Optimize the EXISTS sub-query by Metadata RowCount

As same as the UNIQUE sub-query, before we convert the sub-query to RelNode. we use the Metadata to optimize it.

EXISTS sub-query, If the sub-query is guaranteed to produce at least one row, just return TRUE. If the sub-query is guaranteed to produce no row, just return FALSE.

For example:

select *
from dept as d
where EXISTS (select count(*) from emp e where d.deptno = e.deptno)

We can optimize it to:

EnumerableTableScan(table=[[scott, DEPT]])