oracle 压缩索引
创建数据：
SQL> create table tb_compress_index
2 (
3 user_year int not null,
4 user_month int not null,
5 user_date int not null,
6 user_name varchar2(20) null
7 );
表已创建。
SQL> DECLARE
2 V_DATE INT;
3 V_MONTH INT;
4 BEGIN
5 FOR I IN 1..100 LOOP
6 V_DATE := I mod 30;
7 V_DATE := V_DATE + 1;
8 V_MONTH := I mod 12;
9 V_MONTH := V_MONTH + 1;
10 INSERT INTO tb_compress_index VALUES (2010, V_MONTH, V_DATE, 'user_' || I);
11 END LOOP;
12 COMMIT;
13 END;
14 /
PL/SQL 过程已成功完成。
查询：
SQL> WITH compress_index AS (SELECT * FROM tb_compress_index) SELECT * FROM compress_index WHERE ROWNUM < 10;
USER_YEAR USER_MONTH USER_DATE USER_NAME
---------- ---------- ---------- --------------------
2010 2 2 user_1
2010 3 3 user_2
2010 4 4 user_3
2010 5 5 user_4
2010 6 6 user_5
2010 7 7 user_6
2010 8 8 user_7
2010 9 9 user_8
2010 10 10 user_9
已选择9行。
SQL>创建压缩索引，然后看执行计划：
SQL> create index idx_compress_index on tb_compress_index(user_year, user_month, user_date) compress 3;
索引已创建。
SQL> set autotrace traceonly
SQL> set linesize 120
SQL> select user_year, user_month, user_date from tb_compress_index where user_month = 12;
已选择8行。
执行计划
----------------------------------------------------------
Plan hash value: 2921924755
---------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 8 | 312 | 1 (0)| 00:00:01 |
|* 1 | INDEX FULL SCAN | IDX_COMPRESS_INDEX | 8 | 312 | 1 (0)| 00:00:01 |
---------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("USER_MONTH"=12)
filter("USER_MONTH"=12)
Note
-----
- dynamic sampling used for this statement
统计信息
----------------------------------------------------------
4 recursive calls
0 db block gets
11 consistent gets
0 physical reads
0 redo size
613 bytes sent via SQL*Net to client
385 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
8 rows processed
使用普通索引：
SQL> drop index idx_compress_index;
索引已删除。
SQL> create index idx_compress_index on tb_compress_index(user_year, user_month, user_date);
索引已创建。
SQL> select user_year, user_month, user_date from tb_compress_index where user_month = 12;
已选择8行。
执行计划
----------------------------------------------------------
Plan hash value: 2921924755
---------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 8 | 312 | 1 (0)| 00:00:01 |
|* 1 | INDEX FULL SCAN | IDX_COMPRESS_INDEX | 8 | 312 | 1 (0)| 00:00:01 |
---------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("USER_MONTH"=12)
filter("USER_MONTH"=12)
Note
-----
- dynamic sampling used for this statement
统计信息
----------------------------------------------------------
66 recursive calls
0 db block gets
29 consistent gets
0 physical reads
0 redo size
613 bytes sent via SQL*Net to client
385 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
8 rows processed
SQL>多测试几次
使用压缩索引
SQL> drop index idx_compress_index;
索引已删除。
SQL> create index idx_compress_index on tb_compress_index(user_year, user_month, user_date) compress 3;
索引已创建。
SQL> alter system flush buffer_cache;
系统已更改。
SQL> alter system flush shared_pool;
系统已更改。
SQL> select user_year, user_month, user_date from tb_compress_index where user_month = 12;
已选择8行。
执行计划
----------------------------------------------------------
Plan hash value: 2921924755
---------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 8 | 312 | 1 (0)| 00:00:01 |
|* 1 | INDEX FULL SCAN | IDX_COMPRESS_INDEX | 8 | 312 | 1 (0)| 00:00:01 |
---------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("USER_MONTH"=12)
filter("USER_MONTH"=12)
Note
-----
- dynamic sampling used for this statement
统计信息
----------------------------------------------------------
1341 recursive calls
0 db block gets
264 consistent gets
43 physical reads
0 redo size
613 bytes sent via SQL*Net to client
385 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
33 sorts (memory)
