oracle 表压缩技术

   压缩表是我们维护管理中常常会用到的。以下我们看都oracle给我们提供了哪些压缩方式。

文章摘自“Oracle® Database Administrator's Guide11g Release 2 (11.2)”。由于Hybrid Columnar Compression压缩仅仅有在Exadata上才支持，眼下用的比較少，我们会重点说一下basic和oltp两种压缩方式。

 

Consider Using Table Compression

As your database grows in size, consider using table compression. Compression saves disk space, reduces memory use in the database buffer cache, and can significantly speed query execution during reads. Compression has a cost in CPU overhead for data loading and DML. However, this cost might be offset by reduced I/O requirements.

##随着数据的添加。能够考虑使用表压缩技术。

压缩能够节省磁盘空间，降低SGA中buffer cache的使用，显著提高查询时读数据的效率。对压缩过后的表进行数据导入和DML须要消耗很多其它的cpu资源，可是压缩的使用降低了I/O的开销，能够抵消掉额外的cpu开销成本(依据系统的情况，假设你的库本来cpu资源已经不足，那么这样做就不合适了)

Table compression is completely transparent to applications. It is useful in decision support systems (DSS), online transaction processing (OLTP) systems, and archival systems.

##压缩技术相应用来说全然是透明的。它对dss，oltp。archival system等系统等非常实用

You can specify compression for a tablespace, a table, or a partition. If specified at the tablespace level, then all tables created in that tablespace are compressed by default.

##你能够为表空间。表或者分区指定压缩属性。假设你为表空间指定了压缩属性，那么在该表空间下建的表会默认的继承压缩属性。

Compression can occur while data is being inserted, updated, or bulk loaded into a table. Operations that permit compression include:

##压缩在insert，update和批量载入时都可能发生(详细要看你使用的是哪种压缩方式)

• Single-row or array inserts and updates

• The following direct-path INSERT methods:

  • Direct path SQL*Loader

  • CREATE TABLE AS SELECT statements

  • Parallel INSERT statements

  • INSERT statements with an APPEND or APPEND_VALUES hint

Oracle Database supports several methods of table compression. They are summarized in Table 20-1.

##oracle数据库支持例如以下几种压缩方式(注意，Hybrid Columnar Compression模式的压缩仅仅有在Exadata上才支持)

Table 20-1 Table Compression Methods

Table Compression Method	Compression Level	CPU Overhead	Applications	Notes
Basic compression	High	Minimal	DSS	None.
OLTP compression	High	Minimal	OLTP, DSS	None.
Warehouse compression (Hybrid Columnar Compression)	Higher	Higher	DSS	The compression level and CPU overhead depend on compression level specified (LOW or HIGH).
Archive compression (Hybrid Columnar Compression)	Highest	Highest	Archiving	The compression level and CPU overhead depend on compression level specified (LOW or HIGH).

When you use basic compression, warehouse compression, or archive compression, compression only occurs when data is bulk loaded into a table.

##当你使用的是basic，warehouse或者archive压缩时，仅仅有批量载入的数据才会被压缩(此处我认为有点问题，应该是仅仅有直接路径载入的数据才会被压缩。不知道官方文档这里说的批量载入是否指的就是直接路径载入)

When you use OLTP compression, compression occurs while data is being inserted, updated, or bulk loaded into a table. Operations that permit compression include:

##当你使用的是oltp压缩时。传统路径插入和直接路径插入的数据都会被压缩

• Single-row or array inserts and updates##这里就是指的传统路径插入

• The following direct-path INSERT methods:##这里指的是直接路径插入

  • Direct path SQL*Loader

  • CREATE TABLE AS SELECT statements

  • Parallel INSERT statements

  • INSERT statements with an APPEND or APPEND_VALUES hint

Basic compression compresses data inserted by direct path load only and supports limited data types and SQL operations. OLTP compression is intended for OLTP applications and compresses data manipulated by any SQL operation.

##Basic压缩方式仅支持有限的数据类型和sql操作，当数据使用直接路径插入时，这些数据会被压缩。OLTP压缩一般用在oltp系统上。可以压缩各种sql操作产生的数据

Warehouse compression and archive compression achieve the highest compression levels because they use Hybrid Columnar Compression technology. Hybrid Columnar Compression technology uses a modified form of columnar storage instead of row-major storage. This enables the database to store similar data together, which improves the effectiveness of compression algorithms. For data that is updated, Hybrid Columnar Compression uses more CPU and moves the updated rows to row format so that future updates are faster. Because of this optimization, you should use it only for data that is updated infrequently.

##由于使用了混合列模式压缩技术。Warehouse 和 archive可以提供最高的压缩等级。

混合列模式压缩技术使用列模式存储取代了之前的行模式存储。列模式存储使用数据库可以把相似的数据存储在一起从而提高压缩的效率。

假设混合列模式压缩的数据被更新，那么会消耗额外的cpu把被更新的行转变成行模式(行的rowid会发生变化)，速度也是比較快的(不知道此处的快是跟行模式压缩相比。还是同非压缩表的update对照？？？)。

基于混合列模式压缩的优化原理。我们应该在那些非常少被update的表上使用这样的压缩方式。

The higher compression levels of Hybrid Columnar Compression are achieved only with data that is direct-path inserted. Conventional inserts and updates are supported, but cause rows to be moved from columnar to row format, and reduce the compression level.

Table 20-2 lists characteristics of each table compression method.

