Major Description
SqlDialect contains a 'supportsFunction' that can be used by rules to know whether a certain function is supported in the given dialect, e.g., to choose whether to push a Filter expression to JDBC, etc.
The default implementation of 'supportsFunction' always returns true.
I believe a better idea would be to support in the default implementation for the method the most common SQL functions. Then each dialect can override that behavior and expand/limit the supported functions, e.g., JethroDataDialect already does that.