Recommended design for accessing context/session information in UDF implementation

[sunng87]

As for now, most datafusion udfs are focusing on data. When I'm going to implement some functions like database() or version(), I just realized there is no simple way to access information of current session.

For example, in MySQL SELECT database() returns current selected database. If we implement database as a UDF, we will need to access current session on calling ScalarFunctionImplementation. However, the function signature only accepts input data.

Another use case is to access time-zone or locale settings from current session when implementing a date/time related function.

While the function system is under a major refactoring, you may want to add this feature into consideration. Thank you!

[alamb]

Hi @sunng87 -- I think the built in now() function does something similar -- specifically it needs to know the session start time from the session context.

In particular it gets access to https://docs.rs/datafusion/latest/datafusion/execution/context/struct.ExecutionProps.html

I wonder if you could use the var_providers mechanism or maybe we need to plumb the ExecutionProps into UDFs somehow

[sunng87]

Thank you, @alamb ! I will look into ExecutionProps and will provide feedback if there is any issue.