When we have a limit operator on top of project, left outer join, union all operator, we should push the limit operator, until the limit is put into scan operator. Doing so, it will reduce the amount of data scanned by the scan operator.
Select COL1, COL2 FROM T1 LIMIT 5;
PROJECT (COL1, COL2)
We need push limit past project, down into scan.
Similar applies to left outer join, or union all.