Optimizer chooses loose index scan (QUICK_GROUP_MIN_MAX_SELECT)
for a group by even when there exists a predicate with disjunction.
This is due to that either of the below two cases are encountered
and not handled properly.

1. A range tree created for index merge scan is not taken
into account while checking for the presence of disjuncted
conditions in get_best_group_min_max().

2. A disjunction condition could lead to null tree if an index
merge scan is not possible. A NULL tree will cause
get_best_group_min_max() to skip many relevent checks
like WA2.

There are two scenarios here,
  (a) The WHERE clause is a disjunction of conditions on
     MIN/MAX column only.
    => We can use min_max optimization considering all
    other criteria (eg:SA2) are met.

 (b) MIN/MAX column is participating in a disjunctive WHERE
     clause along with other columns.
     => min_max optimization is not applicable in this scenario.

Fix:
Skip loose index scan in below scenarios involving disjoint conditions
in WHERE clause.

1. When the condition in WHERE clause results in more than
one disjoint range trees (when index merge scan is possible).

2. The range tree is null and MIN/MAX column participates
in the WHERE clause along with other columns.
[There is no range tree if WHERE condition can't be represented
in a single range tree and index merge is not possible]

Change-Id: I2ec8ef815d9dfa64e0bf60e97d3b6129c070e24b