Hi, Apache Calcite has a powerful but complicated and non-documented function library. Some projects may require overriding some of the existing operators to introduce custom type deduction, custom validation, etc. This includes the base arithmetic functions (e.g, disallow INT + VARCHAR), aggregate functions (e.g., custom precision extension), etc.
One convenient way of doing this is to re-define the function in your custom operator table. However, this doesn't work because Apache Calcite core uses the direct references to SqlStdOperatorTable. It starts with the parser [1] and validator [2]. If you manage to inject your functions at this stage (e.g., using a custom validator implementation or a custom SqlVisitor), the sql-to-rel converter will overwrite your functions still [3]. And even when you get the RelNode, optimization rules would silently replace your custom functions with the default ones [4]. Alternatively, you may try extending some base interface, such as the TypeCoercion, but this doesn't give fine-grained control over the function behavior because you have to retain the existing function definitions to do coercion works. A better solution might be is to abstract out the function references through some sort of "factory"/"resolver", somewhat similar to the one used to resolve user-provided operators. For instance, the user may pass an optional desired operator table to parser/validator/converter configs and RelOptCluster. Then the "core" codebase could be refactored to dereference functions by SqlKind instead of SqlStdOperatorTable. If some required function types are missing from the SqlKind enum, we can add them. The default behavior would delegate to SqlStdOperatorTable, so the existing apps would not be affected. A "small" problem is that there are ~1500 usages of the SqlStdOperatorTable in the "core" module, but most of the usages are very straightforward to replace. This way, we would ensure a consistent function resolution throughout all query optimization phases. WDYT? Regards, Vladimir. [1] https://github.com/apache/calcite/blob/calcite-1.30.0/core/src/main/codegen/templates/Parser.jj#L7164 [2] https://github.com/apache/calcite/blob/calcite-1.30.0/core/src/main/java/org/apache/calcite/sql/validate/SqlValidatorImpl.java#L6788 [3] https://github.com/apache/calcite/blob/calcite-1.30.0/core/src/main/java/org/apache/calcite/sql2rel/SqlToRelConverter.java#L1438 [4] https://github.com/apache/calcite/blob/calcite-1.30.0/core/src/main/java/org/apache/calcite/rel/rules/AggregateReduceFunctionsRule.java#L357
