[SPARK-56883][SQL] DESCRIBE FUNCTION for SQL UDFs

[srielau]

What changes were proposed in this pull request?

Renders a structured DESCRIBE FUNCTION [EXTENDED] output for SQL user-defined functions (temporary and persistent) in place of the generic Function / Class / Usage:<json blob> dump that DescribeFunctionCommand produces today for any function whose ExpressionInfo.className != null.

For SQL UDFs the output becomes:

• Function: qualified name
• Type: SCALAR or TABLE
• Input: parameter list (name + SQL type, column-aligned; DEFAULT <expr> and 'comment' annotations are added in EXTENDED mode)
• Returns: scalar return type, or the table return columns (column comments and defaults are added in EXTENDED mode)
• EXTENDED only: Comment, Collation, Deterministic, Data Access (CONTAINS SQL / READS SQL DATA), Configs, Owner, Create Time, Body, and SQL Path.

SQL Path: is emitted only when both spark.sql.path.enabled = true and a frozen path was persisted on the function at CREATE FUNCTION time (SPARK-56639 / SPARK-56520). The path is read from the function's function.resolutionPath property and rendered through SqlPathFormat.formatForDisplay, producing the same `catalog`.`namespace` format used elsewhere in DESCRIBE output. This shows the resolution path that the function will use during analysis — the creator's PATH frozen at CREATE time, not the invoker's current PATH.

Behavior for builtin functions and non-SQL UDFs is unchanged.

Class hierarchy / dispatch:

• SQLFunction (catalyst): adds the SCALAR / TABLE constants and a new fromExpressionInfo(info, parser) constructor that reconstructs a SQLFunction from the JSON usage blob produced by toExpressionInfo. This is the same path used by both temp UDFs (which are not in the catalog) and persistent UDFs.
• DescribeFunctionCommand (sql/core): when SQLFunction.isSQLFunction(info.getClassName) is true, dispatches to a new describeSQLFunction(info, parser) helper that emits the column-aligned key/value rows shown above. The frozen SQL PATH is rendered inline through SqlPathFormat; the temporary DescribeFunctionCommandUtils helper introduced for that purpose by SPARK-56639 is removed (its single responsibility is now absorbed by describeSQLFunction).
• SessionCatalog.registerFunction: when a persistent SQL UDF is invoked for the first time, the function registry caches it. Previously the cached ExpressionInfo was always built via makeExprInfoForHiveFunction, which sets usage = null. That worked for the pre-existing DESCRIBE FUNCTION codepath (which doesn't read usage), but breaks the new describeSQLFunction path: after a SQL UDF has been invoked once, DESCRIBE FUNCTION reads back the cached info and SQLFunction.fromExpressionInfo cannot parse null. registerFunction now branches on funcDefinition.isUserDefinedFunction and builds the structured ExpressionInfo via UserDefinedFunction.fromCatalogFunction(funcDefinition, parser).toExpressionInfo for SQL UDFs (matching the lookup-side build in lookupPersistentFunction), so the cached info has the right usage blob for DESCRIBE.

Why are the changes needed?

DESCRIBE FUNCTION is intended to give users a human-readable description of a routine, analogous to DESCRIBE TABLE for tables. For SQL UDFs the current output instead exposes the internal serialization format:

> DESCRIBE FUNCTION EXTENDED area;
 Function: default.area
 Class: sqlFunction.
 Usage: {"sqlFunction.inputParam":"width DOUBLE,height DOUBLE","sqlFunction.returnType":"DOUBLE","sqlFunction.expression":"width * height","sqlFunction.isTableFunc":"false",...}
 Extended Usage:

That JSON blob is not part of any public surface, and the literal string sqlFunction. for Class: is meaningless to users. All of the structured metadata we need — signature, return type, body, characteristics, frozen SQL PATH — is already serialized in ExpressionInfo; this PR just formats it.

Does this PR introduce any user-facing change?

Yes — the rows returned by DESCRIBE FUNCTION [EXTENDED] <sql_udf> change.

Before:

> DESCRIBE FUNCTION EXTENDED area;
 Function: default.area
 Class: sqlFunction.
 Usage: {"sqlFunction.inputParam":"width DOUBLE,height DOUBLE", ...}
 Extended Usage:

After (simple case):

> DESCRIBE FUNCTION EXTENDED area;
 Function:      default.area
 Type:          SCALAR
 Input:         width  DOUBLE 'width'
                height DOUBLE 'height'
 Returns:       DOUBLE
 Comment:       compute area
 Deterministic: true
 Data Access:   CONTAINS SQL
 Owner:         <owner>
 Create Time:   <timestamp>
 Body:          width * height

After (function created under spark.sql.path.enabled = true with a non-default PATH at CREATE time):

> SET spark.sql.path.enabled = true;
> SET PATH = spark_catalog.path_func_db_a, system.builtin;
> CREATE FUNCTION frozen_fn() RETURNS INT RETURN (SELECT MAX(id) FROM frozen_t);
> SET PATH = spark_catalog.path_func_db_b, system.builtin;
> DESCRIBE FUNCTION EXTENDED default.frozen_fn;
 Function:      default.frozen_fn
 Type:          SCALAR
 Input:         ()
 Returns:       INT
 ...
 Body:          (SELECT MAX(id) FROM frozen_t)
 SQL Path:      `spark_catalog`.`path_func_db_a`, `system`.`builtin`

SQL Path reflects the creator's frozen PATH, not the session's current PATH at describe time. Output for builtin functions, Hive UDFs, and other non-SQL UDFs is unchanged.

How was this patch tested?

Added four unit tests to SQLFunctionSuite (sql/core):

• describe SQL scalar functions — temporary and persistent scalar UDFs with comments, defaults, and EXTENDED mode. Asserts Function, Type, Input (column-aligned, with DEFAULT and 'comment' in extended mode), Returns, Deterministic, Data Access, Comment, Create Time, Body.
• describe SQL table functions — table UDFs with explicit return columns; asserts Type: TABLE, Returns columns, and the EXTENDED-only fields.
• describe SQL functions with derived routine characteristics — checks that Deterministic and Data Access reflect derived values for functions that read tables / call non-deterministic builtins, and that user-supplied characteristics are preserved.
• The existing SPARK-56639: SQL function uses frozen SQL path test is extended: after switching PATH to a different namespace it invokes default.frozen_fn (populating the function-registry cache) and then runs DESCRIBE FUNCTION EXTENDED default.frozen_fn, asserting the SQL Path: row shows the creator's frozen path (`spark_catalog`.`path_func_db_a`, `system`.`builtin`) and does not mention the invoker's current path namespace. This extension also exercises the SessionCatalog.registerFunction fix above: prior to the fix, the DESCRIBE after the invocation hit CORRUPTED_CATALOG_FUNCTION because the cached ExpressionInfo had usage = null.

Each describe test uses checkKeywordsExist against DESCRIBE FUNCTION [EXTENDED] <name> output.

Was this patch authored or co-authored using generative AI tooling?

Generated-by: Claude (claude-opus-4-7)