Module arg 

Window-function argument evaluation.

A window function’s argument is an arbitrary expression — SUM(price * qty), LAG(10 / denom), SUM(1) — not merely a column reference. Every argument is evaluated per row through the shared row evaluator, so a division/modulo-by-zero inside an argument raises EvalError::DivisionByZero (SQLSTATE 22012) exactly as it does in a projection or WHERE clause, and a non-column argument yields its computed value rather than a NULL placeholder.