orcl>select *
2  from t
3  where gender='M'
4  and location in (1,10,30)
5  and age_group='child';

663 rows selected.

Execution Plan
----------------------------------------------------------
Plan hash value: 1601196873

--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |   575 | 27025 |   138   (3)| 00:00:02 |
|*  1 |  TABLE ACCESS FULL| T    |   575 | 27025 |   138   (3)| 00:00:02 |
--------------------------------------------------------------------------

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

1 - filter("GENDER"='M' AND ("LOCATION"=1 OR "LOCATION"=10 OR
"LOCATION"=30) AND "AGE_GROUP"='child')

Note
-----
- dynamic sampling used for this statement

Statistics
----------------------------------------------------------
0  recursive calls
0  db block gets
653  consistent gets
0  physical reads
0  redo size
13755  bytes sent via SQL*Net to client
865  bytes received via SQL*Net from client
46  SQL*Net roundtrips to/from client
0  sorts (memory)
0  sorts (disk)
663  rows processed

4.强制执行联合索引，终于明白为什么走全表扫描了。

SYS@ orcl>select /*+index(t,idx_union)*/*
2  from t
3  where gender='M'
4  and location in (1,10,30)
5  and age_group='child';

663 rows selected.

Execution Plan
----------------------------------------------------------
Plan hash value: 306189815

------------------------------------------------------------------------------------------
| Id  | Operation                    | Name      | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |           |   575 | 27025 | 38152   (1)| 00:07:38 |
|   1 |  INLIST ITERATOR             |           |       |       |            |          |
|   2 |   TABLE ACCESS BY INDEX ROWID| T         |   575 | 27025 | 38152   (1)| 00:07:38 |
|*  3 |    INDEX RANGE SCAN          | IDX_UNION | 49691 |       |   151   (2)| 00:00:02 |
------------------------------------------------------------------------------------------

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

3 - access("GENDER"='M' AND ("LOCATION"=1 OR "LOCATION"=10 OR "LOCATION"=30)
AND "AGE_GROUP"='child')

Note
-----
- dynamic sampling used for this statement

Statistics
----------------------------------------------------------
0  recursive calls
0  db block gets
564  consistent gets
0  physical reads
0  redo size
34169  bytes sent via SQL*Net to client
865  bytes received via SQL*Net from client
46  SQL*Net roundtrips to/from client
0  sorts (memory)
0  sorts (disk)
663  rows processed

5.本次的主角：位图索引。在gender,location,age_group三个字段分别建位图索引。

SYS@ orcl>create bitmap index gender_idx on t(gender);

Index created.

SYS@ orcl>create bitmap index location_idx on t(location);

Index created.

SYS@ orcl>create bitmap index age_group_idx on t(age_group);

Index created.

SYS@ orcl>select *
2  from t
3  where gender='M'
4  and location in (1,10,30)
5  and age_group='41 and over';

663 rows selected.

Execution Plan
----------------------------------------------------------
Plan hash value: 687389132

-----------------------------------------------------------------------------------------------
| Id  | Operation                     | Name          | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT              |               |   575 | 27025 |     9   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID  | T             |   575 | 27025 |     9   (0)| 00:00:01 |
|   2 |   BITMAP CONVERSION TO ROWIDS |               |       |       |            |          |
|   3 |    BITMAP AND                 |               |       |       |            |          |
|*  4 |     BITMAP INDEX SINGLE VALUE | GENDER_IDX    |       |       |            |          |
|   5 |     BITMAP OR                 |               |       |       |            |          |
|*  6 |      BITMAP INDEX SINGLE VALUE| LOCATION_IDX  |       |       |            |          |
|*  7 |      BITMAP INDEX SINGLE VALUE| LOCATION_IDX  |       |       |            |          |
|*  8 |      BITMAP INDEX SINGLE VALUE| LOCATION_IDX  |       |       |            |          |
|*  9 |     BITMAP INDEX SINGLE VALUE | AGE_GROUP_IDX |       |       |            |          |
-----------------------------------------------------------------------------------------------

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

4 - access("GENDER"='M')
6 - access("LOCATION"=1)
7 - access("LOCATION"=10)
8 - access("LOCATION"=30)
9 - access("AGE_GROUP"='child')

Note
-----
- dynamic sampling used for this statement

Statistics
----------------------------------------------------------
0  recursive calls
0  db block gets
426  consistent gets
0  physical reads
0  redo size
34169  bytes sent via SQL*Net to client
865  bytes received via SQL*Net from client
46  SQL*Net roundtrips to/from client
0  sorts (memory)
0  sorts (disk)
663  rows processed

总结：

全表扫描：花费138

组合索引：花费38152（TABLE ACCESS BY INDEX ROWID），花费151（INDEX RANGE SCAN）。即使走索引扫描也比全表扫描高。

位图索引：花费9（全表扫描是其15倍，组合索引是其4239倍。如果多个字段差别更加明显。）