Oracle 压缩表的一点测试

---压缩表可减少数据量，从而减少IO

DROP TABLE t purge;

CREATE TABLE t NOCOMPRESS AS
SELECT rownum AS n, rpad(' ',500,mod(rownum,15)) AS pad
FROM dual
CONNECT BY level <= 200000;

--收集表统计信息

admin@ORCL> execute dbms_stats.gather_table_stats('ADMIN','T');

PL/SQL 过程已成功完成。



--未压缩的表当前情况
admin@ORCL> SELECT table_name, blocks,compression  FROM user_tables WHERE table_name = 'T';

TABLE_NAME                         BLOCKS COMPRESS
------------------------------ ---------- --------
T                                   14449 DISABLED
admin@ORCL> set autotrace on
--查看资源消耗，COST 为3185,逻辑读为14297
admin@ORCL> select count(*) from t;

  COUNT(*)
----------
    200000


执行计划
----------------------------------------------------------
Plan hash value: 2966233522

-------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Cost (%CPU)| Time     |
-------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |     1 |  3185   (1)| 00:00:39 |
|   1 |  SORT AGGREGATE    |      |     1 |            |          |
|   2 |   TABLE ACCESS FULL| T    |   199K|  3185   (1)| 00:00:39 |
-------------------------------------------------------------------


统计信息
----------------------------------------------------------
          1  recursive calls
          0  db block gets
      14297  consistent gets
          0  physical reads
          0  redo size
        408  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)
          1  rows processed


--开始压缩表

ALTER TABLE t MOVE COMPRESS;

execute dbms_stats.gather_table_stats('ADMIN','T');

admin@ORCL> SELECT table_name, blocks,compression  FROM user_tables WHERE table_name = 'T';

TABLE_NAME                         BLOCKS COMPRESS
------------------------------ ---------- --------
T                                    2639 ENABLED

set autotrace on 
admin@ORCL> select count(*) from t;

  COUNT(*)
----------
    200000


执行计划
----------------------------------------------------------
Plan hash value: 2966233522

-------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Cost (%CPU)| Time     |
-------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |     1 |   587   (2)| 00:00:08 |
|   1 |  SORT AGGREGATE    |      |     1 |            |          |
|   2 |   TABLE ACCESS FULL| T    |   200K|   587   (2)| 00:00:08 |
-------------------------------------------------------------------


统计信息
----------------------------------------------------------
          0  recursive calls
          0  db block gets
       2596  consistent gets
          0  physical reads
        116  redo size
        408  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)
          1  rows processed
/

注意：压缩后的表更新的开销会更大，查询耗费的CPU也更多。所以压缩表一般适合在更新比较少，且CPU消耗不大，IO消耗很大系统中试用。