Zone maps

Zone maps are automatically generated, internal tables that summarize the range of column values (minimum and maximum) of rows that fall into the same extent, that is, an addressable storage block on disk.

Zone maps are always created for columns that were selected as organizing keys. Furthermore, you can only select a column as an organizing key if the data type of this column allows IBM® DB2® Analytics Accelerator for z/OS® to create a zone map.

Using the information in the zone maps, IBM Db2 Analytics Accelerator for z/OS “knows” which extents it needs to “look at” during a disk scan. Extents with relevant data (table rows containing the values demanded by a query) must be scanned; those that contain just irrelevant data can be skipped.

Zone maps thus tell the query engine whether it is worth to look at a certain extent on disk. If it is, then the clusters within an extent allow the query engine to scan the table rows in the extent much faster because rows with relevant data will be found in the same block, while blocks that do not contain relevant data can be skipped entirely.