Extent and Zonemaps

Extents and Zonemaps in Netezza

An extent is the smallest unit of disk allocation on a SPU. One extent is equal to 3MB of a disk.

A zonemap is an internal mapping structure to extents that take advantage of the internal ordering of data to eliminate extents that do not need to be scanned. Zonemaps transparently avoid scanning of unreferenced rows.

Zonemaps are created for every column in the table and contain the minimum and maximum values for every extent. In layman terms zone maps contains the minimum and maximum values of a column. Whenever you issues an sql query with where clause, the netezza system first scans the zonemaps to find the addresses of the minimum and maximum extents that contains the column values. After that the netezza system scans the column values in sequential order, finds for a match and then displays the result.

Zonemaps are created and refreshed for every SPU when you

1. Generate statistics
2. Nzload operation
3. Insert, update operations
4. Nzreclaim operation