@Override
public boolean equals(Relation arg) {
if (arg == this) return true;
if (arg == null || !(arg instanceof Relation)) return false;
Relation other = (Relation) arg;
return TYPE.equals(other.getType()) && other.stream().findFirst().isPresent();
}
/**
* we look for keys that are preserved by the projection and if so these keys are the new keys,
* otherwise the 'on' attributes form the new key (key of the projection)
*/
@Override
public Keys lazyComputeKey() {
Keys keys = operand.getKeys();
Predicate<Key> p = (k) -> isKeyPreserving(k);
Set<Key> kept = keys.stream().filter(p).collect(Collectors.toSet());
if (kept.isEmpty()) {
return new Keys(type.getLargestKey());
} else {
return new Keys(kept);
}
}
public boolean isKeyPreserving() {
Predicate<Key> pred = (k) -> isKeyPreserving(k);
return operand.getKeys().stream().anyMatch(pred);
}
@Override
protected RelationType typeCheck() {
return operand.getType().project(attributes);
}