Optimization for existential subquery

[sileix]

Existential subqueries can be optimized as operations like sorting won't affect the value of the boolean expression.

Operations that have effect:

• filter
• projection: the output parameter could be optional, the semantics is at least one row has all projections are not null or undefined.
• aggregation: similar to projection, as long as the fields used are not null or undefined, this should be true, so this can probably be optimized to projection.
• slice: the semantics is at least any of x rows are returned, where x is the length of the slice

Operations that have no effect:

• sort
• index
• argmin/argmax
• alias