Function is_udtf_function 

pub fn is_udtf_function(name: &str) -> bool

Checks if a function is a table-generating function / UDTF (e.g., UNNEST, EXPLODE).

UDTFs return multiple rows for each input row, expanding the result set. This classification affects how lineage is tracked through these functions.

The check is case-insensitive. Uses ASCII lowercase for performance since SQL function names are always ASCII.