DB2 10.5 for Linux, UNIX, and Windows

Distribution maps

In a partitioned database environment, the database manager must know where to find the data that it needs. The database manager uses a map, called a distribution map, to find the data.

A distribution map is an internally generated array containing either 32 768 entries for multiple-partition database partition groups, or a single entry for single-partition database partition groups. For a single-partition database partition group, the distribution map has only one entry containing the number of the database partition where all the rows of a database table are stored. For multiple-partition database partition groups, the numbers of the database partition group are specified in a way such that each database partition is used one after the other to ensure an even distribution across the entire map. Just as a city map is organized into sections using a grid, the database manager uses a distribution key to determine the location (the database partition) where the data is stored.

For example, assume that you have a database on four database partitions (numbered 0-3). The distribution map for the IBMDEFAULTGROUP database partition group of this database is:

   0 1 2 3 0 1 2 ...

If a database partition group had been created in the database using database partitions 1 and 2, the distribution map for that database partition group is:

   1 2 1 2 1 2 1 ...

If the distribution key for a table to be loaded into the database is an integer with possible values between 1 and 500 000, the distribution key is hashed to a number between 0 and 32 767. That number is used as an index into the distribution map to select the database partition for that row.

Figure 1 shows how the row with the distribution key value (c1, c2, c3) is mapped to number 2, which, in turn, references database partition n5.

Figure 1. Data distribution using a distribution map

Graphic showing data distribution using a distribution map.

A distribution map is a flexible way of controlling where data is stored in a multi-partition database. If you must change the data distribution across the database partitions in your database, you can use the data redistribution utility. This utility allows you to rebalance or introduce skew into the data distribution.

You can use the db2GetDistMap API to obtain a copy of a distribution map that you can view. If you continue to use the sqlugtpi API to obtain the distribution information, this API might return error message SQL2768N, because it can only retrieve distribution maps containing 4096 entries.