Adjusting Skew Allowance
The DBS Control SkewAllowance field specifies a percentage factor value used by the Optimizer in choosing the size of each hash join partition. SkewAllowance reduces the memory size for the hash join specified by HTMemAlloc. This allows the Optimizer to take into account a potential skew of the data that could make the hash join run slower than a merge join.
If you know your data very well and do not expect skewing at this extreme, you can set this value to 50%, which still allows for a skew that is double the size the Optimizer uses in its estimates.
Values 20 through 74% can improve hash join performance in some single-user situations but should not be used on production systems. Do not use these values whenever more than one user is logged on. Most end users never have a need to use settings in these ranges.
A setting higher than 75% might improve hash join performance when the data is so severely skewed that it degrades performance. In this case, turn SkewAllowance off or try increasing the value to 80%.