#434

backend/app/services/counts_axes.rb

@@ -72,14 +72,27 @@ class CountsAxes
     end
   end
 
-  def count_expression
-    return 'COUNT(DISTINCT v.id)' unless rows.any? { _1['field'] == 'tags' }
-
-    # Keep tag occurrences even when several private tags share one masked label.
-    values = ['v.id', "COALESCE(CONCAT('internal:', tag_entry.tag_id), " +
-        "CONCAT('external:', tag_entry.external_tag_id))"]
-    values << 'changed.field_key' if rows.any? { _1['field'] == 'changed_field' }
-    "COUNT(DISTINCT #{ values.join(', ') })"
+  def grouped_sql(scope, indexes = all.each_index.to_a)
+    keys = indexes.map { "a#{ _1 }" }
+    selected = indexes.map { "#{ expressions[_1] } AS a#{ _1 }" }
+    identities = ['v.id']
+    if rows.any? { _1['field'] == 'tags' }
+      # Raw identities stay inside COUNT; masked tag occurrences remain additive.
+      identities << "COALESCE(CONCAT('internal:', tag_entry.tag_id), " +
+          "CONCAT('external:', tag_entry.external_tag_id))"
+      identities << 'changed.field_key' if rows.any? { _1['field'] == 'changed_field' }
+    end
+    members = identities.each_index.map { "counts_member_#{ _1 }" }
+    selected += identities.each_with_index.map { |expression, index|
+      "#{ expression } AS #{ members[index] }"
+    }
+    # Evaluate correlated visibility subqueries before grouping. LIMIT prevents
+    # derived-table merging, so ONLY_FULL_GROUP_BY sees ordinary projected columns.
+    projection = scope.except(:select, :order).select(Arel.sql(selected.join(', ')))
+        .limit(18_446_744_073_709_551_615).to_sql
+    fields = keys + ["COUNT(DISTINCT #{ members.join(', ') }) AS count"]
+    grouping = keys.empty? ? '' : " GROUP BY #{ keys.join(', ') }"
+    "SELECT #{ fields.join(', ') } FROM (#{ projection }) counts_values#{ grouping }"
   end
 
   def joins(fields = all.map { _1['field'] })