Add _vovw_count columnt to tables for which overviews are created

Initially we planned to add this column to the queries sent to the
tiler only, but that makes the column hard to access from the editor.

scripts-available/CDB_Overviews.sql

@@ -718,6 +718,7 @@ DECLARE
   overview_z integer;
   overview_tables REGCLASS[];
   overviews_step integer := 1;
+  has_counter_column boolean;
 BEGIN
   -- Determine the referece zoom level
   EXECUTE 'SELECT ' || quote_ident(refscale_strategy::text) || Format('(''%s'', %s);', reloid, tolerance_px) INTO ref_z;
@@ -743,6 +744,17 @@ BEGIN
     SELECT array_append(overview_tables, base_rel) INTO overview_tables;
   END LOOP;
 
+  IF overview_tables IS NOT NULL AND array_length(overview_tables, 1) > 0 THEN
+    SELECT EXISTS (
+      SELECT * FROM CDB_ColumnNames(reloid)  as colname WHERE colname = '_vovw_count'
+    ) INTO has_counter_column;
+    IF NOT has_counter_column THEN
+      EXECUTE Format('
+        ALTER TABLE %s ADD COLUMN _vovw_count integer DEFAULT 1;
+      ', reloid);
+    END IF;
+  END IF;
+
   RETURN overview_tables;
 END;
 $$ LANGUAGE PLPGSQL;