Determines how block-level compression (BLC) is managed for Permanent storage tables that have the BLOCKCOMPRESSION option set to DEFAULT, or that do not have the BLOCKCOMPRESSION option explicitly specified when the table is created.
|MANUAL||BLC is applied based on the settings of these DBS Control fields: CompressPermPrimaryDBs, CompressPermFallbackDBs, CompressPermPrimaryCLOBDBs, CompressPermFallbackCLOBDBs which determine the default BLC for these table types at the time the table is created. These defaults can be overridden using the BLOCKCOMPRESSION query band at load time.
Tables can be compressed or uncompressed at any time after loading by using the Ferret COMPRESS and UNCOMPRESS commands.
Data inserted into existing tables inherits the current compression status of the table at the time the data is inserted.
|AUTOTEMP||BLC is automatically performed by Vantage based on the frequency of access to the table data (the data temperature). Temperature-based BLC (TBBLC) compresses infrequently accessed data, and uncompresses data that it is accessed frequently.
If temperature-based BLC (TBBLC) is disabled, tables that are set to use AUTOTEMP will behave like tables set to use MANUAL compression until TBBLC is enabled again. TBBLC is enabled and disabled using the EnableTempBLC field.
|ALWAYS||Data in the table is always compressed at the data block level.|
|NEVER||Data in the table is never compressed at the data block level.|
Changes Take Effect
After the DBS Control Record has been written.
- Hot data is frequently accessed.
- Cold data is infrequently accessed.
- Warm data is accessed moderately frequently.
Data temperature is measured separately for each AMP at a cylinder level and is a relative measure because the temperature of cylinders can change even if the data it is not directly accessed. This is because it depends on how often other cylinders are accessed in comparison. TBBLC automatically compresses cold data to save storage space and decompresses Warm or Hot data if it was previously Cold (and thus compressed). At any time, different portions of a table, which are spread among many cylinders, may exist in different states of compression depending on their temperatures.
BLC enables data compression at the data block (DB) level of the file system. Compression reduces the amount of storage required for a given amount of data. The BlockLevelCompression field of DBS Control enables and disables BLC.
When changed, causes TBBLC to immediately start a new cycle.
|For more information on…||See…|
|Block-level compression||BlockLevelCompression and Teradata Vantage™ - Database Design, B035-1094.|
|BLOCKCOMPRESSION table attribute||Teradata Vantage™ - SQL Data Definition Language Syntax and Examples, B035-1144.|
|Ferret COMPRESS, UNCOMPRESS, and FORCE commands||Ferret Utility (ferret).|
|Query bands for compression||Teradata Vantage™ - SQL Data Definition Language Syntax and Examples, B035-1144.|
|Storage group of DBS Control fields||Storage Group.|