##改变分区压缩方式的方法如下：

To change the compression method for new data only, use ALTER TABLE ... MODIFY PARTITION ... COMPRESS ...##仅对新数据有效

To change the compression method for both new and existing data, use either ALTER TABLE ... MOVE PARTITION ... COMPRESS ... or online table redefinition.##使用alter table...move partition...compress或者表的在线重定义方式改变表的压缩方式，对新的数据和旧的数据都是有效的。

Determining If a Table Is Compressed

In the *_TABLES data dictionary views, compressed tables have ENABLED in the COMPRESSION column. For partitioned tables, this column is null, and the COMPRESSION column of the *_TAB_PARTITIONS views indicates the partitions that are compressed. In addition, the COMPRESS_FOR column indicates the compression method in use for the table or partition.

##在*_TABLES视图中压缩表的compress列被标记为"ENABLED"，当然如果是分区表的话，那么这一列被标记为空。分区表及组合分区表的压缩情况我们相应的应该查看*_TAB_PARTITIONS及*_TAB_SUBPARTITIONS视图

SQL> SELECT table_name, compression, compress_for FROM user_tables;
 
TABLE_NAME       COMPRESSION   COMPRESS_FOR
---------------- ------------  -----------------
T1               DISABLED
T2               ENABLED       BASIC
T3               ENABLED       OLTP
T4               ENABLED       QUERY HIGH
T5               ENABLED       ARCHIVE LOW

SQL> SELECT table_name, partition_name, compression, compress_for
  FROM user_tab_partitions;

TABLE_NAME  PARTITION_NAME   COMPRESSION   COMPRESS_FOR
----------- ---------------- -----------   ------------------------------
SALES       Q4_2004          ENABLED       ARCHIVE HIGH
  ...
SALES       Q3_2008          ENABLED       QUERY HIGH
SALES       Q4_2008          ENABLED       QUERY HIGH
SALES       Q1_2009          ENABLED       OLTP
SALES       Q2_2009          ENABLED       OLTP

Determining Which Rows Are Compressed

When Hybrid Columnar Compression tables are updated, the rows change to a lower level of compression, such as from warehouse compression (QUERY HIGH) to OLTP compression or no compression. To determine the compression level of a row, use the GET_COMPRESSION_TYPE function in the DBMS_COMPRESSION package.

##当混合列模式压缩的表被更新，被更新的行的压缩等级会降低，如从warehouse compression (QUERY HIGH)降至oltp或者非压缩。我们可以使用DBMS_COMPRESSION宝中的GET_COMPRESSION_TYPE function去查询某一列的压缩等级。

For example, the following query returns the compression type for a row in the hr.employees table:

SELECT DECODE(DBMS_COMPRESSION.GET_COMPRESSION_TYPE(
                 ownname => 'HR', 
                 tabname => 'EMPLOYEES', 
                 row_id  => 'AAAVEIAAGAAAABTAAD'), 
   1,  'No Compression',
   2,  'Basic or OLTP Compression', 
   4,  'Hybrid Columnar Compression for Query High',
   8,  'Hybrid Columnar Compression for Query Low',
   16, 'Hybrid Columnar Compression for Archive High',
   32, 'Hybrid Columnar Compression for Archive Low',
   'Unknown Compression Type') compression_type
FROM DUAL;

By sampling the table rows, you can determine the percentage of rows that are no longer at the higher compression level. You can use ALTER TABLE or MOVE PARTITION to specify a higher compression level. For example, if 10 percent of the rows are no longer at the highest compression level, then you might alter the table or move the partition to specify a higher compression level.

See Also:

Changing the Compression Level