@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);
}
}
/** Dee, the relation with no attribute but one tuple. */
public class Dee extends AbstractRelation {
private static final RelationType TYPE = RelationType.dress();
private static Relation INSTANCE;
private static final Tuple EMPTY_TUPLE = Tuple.dress();
private Dee() {}
public static synchronized Relation instance() {
if (INSTANCE == null) {
INSTANCE = new Dee();
}
return INSTANCE;
}
@Override
public RelationType getType() {
return TYPE;
}
@Override
public <R> R accept(Visitor<R> visitor) {
return visitor.visit(this);
}
public Cog toCog() {
return new BaseCog(this, () -> Collections.singleton(EMPTY_TUPLE).stream());
}
@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();
}
}