Table 20-2 Table Compression Characteristics

Table Compression Method	CREATE/ALTER TABLE Syntax	Direct-Path INSERT	Notes
Basic compression	COMPRESS [BASIC]	Rows are compressed with basic compression.	COMPRESS and COMPRESS BASIC are equivalent. Rows inserted without using direct-path insert and updated rows are uncompressed.
OLTP compression	COMPRESS FOR OLTP	Rows are compressed with OLTP compression.	Rows inserted without using direct-path insert and updated rows are compressed using OLTP compression.
Warehouse compression (Hybrid Columnar Compression)	COMPRESS FOR QUERY [LOW|HIGH]	Rows are compressed with warehouse compression.	This compression method can result in high CPU overhead. Updated rows and rows inserted without using direct-path insert are stored in row format instead of column format, and thus have a lower compression level.
Archive compression (Hybrid Columnar Compression)	COMPRESS FOR ARCHIVE [LOW|HIGH]	Rows are compressed with archive compression.	This compression method can result in high CPU overhead. Updated rows and rows inserted without using direct-path insert are stored in row format instead of column format, and thus have a lower compression level.

You specify table compression with the COMPRESS clause of the CREATE TABLE statement. You can enable compression for an existing table by using these clauses in an ALTER TABLE statement. In this case, only data that is inserted or updated after compression is enabled is compressed. Similarly, you can disable table compression for an existing compressed table with the ALTER TABLE...NOCOMPRESS statement. In this case, all data that was already compressed remains compressed, and new data is inserted uncompressed.

##你能够在建表的时候指定compress字句。也能够在表创建后使用alter table语句改变表的压缩属性。假设是后者，那么表中已存在的数据不会被压缩，仅仅有新插入或者update的数据有可能被压缩。相同的假设你改变一个压缩表的属性为非压缩表，表中已经存在的被压缩的数据还是保持压缩的状态，不会被解压，但新插入的数据将不会再被压缩。

The COMPRESS FOR QUERY HIGH option is the default data warehouse compression mode. It provides good compression and performance when using Hybrid Columnar Compression on Exadata storage. The COMPRESS FOR QUERY LOW option should be used in environments where load performance is critical. It loads faster than data compressed with the COMPRESS FOR QUERY HIGH option.

The COMPRESS FOR ARCHIVE LOW option is the default archive compression mode. It provides a high compression level and is ideal for infrequently-accessed data. The COMPRESS FOR ARCHIVE HIGH option should be used for data that is rarely accessed.

A compression advisor, provided by the DBMS_COMPRESSION package, helps you determine the expected compression level for a particular table with a particular compression method.

Note:

Hybrid Columnar Compression is dependent on the underlying storage system. See Oracle Database Licensing Information for more information.

See Also:

• Oracle Database Concepts for an overview of table compression

• "Compressed Tablespaces"

•  

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"

Example 20-1 Creating a Table with OLTP Table Compression

##新建一个oltp类型的压缩表

The following example enables OLTP table compression on the table orders:

CREATE TABLE orders  ...  COMPRESS FOR OLTP;

Data for the orders table is compressed during both direct-path INSERT and conventional DML.

##此种压缩方式下。直接路径插入和传统路径插入的数据都会被压缩。

Example 20-2 Creating a Table with Basic Table Compression

##新建一个basic类型的压缩表

The following statements, which are equivalent, enable basic table compression on the sales_history table, which is a fact table in a data warehouse:

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

This example demonstrates using the APPEND hint to insert rows into the sales_history table using direct-path INSERT.

##使用append提示符激活直接路径插入

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

Example 20-4 Creating a Table with Warehouse Compression

This example enables Hybrid Columnar Compression on the table sales_history:

CREATE TABLE sales_history  ...  COMPRESS FOR QUERY;

The table is created with the default COMPRESS FOR QUERY HIGH option. This option provides a higher level of compression than basic or OLTP compression. It works well when load performance is critical, frequent queries are run against this table, and no DML is expected.

Example 20-5 Creating a Table with Archive Compression

The following example enables Hybrid Columnar Compression on the table sales_history:

CREATE TABLE sales_history  ...  COMPRESS FOR ARCHIVE;

The table is created with the default COMPRESS FOR ARCHIVE LOW option. This option provides the highest level of compression and works well for infrequently-accessed data.

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。

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:

Oracle Database PL/SQL Packages and Types Reference for additional information about GET_COMPRESSION_TYPE

 

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。

See Also:

• "Moving a Table to a New Segment or Tablespace" for additional information about the ALTER TABLE command

• Oracle Database PL/SQL Packages and Types Reference for additional information about the DBMS_REDEFINITION package

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。

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.

See Also:

• Oracle Database Utilities for additional information about the Data Pump Import utility

• Oracle Database SQL Language Reference for additional information about the ALTER TABLESPACE command

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;
     

See ALso:

• Oracle Database Backup and Recovery User's Guide for additional information about RMAN

• Oracle Database SQL Language Reference for additional information about the ALTER TABLE command

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)

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.

See Also:

• Oracle Database SQL Language Reference for more details on the CREATE TABLE...COMPRESS, ALTER TABLE...COMPRESS, and ALTER TABLE...MOVE statements, including restrictions

• Oracle Database VLDB and Partitioning Guide for more information on table partitioning

• "Improving INSERT Performance with Direct-Path INSERT"

• "Redefining Tables Online"