Gemäß dem Quellcode, der sich hier befindet, begrenzt PostgreSQL ab Zeile 850 die Anzahl der Argumente nicht explizit.
Das Folgende ist ein Codekommentar aus Zeile 870:
/*
* We try to generate a ScalarArrayOpExpr from IN/NOT IN, but this is only
* possible if the inputs are all scalars (no RowExprs) and there is a
* suitable array type available. If not, we fall back to a boolean
* condition tree with multiple copies of the lefthand expression.
* Also, any IN-list items that contain Vars are handled as separate
* boolean conditions, because that gives the planner more scope for
* optimization on such clauses.
*
* First step: transform all the inputs, and detect whether any are
* RowExprs or contain Vars.
*/
