planEq

Structural equality of two plan subtrees: same node kind, same payload (expressions compared with exprEq, aggregation specifications field-wise) and recursively equal children. Source leaves compare by buffer identity - same columns, dtypes, heights and the same underlying immutable chunk storage - never by content: two frames with equal values but separate buffers do not share (conservative and cheap; copies of one frame, the shape reuse actually produces, always share).