Table 20-1 Table Compression Methods

Table 20-2 Table Compression Characteristics

Note:

See Also:

• Oracle Database Concepts for an overview of table compression

• "Compressed Tablespaces"

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Examples Related to Table Compression

The following examples are related to table compression:

• Example 20-1, "Creating a Table with OLTP Table Compression"

• Example 20-2, "Creating a Table with Basic Table Compression"

• Example 20-3, "Using Direct-Path Insert to Insert Rows Into a Table"

• Example 20-4, "Creating a Table with Warehouse Compression"

• Example 20-5, "Creating a Table with Archive Compression"

CREATE TABLE orders  ...  COMPRESS FOR OLTP;

CREATE TABLE sales_history  ...  COMPRESS BASIC;

CREATE TABLE sales_history  ...  COMPRESS;

Frequent queries are run against this table, but no DML is expected.

##这样的压缩方式一般被用在频繁查询，但不被dml的表上

Example 20-3 Using Direct-Path Insert to Insert Rows Into a Table

INSERT /*+ APPEND */ INTO sales_history SELECT * FROM sales WHERE cust_id=8890;
COMMIT;

CREATE TABLE sales_history  ...  COMPRESS FOR QUERY;

CREATE TABLE sales_history  ...  COMPRESS FOR ARCHIVE;

Compression and Partitioned Tables

A table can have both compressed and uncompressed partitions, and different partitions can use different compression methods. If the compression settings for a table and one of its partitions do not match, then the partition setting has precedence for the partition.

##一个分区表能够同一时候包括压缩的和非压缩的分区。而且能够给压缩分区指定不同的压缩方式。我们能够给分区表以及分区指定不同的分区方式，而且分区上的指定具有较高的优先级。

To change the compression method for a partition, do one of the following:

##改变分区压缩方式的方法例如以下：

• 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。

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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"，当然假设是分区表的话。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对应的应该查看*_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减少。如从warehouse compression (QUERY HIGH)降至oltp或者非压缩。我们能够使用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

You can change the compression level for a partition, table, or tablespace. For example, suppose a company uses warehouse compression for its sales data, but sales data older than six months is rarely accessed. If the sales data is stored in a table that is partitioned based on the age of the data, then the compression level for the older data can be changed to archive compression to free disk space.

If a table is partitioned, then the DBMS_REDEFINITION package can change the compression level of the table. This package performs online redefinition of a table by creating a temporary copy of the table that holds the table data while it is being redefined. The table being redefined remains available for queries and DML statements during the redefinition. The amount of free space for online table redefinition depends on the relative compression level of the existing table and the new table. Ensure you have enough hard disk space on your system before using the DBMS_REDEFINITION package.

If a table is not partitioned, then you can use the ALTER TABLE...MOVE...COMPRESS FOR... statement to change the compression level. The ALTER TABLE...MOVE statement does not permit DML statements against the table while the command is running.

To change the compression level for a partition, use the ALTER TABLE...MODIFY PARTITION statement. To change the compression level for a tablespace, use the ALTER TABLESPACE statement.

##对于非分区表我们仅仅能通过ALTER TABLE...MOVE...COMPRESS FOR命令来改变表的压缩等级。在变更的过程中不同意对表进行DML操作(假设表非常大的话，就会长时间的影响应用)。假设是分区表我们能够通过上面的方式来做。也能够通过使用DBMS_REDEFINITION包来改变表的压缩等级，使用在线重定义的时候会为目标表建一个暂时备份来保存表中的数据，在线重定义过程中目标是一直能够訪问和dml的。

Adding and Dropping Columns in Compressed Tables

The following restrictions apply when adding columns to compressed tables:

##向压缩表中加入列的时候有例如以下限制：

• Basic compression—You cannot specify a default value for an added column.##对于basic压缩方式你不能为加入的列指定默认值

• OLTP compression—If a default value is specified for an added column, then the column must be NOT NULL. Added nullable columns with default values are not supported.##

The following restrictions apply when dropping columns in compressed tables:

