goldmedal opened a new issue, #15363:
URL: https://github.com/apache/datafusion/issues/15363
### Is your feature request related to a problem or challenge?
In my case, I would like to extend a default function's supporting
signature. For example, the `avg` function in DataFusion doesn't support
calculating the Duration average.
However, suppose I want to register a custom `avg` for Duration. In that
case, it means that I need to re-implement the whole `avg` function because the
new implementation will override the previous one when registered. I want to
propose an approach to register the custom implementation and keep the original
implementation at the same time.
### Describe the solution you'd like
In Postgres or DuckDB, their function definitions have catalog and schema.
We can create a macro or UDF with the specific catalog or schema prefix. Then,
we can set up the search_path to decide where to find the functions. If no
matching candidates are found in the search path, we can find the candidate in
the default path.
If we have similar features, I imagine I can follow these steps to extend an
existing function easily:
1. Create a schema `self`.
2. Implement an `avg` function for Duration.
3. Register the `self.avg` function.
4. Set `search_path` to `self`.
5. Query the SQL `select avg(time_col1 - time_col2) from table1` (Invoke
self.avg internally).
6. Query the SQL `select avg(int_col) from table1` (Invoke default avg
internally).
To approach this proposal, I think we need two new features:
- Register and use UDFs with the catalog and schema. I think we can do
something at:
https://github.com/apache/datafusion/blob/d460abb63f6ff3abce6de022834f6f17c77db411/datafusion/core/src/execution/session_state.rs#L1773-L1778
- Implement `search_path` for searching function candidates.
### Describe alternatives you've considered
_No response_
### Additional context
_No response_
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