Struct Membership 

pub struct Membership {
    pub over: Core,
    pub key: Core,
    pub param: VarId,
    pub keep: Presence,
}

One membership test, as the restriction that answers it.

Lookup’s sibling, and the difference is what comes out: a lookup produces a row and this produces the element the filter was given, kept or dropped. So there is no rewritten body here — the operator has no per-element function beyond the key, because a predicate the index answers is not a predicate any more.

Fields

over: Core

The collection whose keys decide, in the caller’s variables: the first argument of the map_contains.

key: Core

The key to probe it by, as a function of the element. This is the Fun body of crate::plan::Op::Restrict.

param: VarId

The parameter key is written over — the filtered element.

keep: Presence

Which answer keeps the row: the predicate as written, or its negation.