##从压缩表中删除列时有例如以下限制：

• Basic compression—Dropping a column is not supported.##对于basic压缩来说不支持列删除

• OLTP compression—DROP COLUMN is supported, but internally the database sets the column UNUSED to avoid long-running decompression and recompression operations.##对于oltp压缩也是不支持列删除，可是我们能够把列设置为UNUSED的。避免解压缩和又一次压缩操作队它的影响。

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Exporting and Importing Hybrid Columnar Compression Tables

Hybrid Columnar Compression tables can be imported using the impdp command of the Data Pump Import utility. By default, the impdp command preserves the table properties, and the imported table is a Hybrid Columnar Compression table. On tablespaces not supporting Hybrid Columnar Compression, the impdp command fails with an error. The tables can also be exported using the expdp command.

You can import the Hybrid Columnar Compression table as an uncompressed table using the TRANSFORM:SEGMENT_ATTRIBUTES=n option clause of the impdp command.

An uncompressed or OLTP-compressed table can be converted to Hybrid Columnar Compression format during import. To convert a non-Hybrid Columnar Compression table to a Hybrid Columnar Compression table, do the following:

1. Specify default compression for the tablespace using the ALTER TABLESPACE ... SET DEFAULT COMPRESS command.

2. Override the SEGMENT_ATTRIBUTES option of the imported table during import.

Restoring a Hybrid Columnar Compression Table

There may be times when a Hybrid Columnar Compression table must be restored from a backup. The table can be restored to a system that supports Hybrid Columnar Compression, or to a system that does not support Hybrid Columnar Compression. When restoring a table with Hybrid Columnar Compression to a system that supports Hybrid Columnar Compression, restore the file using Oracle Recovery Manager (RMAN) as usual.

When a Hybrid Columnar Compression table is restored to a system that does not support Hybrid Columnar Compression, you must convert the table from Hybrid Columnar Compression to OLTP compression or an uncompressed format. To restore the table, do the following:

1. Ensure there is sufficient storage in environment to hold the data in uncompressed or OLTP compression format.

2. Use RMAN to restore the Hybrid Columnar Compression tablespace.

3. Complete one of the following actions to convert the table from Hybrid Columnar Compression to OLTP compression or an uncompressed format:

   • Use the following statement to change the data compression from Hybrid Columnar Compression to COMPRESS FOR OLTP:

     ALTER TABLE table_name MOVE COMPRESS FOR OLTP;
     

   • Use the following statement to change the data compression from Hybrid Columnar Compression to NOCOMPRESS:

     ALTER TABLE table_name MOVE NOCOMPRESS;
     

   • Use the following statement to change each partition to NOCOMPRESS:

     ALTER TABLE table_name MOVE PARTITION partition_name NOCOMPRESS;
     

     Change each partition separately.

   • Use the following statement to move the data to NOCOMPRESS in parallel:

     ALTER TABLE table_name MOVE NOCOMPRESS PARALLEL;
     

Notes and Other Restrictions for Compressed Tables

The following are notes and restrictions related to compressed tables:

• Online segment shrink is not supported for compressed tables.##压缩表不支持在线segment shrink操作

• The table compression methods described in this section do not apply to SecureFiles large objects (LOBs). SecureFiles LOBs have their own compression methods. See Oracle Database SecureFiles and Large Objects Developer‘s Guide for more information.##本节所说的压缩方式不能再SecureFiles large objects上使用。SecureFiles LOBs有自己的压缩方式

• Compression technology uses CPU. Ensure that you have enough available CPU to handle the additional load.##压缩操作对cpu的消耗比較高，确保你的cpu资源够用

• Tables created with basic compression have the PCT_FREE parameter automatically set to 0 unless you specify otherwise.##建表时指定basic压缩方式PCT_FREE值为0，当然你也能够特别指定其它的你觉得合适的值。(建表时指定oltp压缩方式，PCT_FREE值默认会被设置为10)

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Packing Compressed Tables

If you use conventional DML on a table compressed with basic compression or Hybrid Columnar Compression, then all inserted and updated rows are stored uncompressed or in a less-compressed format. To "pack" the compressed table so that these rows are compressed, use an ALTER TABLE MOVE statement. This operation takes an exclusive lock on the table, and therefore prevents any updates and loads until it completes. If this is not acceptable, then you can use online table redefinition.