parse_item() in execplan code now always get an actual GWI structure

to avoid accedental crashes.

Add check for Conversion of Big IN Predicates Into Subqueries optimization
conditions.

Enabled derivedTableOptimization() for group by and derived handlers.

Disabled Conversion of Big IN Predicates Into Subqueries optimization.

Disabled most of optimizer_flags for now.

RowGroup + operator now correctly sets useStringTable flag that
instructs code to check StringStore instead of plain data buffer.

utils/rowgroup/rowgroup.cpp

@@ -1591,6 +1591,7 @@ RowGroup& RowGroup::operator+=(const RowGroup& rhs)
     }
 
     hasLongStringField = rhs.hasLongStringField || hasLongStringField;
+    useStringTable = rhs.useStringTable || useStringTable;
     offsets = (useStringTable ? &stOffsets[0] : &oldOffsets[0]);
 
     return *this;