Speeding up heterogeneous data access by converting and pushing down string comparisons

Pushing down predicates to an external data source is critical to the performance for querying heterogeneous data sources. However, predicate-containing string comparisons cannot be pushed down unchanged if the external data source uses a different collating sequence. We describe a table-driven technique for rewriting such predicates to account for the differences in collation. In addition to precise conversion, we also consider imprecise conversion.