Distribution schemes for table fragmentation
A distribution scheme is a method that the database
server uses to distribute rows or index entries to fragments. HCL®
OneDB® database
servers support the following distribution schemes:
- Expression-based
- This distribution scheme puts rows that contain specified values in the same fragment. You specify a fragmentation expression that defines criteria for assigning a set of rows to each fragment, either as a range rule or some arbitrary rule. You can specify a remainder fragment that holds all rows that do not match the criteria for any other fragment, although a remainder fragment reduces the efficiency of the expression-based distribution scheme.
- Round-robin
- This distribution scheme places rows one after another in fragments,
rotating through the series of fragments to distribute the rows evenly.
The database server defines the rule internally.
For INSERT statements, the database server uses a hash function on a random number to determine the fragment in which to place the row. For INSERT cursors, the database server places the first row in a random fragment, the second in the next sequential fragment, and so on. If one of the fragments is full, it is skipped.
For complete descriptions of the SQL syntax you use to specify a distribution scheme, see the CREATE TABLE and CREATE INDEX statements in the HCL OneDB Guide to SQL: Syntax. For an explanation about the performance aspects of fragmentation, see your HCL OneDB Performance Guide.