Purpose
Specifies the percentage of cache memory to be allocated to a hash table that is used for a hash join. Hash joins are used as optimizations to a merge joins under specific conditions.
This field should be changed only under the direction of the Teradata Support Center.
Field Group
Performance
Valid Range
0 through 10
A setting of zero prevents hash joins from being used as optimizations.
Default
10
Changes Take Effect
After the DBS Control Record has been written.
Usage Notes
Hash joins can benefit performance when skew is low.
Larger values for HTMemAlloc allow hash join optimizations to be applied to larger tables.
The Optimizer uses the value of HTMemAlloc to determine the size of the hash table as follows:
Hash Table Size = (HTMemAlloc/100) * 20 MB
20 MB is the maximum amount of memory set aside by Teradata Database for hash joins.
Related Topics
| For more information on… | See… |
|---|---|
| SkewAllowance field | SkewAllowance. |
| hash table size calculations and possible values | SQL Request and Transaction Processing. |