Execution plan reproducibility

[stasinos]

In a situation like this:

BindJoin
  Plan@local-semagrow[costs [5020.05257,0] 2005257 tuples]
    ...
  Plan@local-semagrow[costs [5020.05257,0] 2005257 tuples]
    ...

where both subqueries have exactly the same cost, the optimizer can order differently the sub-trees of the BindJoin in different runs. Although strictly speaking not a mistake, it would be preferable to produce identical plans for the same inputs every time the planner is executed.

[stasinos]

We observed this behaviour while looking into Issue #49 . Although that issue was resolved by correcting the cost estimator, the reproducibility issue remains should the two sub-plans really have the same cost.