Support UDTFs in information_schema.routines / SHOW FUNCTIONS

[zhuqi-lucas]

Which issue does this PR close?

Closes #23437.

Rationale for this change

information_schema.routines and SHOW FUNCTIONS currently enumerate only scalar / aggregate / window UDFs. Table functions (UDTFs) registered via SessionContext::register_udtf are omitted, making them undiscoverable through SQL. Discovery matters because downstream tooling (DataFusion CLI, Massive's atlas SQL surface, dbt-datafusion, etc.) uses these SQL surfaces to list available functions.

What changes are included in this PR?

1. Snapshot table functions into the information_schema provider.

TaskContext cannot expose table_functions without introducing a crate cycle (TableFunction lives in datafusion-catalog, which depends on datafusion-execution where TaskContext lives). Instead, snapshot SessionState.table_functions into InformationSchemaProvider at construction time. The provider is built per-query via schema_for_ref, so the snapshot stays fresh.

2. Emit UDTF rows.

• make_routines: one row per UDTF with routine_type = \"FUNCTION\", function_type = \"TABLE\", data_type = \"TABLE\".
• make_parameters: a single synthetic OUT row per UDTF (data_type = \"TABLE\") so the JOIN in SHOW FUNCTIONS resolves. UDTFs have no scalar signature, so no IN rows are emitted.

3. Rewrite the SHOW FUNCTIONS SQL.

The old query joined parameters p (INNER) requiring both IN and OUT rows before joining routines. UDTFs have no IN parameters, so they fell out. New shape:

```
FROM (parameters WHERE mode='OUT') o
LEFT JOIN (parameters WHERE mode='IN') i
ON matching keys
JOIN information_schema.routines r
ON name matches
```

Are these changes tested?

Verified locally by registering a UDTF and running SHOW FUNCTIONS:

```

SHOW FUNCTIONS LIKE 'stale%'
+---------------+-------------+---------------+
| function_name | return_type | function_type |
| stale_files | TABLE | TABLE |
+---------------+-------------+---------------+
```

Happy to add sqllogictests in datafusion/sqllogictest/test_files/information_schema.slt if maintainers prefer.

Are there any user-facing changes?

Yes: SHOW FUNCTIONS output now includes table functions. information_schema.routines and information_schema.parameters gain rows for each registered UDTF. Existing rows are unchanged.

[Copilot]

Pull request overview

This PR extends DataFusion’s SQL discovery surfaces to include table functions (UDTFs) in information_schema.routines and SHOW FUNCTIONS, addressing the gap where only scalar/aggregate/window UDFs were listed.

Changes:

• Snapshot SessionState.table_functions into InformationSchemaProvider so UDTFs can be exposed without introducing crate cycles.
• Emit information_schema.routines and information_schema.parameters rows for UDTFs (synthetic OUT parameter row with data_type = 'TABLE').
• Rewrite SHOW FUNCTIONS SQL to start from OUT parameter rows and LEFT JOIN IN parameter rows so functions with no inputs can appear.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
datafusion/sql/src/statement.rs	Rewrites SHOW FUNCTIONS to be compatible with functions that lack IN parameters (e.g., UDTFs).
datafusion/core/src/execution/session_state.rs	Passes the session’s registered table functions into the per-query InformationSchemaProvider.
datafusion/catalog/src/information_schema.rs	Stores table functions in InformationSchemaConfig and emits corresponding routines / parameters rows.

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