Add system.metadata.functions table for queryable function metadata

[mbasmanova]

Today, function metadata is only available via SHOW FUNCTIONS, which returns all function signatures as a flat list. There is no way to query function metadata using standard SQL — e.g., to get distinct function names, filter by function type, or find functions owned by a specific team.

Proposal

Add a system.metadata.functions table to the existing system catalog, alongside system.runtime.queries.

Schema

Column	Type	Description
function_name	VARCHAR	Function name
function_type	VARCHAR	scalar, aggregate, or window
return_type	VARCHAR	Return type signature (e.g., bigint, array(T))
argument_types	ARRAY(VARCHAR)	Argument type signatures
is_variadic	BOOLEAN	Whether the function accepts a variable number of arguments
owner	VARCHAR	Responsible team/person
properties	JSON	Type-specific metadata (see below)

One row per function signature (overload).

Properties JSON

Type-specific metadata that doesn't apply uniformly to all function types:

Scalar functions:

{"deterministic": true, "null_on_null_input": true}

Aggregate functions:

{"order_sensitive": false, "ignore_duplicates": true}

Window functions:

{}

Example queries

-- Distinct function names
SELECT DISTINCT function_name FROM system.metadata.functions ORDER BY 1;

-- All aggregate functions
SELECT DISTINCT function_name FROM system.metadata.functions WHERE function_type = 'aggregate';

-- Functions owned by a specific team
SELECT DISTINCT function_name FROM system.metadata.functions WHERE owner = 'team_name';

-- Non-deterministic scalar functions
SELECT function_name FROM system.metadata.functions
WHERE json_extract_scalar(properties, '$.deterministic') = 'false';

-- Functions that accept a BIGINT first argument
SELECT function_name FROM system.metadata.functions WHERE argument_types[1] = 'bigint';

-- Functions with exactly 2 arguments
SELECT DISTINCT function_name FROM system.metadata.functions WHERE cardinality(argument_types) = 2;

Implementation notes

Function metadata is available via existing Velox APIs:

• getFunctionSignatures() — scalar functions (simple + vector)
• getAggregateFunctionSignatures() — aggregate functions
• windowFunctions() — window functions

Each registration includes signatures (FunctionSignature) and metadata (VectorFunctionMetadata for scalars, AggregateFunctionMetadata for aggregates). The owner field is currently only available on scalar function metadata; extending it to aggregate and window metadata would be a separate Velox change.

The implementation can follow the existing pattern of system.runtime.queries — add a new SystemTable subclass under the metadata schema.

[mbasmanova]

CC: @peterenescu @amitkdutta @natashasehgal @kagamiori