Version 11.5 contains numerous compression and space reclamation enhancements for
column-organized tables that continue to make Db2 an
industrial-strength database solution that is suitable for any size of organization.
The following table displays a list of compression and space reclamation enhancements for
column-organized tables in Version 11.5
Table 1. Compression and space reclamation enhancements in Version 11.5
|New compression daemon
||EDU db2cmpd executes compression related tasks in the background.
|Automatic Recompress feature for tables
||If an insert operation is used to populate a table, automatic creation of the compression
dictionaries begin once a threshold count of rows are inserted into the table. This design ensures
that a large enough sample of rows exists to build dictionaries that yield an adequate compression
ratio. Rows that are inserted before the compression dictionaries are created populated and are not
initially compressed. Rows inserted after dictionary creation will be compressed. The Automatic
Recompress feature for tables uses the new compression daemon to asynchronously check for tables
that contain rows that have not been compressed at the front of the table. It then recompresses
those rows in place which may leave empty extents in the table. These empty extents can be reclaimed
later. Only the values at the front of the table that were inserted before the dictionary was
created will be recompressed. The Automatic Recompress feature runs online, concurrent with other
insert, update, delete, and select transactions. In addition to improving space utilization, it will
also improve query performance which is more efficient processing encoded data. As a result, queries
will run faster on a table where the front part of the table has been recompressed with the
dictionary. The Automatic Recompress feature requires no user intervention, and uses the REORG TABLE
command with a new RECOMPRESS option to recompress the rows. The user may notice the REORG TABLE ...
RECOMPRESS command while monitoring the table. For simplicity, this feature is fully automated and
manual execution of this RECOMPRESS option is not supported
|Enhancements to REORG TABLE... RECLAIM EXTENTS
||REORG TABLE... RECOMPRESS encodes and compresses the data in place and may leave empty
extents in table. REORG TABLE... RECLAIM EXTENTS has been enhanced to find these empty extents and
reclaim them so that other tables in the same table space may reuse them.