Fix inner query labels for Vertica (#2041)

In Vertica, inner queries require different aliases than the main query.

This is an example of query generated before this patch:
SELECT chain AS chain,
                weekstartday AS __timestamp,
                SUM(inventory) AS "Inventory"
FROM mytable
JOIN
  (SELECT chain AS chain__,
                   SUM(inventory) AS "Inventory"
   FROM mytable
   WHERE weekstartday >= '2016-01-24 00:00:00'
     AND weekstartday <= '2017-01-17 00:00:00'
   GROUP BY chain
   ORDER BY "Inventory" DESC LIMIT 50) AS anon_1 ON chain = chain__
WHERE weekstartday >= '2016-01-24 00:00:00'
  AND weekstartday <= '2017-01-17 00:00:00'
GROUP BY chain,
         weekstartday
ORDER BY "Inventory" DESC LIMIT 50000

Which in Vertica produces the error:
Error: ('42702', '[42702] ERROR 2671: Column reference "inventory" is ambiguous\n (2671) (SQLExecDirectW)')


And this is the same example after the patch:
SELECT chain AS chain,
                weekstartday AS __timestamp,
                SUM(inventory) AS "Inventory"
FROM mytable
JOIN
  (SELECT chain AS chain__,
                   SUM(inventory) AS mme_inner__
   FROM mytable
   WHERE weekstartday >= '2016-01-24 00:00:00'
     AND weekstartday <= '2017-01-17 00:00:00'
   GROUP BY chain
   ORDER BY mme_inner__ DESC LIMIT 50) AS anon_1 ON chain = chain__
WHERE weekstartday >= '2016-01-24 00:00:00'
  AND weekstartday <= '2017-01-17 00:00:00'
GROUP BY chain,
         weekstartday
ORDER BY "Inventory" DESC LIMIT 50000

Related PR:
19f5371

superset/models.py

@@ -1386,8 +1386,10 @@ def visit_column(element, compiler, **kw):
 
         if is_timeseries and timeseries_limit and groupby:
             # some sql dialects require for order by expressions
-            # to also be in the select clause
-            inner_select_exprs += [main_metric_expr]
+            # to also be in the select clause -- others, e.g. vertica,
+            # require a unique inner alias
+            inner_main_metric_expr = main_metric_expr.label('mme_inner__')
+            inner_select_exprs += [inner_main_metric_expr]
             subq = select(inner_select_exprs)
             subq = subq.select_from(tbl)
             inner_time_filter = dttm_col.get_time_filter(
@@ -1396,7 +1398,7 @@ def visit_column(element, compiler, **kw):
             )
             subq = subq.where(and_(*(where_clause_and + [inner_time_filter])))
             subq = subq.group_by(*inner_groupby_exprs)
-            ob = main_metric_expr
+            ob = inner_main_metric_expr
             if timeseries_limit_metric_expr is not None:
                 ob = timeseries_limit_metric_expr
             subq = subq.order_by(desc(ob))