Sets the maximum number of rows that the IQ optimizer considers for a hash algorithm.
Option can be set at the database (PUBLIC) or user level. When set at the database level, the value becomes the default for any new user, but has no impact on existing users. When set at the user level, overrides the PUBLIC value for that user only. No system privilege is required to set option for self. System privilege is required to set at database level or at user level for any user other than self.
Requires the SET ANY PUBLIC OPTION system privilege to set this option. Can be set temporary for an individual connection or for the PUBLIC role. Takes effect immediately.
When generating a query plan, the IQ optimizer might have several algorithms (hash, sort, indexed) to choose from when processing a particular part of a query. These choices often depend on estimates of the number of rows to process or generate from that part of the query. This option sets an upper boundary for how many estimated rows are considered for a hash algorithm.
For example, if there is a join between two tables, and the estimated number of rows entering the join from both tables exceeds the value of MAX_HASH_ROWS, the optimizer does not consider a hash join. On systems with more than 50 MB per user of temporary buffer cache space, you might want to consider a higher value for this option.