0 sorts (disk)
8 rows processed
SQL> select user_year, user_month, user_date from tb_compress_index where user_month = 12;
已选择8行。
执行计划
----------------------------------------------------------
Plan hash value: 2921924755
---------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 8 | 312 | 1 (0)| 00:00:01 |
|* 1 | INDEX FULL SCAN | IDX_COMPRESS_INDEX | 8 | 312 | 1 (0)| 00:00:01 |
---------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("USER_MONTH"=12)
filter("USER_MONTH"=12)
Note
-----
- dynamic sampling used for this statement
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
2 consistent gets
0 physical reads
0 redo size
613 bytes sent via SQL*Net to client
385 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
8 rows processed
SQL>使用普通索引
SQL> drop index idx_compress_index;
索引已删除。
SQL> create index idx_compress_index on tb_compress_index(user_year, user_month, user_date);
索引已创建。
SQL> alter system flush buffer_cache;
系统已更改。
SQL> alter system flush shared_pool;
系统已更改。
SQL> select user_year, user_month, user_date from tb_compress_index where user_month = 12;
已选择8行。
执行计划
----------------------------------------------------------
Plan hash value: 2921924755
---------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 8 | 312 | 1 (0)| 00:00:01 |
|* 1 | INDEX FULL SCAN | IDX_COMPRESS_INDEX | 8 | 312 | 1 (0)| 00:00:01 |
---------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("USER_MONTH"=12)
filter("USER_MONTH"=12)
Note
-----
- dynamic sampling used for this statement
统计信息
----------------------------------------------------------
1341 recursive calls
0 db block gets
264 consistent gets
43 physical reads
0 redo size
613 bytes sent via SQL*Net to client
385 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
33 sorts (memory)
0 sorts (disk)
8 rows processed
SQL> select user_year, user_month, user_date from tb_compress_index where user_month = 12;
已选择8行。
执行计划
----------------------------------------------------------
Plan hash value: 2921924755
---------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 8 | 312 | 1 (0)| 00:00:01 |
|* 1 | INDEX FULL SCAN | IDX_COMPRESS_INDEX | 8 | 312 | 1 (0)| 00:00:01 |
---------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("USER_MONTH"=12)
filter("USER_MONTH"=12)
Note
-----
- dynamic sampling used for this statement
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
2 consistent gets
0 physical reads
0 redo size
613 bytes sent via SQL*Net to client
385 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
8 rows processed
SQL>所以，从性能上来看，压缩索引和普通索引没什么区别。
再看存储空间：
SQL> select index_name, compression, leaf_blocks, prefix_length from user_indexes where index_name='IDX_COMPRESS_INDEX';
INDEX_NAME COMPRESS LEAF_BLOCKS PREFIX_LENGTH
------------------------------ -------- ----------- -------------
IDX_COMPRESS_INDEX ENABLED 1 3
SQL>
SQL> select segment_name, bytes from user_segments where segment_name='IDX_COMPRESS_INDEX';
SEGMENT_NAME BYTES
--------------------------------------------------------------------------------- ----------
IDX_COMPRESS_INDEX 65536
SQL> alter index idx_compress_index rebuild nocompress;
索引已更改。
SQL> select index_name, compression, leaf_blocks, prefix_length from user_indexes where index_name='IDX_COMPRESS_INDEX';
INDEX_NAME COMPRESS LEAF_BLOCKS PREFIX_LENGTH
------------------------------ -------- ----------- -------------
IDX_COMPRESS_INDEX DISABLED 1
SQL> select segment_name, bytes from user_segments where segment_name='IDX_COMPRESS_INDEX';
SEGMENT_NAME BYTES
--------------------------------------------------------------------------------- ----------
IDX_COMPRESS_INDEX 65536
SQL>压缩前和压缩后的BYTES，没变。
所以，压缩索引不一定能节省空间，这就像平时用rar来压缩一本pdf，从43MB压缩到42MB，压缩了没多少。
压缩索引也有几个限制条件。例如，对于非唯一索引的情况，所有列都可以进行压缩；对于唯一索引，除了最后一列以外都可以进行压缩。