Why we cannot get all predicates for outer join query?

[Jiajun Xie]

Hello, all.

I try to use RelMetadataQuery#getAllPredicates get predicate,
but I get null for outer join query that left column name is same as right
column name.
```
final RelNode rel = sql("select name as dname from emp left outer join dept"
    + " on emp.deptno = dept.deptno").toRel();
final RelMetadataQuery mq = rel.getCluster().getMetadataQuery();
final RelOptPredicateList r = mq.getAllPredicates(rel);
assertNull(r);
```


After commenting on two pieces of code:
1. RelMdAllPredicates#getAllPredicates
```
if (join.getJoinType().isOuterJoin()) {
  // We cannot map origin of this expression.
  return null;
}
2. RelMdExpressionLineage#getExpressionLineage
```
if (rel.getJoinType().isOuterJoin()) {
  // If we reference the inner side, we will bail out
  if (rel.getJoinType() == JoinRelType.LEFT) {
    ImmutableBitSet rightFields = ImmutableBitSet.range(
        nLeftColumns, rel.getRowType().getFieldCount());
    if (inputFieldsUsed.intersects(rightFields)) {
      // We cannot map origin of this expression.
      return null;
    }
  } else if (rel.getJoinType() == JoinRelType.RIGHT) {
    ImmutableBitSet leftFields = ImmutableBitSet.range(
        0, nLeftColumns);
    if (inputFieldsUsed.intersects(leftFields)) {
      // We cannot map origin of this expression.
      return null;
    }
  } else {
    // We cannot map origin of this expression.
    return null;
  }
}
I can get the results I need
```
final RelNode rel = sql("select name as dname from emp left outer join dept"
    + " on emp.deptno = dept.deptno").toRel();
final RelMetadataQuery mq = rel.getCluster().getMetadataQuery();
final RelOptPredicateList r = mq.getAllPredicates(rel);
assertThat(r.pulledUpPredicates.get(0).toString(),
    equalTo("=([CATALOG, SALES, EMP].#0.$7, [CATALOG, SALES,
DEPT].#0.$0)"));
```


It seems that we deliberately return null
in RelMetadataQuery#getAllPredicates . Can someone tell me why? Thanks!