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帮助测试一下列压缩
table_compression
The table_compression clause is valid only for heap-organized tables. Use this clause to instruct the database whether to compress data segments to reduce disk use. The COMPRESS keyword enables table compression. The NOCOMPRESS keyword disables table compression. NOCOMPRESS is the default.
When you enable table compression by specifying either COMPRESS or COMPRESS BASIC, you enable basic table compression. Oracle Database attempts to compress data during direct-path INSERT operations when it is productive to do so. The original import utility (imp) does not support direct-path INSERT, and therefore cannot import data in a compressed format.
In earlier releases, this type of compression was called DSS table compression and was enabled using COMPRESS FOR DIRECT_LOAD OPERATIONS. This syntax has been deprecated.
See Also:
"Conventional and Direct-Path INSERT" for information on direct-path INSERT operations, including restrictions
When you enable table compression by specifying COMPRESS FOR OLTP, you enable OLTP table compression. Oracle Database compresses data during all DML operations on the table. This form of compression is recommended for OLTP environments.
In earlier releases, OLTP table compression was enabled using COMPRESS FOR ALL OPERATIONS. This syntax has been deprecated.
Note:
Tables with COMPRESS or COMPRESS BASIC use a PCTFREE value of 0 to maximize compression, unless you explicitly set a value for PCTFREE in the physical_attributes_clause. Tables with COMPRESS FOR OLTP or NOCOMPRESS use the PCTFREE default value of 10, to maximize compress while still allowing for some future DML changes to the data, unless you override this default explicitly.
When you specify COMPRESS FOR QUERY or COMPRESS FOR ARCHIVE, you enable hybrid columnar compression. With hybrid columnar compression, data can be compressed during bulk load operations. During the load process, data is transformed into a column-oriented format and then compressed. Oracle Database uses a compression algorithm appropriate for the level you specify. In general, the higher the level, the greater the compression ratio. Hybrid columnar compression can result in higher compression ratios, at a greater CPU cost. Therefore, this form of compression is recommended for data that is not frequently updated.
COMPRESS FOR QUERY is useful in data warehousing environments. Valid values are LOW and HIGH, with HIGH providing a higher compression ratio. The default is HIGH.
COMPRESS FOR ARCHIVE uses higher compression ratios than COMPRESS FOR QUERY, and is useful for compressing data that will be stored for long periods of time. Valid values are LOW and HIGH, with HIGH providing the highest possible compression ratio. The default is LOW.
See Also:
Oracle Exadata Storage Server Software documentation for more information on hybrid columnar compression, which is a feature of Oracle Exadata
You can specify table compression for the following portions of a heap-organized table:
For an entire table, in the physical_properties clause of relational_table or object_table
For a range partition, in the table_partition_description of the range_partitions clause
For a composite range partition, in the table_partition_description of the range_partition_desc
For a composite list partition, in the table_partition_description of the list_partition_desc
For a list partition, in the table_partition_description of the list_partitions clause
For a system or reference partition, in the table_partition_description of the reference_partition_desc
For the storage table of a nested table, in the nested_table_col_properties clause
See Also:
Oracle Database PL/SQL Packages and Types Reference for information about the DBMS_COMPRESSION package, which helps you choose the correct compression level for an application, and Oracle Database Administrator's Guide for more information about table compression, including examples
Restrictions on Table Compression
Table compression is subject to the following restrictions:
COMPRESS FOR OLTP and COMPRESS BASIC are not supported for tables with more than 255 columns.
Data segments of BasicFile LOBs are not compressed. For information on compression of SecureFile LOBs, see LOB_compression_clause.
You cannot drop a column from a table that uses COMPRESS BASIC, although you can set such a column as unused. All of the operations of the ALTER TABLE ... drop_column_clause are valid for tables that use COMPRESS FOR OLTP, COMPRESS FOR QUERY, and COMPRESS FOR ARCHIVE.
If you specify COMPRESS FOR OLTP, then chained rows are not compressed unless the header for the row remains in the original block and all row columns are moved to another block. If the row chaining results in leaving just the row header in the block and moving all of the row's columns to the next block, and they all fit in the next block, then the columns can be compressed.
You cannot specify any type of table compression for an index-organized table, any overflow segment or partition of an overflow segment, or any mapping table segment of an index-organized table.
You cannot specify any type of table compression for external tables or for tables that are part of a cluster.
You cannot specify hybrid columnar compression on tables with LONG columns, tables with virtual columns, tables that are owned by the SYS schema and reside in the SYSTEM tablespace, and tables with row dependencies enabled.
Logical standby, Streams, and LogMiner are not supported on hybrid columnar compressed tables.
When you update a row in a table compressed with hybrid columnar compression, the ROWID of the row may change.
In tables compressed with hybrid columnar compression, updates to a single row may result in locks on multiple rows. Concurrency for write transactions may therefore be affected.
If a table compressed with hybrid columnar compression has a foreign key constraint, and you insert data using INSERT with the APPEND hint, then the data will be compressed using OLTP. To compress the data with hybrid columnar compression, disable the foreign key constraint, insert the data using INSERT with the APPEND hint, and then reenable the foreign key constraint. |
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楼主: Himmelskorper
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发表于 2009-9-2 15:48
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发表于 2009-9-2 15:56
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