Function is_describe 

pub fn is_describe(sql: &str) -> bool

Returns true if sql is a single DESCRIBE / DESC <table> statement.

DESCRIBE yields a schema listing rather than a row stream and cannot be nested inside an outer SELECT … LIMIT subquery on every backend (notably DataFusion), so callers run it directly instead of through the bounded wrapper. Returns false for anything that does not parse to exactly one DESCRIBE/DESC statement.