BlockLevelCompression
Purpose
Indicates whether the block-level compression (BLC) feature is enabled. This feature allows data blocks (DBs) to be compressed for different categories of tables, subject to other settings in the Compression group of DBS Control fields.
Field Group
Compression
Valid Setting
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Setting |
Description |
|
On |
BLC feature is enabled. DBs from different categories of table may or may not be compressed, subject to the other settings in the Compression group. |
|
Off |
BLC feature, including temperature-based BLC, is disabled for all categories of tables. Newly created data blocks will not be compressed. Existing blocks that have already been compressed will remain compressed. They can be uncompressed using the Ferret utility UNCOMPRESS command. The Ferret COMPRESS command is disabled. |
Default
OFF
Changes Take Effect
After the DBS Control Record has been written.
Usage Notes
BLC enables data compression at the data block (DB) level of the Teradata Database 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.
BLC increases CPU utilization for dynamic compression and decompression operations. This can cause certain operations on compressed tables to use considerably more CPU resources than similar operations on noncompressed tables. Hardware-based BLC (using a compression engine board in every system node) has only a minimal effect on CPU utilization. Unless the system is CPU-rich, these operations can impact other workloads, and could lengthen elapsed response times. However, in some situations, BLC can improve database performance due to reduced I/O bandwidth.
BlocklevelCompression also affects tables that were loaded using the BlockCompression query band to specify data block compression. If BlockLevelCompression is Off, data added to these tables will not be compressed.
The PermDBSize and JournalDBSize settings in DBS Control apply to the uncompressed size of these types of DBs.
Related Topics
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For more information on... |
See... |
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COMPRESS and UNCOMPRESS commands |
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BLOCKCOMPRESSION query band |
SQL Data Definition Language. |