Use DISTINCT ON and translate to MySQL.

app/finders/members_finder.rb

@@ -17,22 +17,7 @@ class MembersFinder
 
       union = Gitlab::SQL::Union.new([project_members, group_members], remove_duplicates: false)
 
-      # We're interested in a list of members without duplicates by user_id.
-      # We prefer project members over group members, project members should go first.
-      #
-      # We could have used a DISTINCT ON here, but MySQL does not support this.
-      sql = <<-SQL
-        SELECT member_numbered.*
-        FROM (
-          SELECT
-          member_union.*,
-          ROW_NUMBER() OVER (
-            PARTITION BY user_id ORDER BY CASE WHEN type = 'ProjectMember' THEN 1 WHEN type = 'GroupMember' THEN 2 ELSE 3 END
-          ) AS row_number
-          FROM (#{union.to_sql}) AS member_union
-       ) AS member_numbered
-       WHERE row_number = 1
-      SQL
+      sql = distinct_on(union)
 
       Member.from("(#{sql}) AS #{Member.table_name}")
     else
@@ -43,4 +28,37 @@ class MembersFinder
   def can?(*args)
     Ability.allowed?(*args)
   end
+
+  private
+
+  def distinct_on(union)
+    # We're interested in a list of members without duplicates by user_id.
+    # We prefer project members over group members, project members should go first.
+    if Gitlab::Database.postgresql?
+      <<~SQL
+          SELECT DISTINCT ON (user_id, invite_email) member_union.*
+          FROM (#{union.to_sql}) AS member_union
+          ORDER BY
+            user_id, invite_email,
+            CASE WHEN type = 'ProjectMember' THEN 1 WHEN type = 'GroupMember' THEN 2 ELSE 3 END
+      SQL
+    else
+      # Older versions of MySQL do not support window functions (and DISTINCT ON is postgres-specific).
+      <<~SQL
+          SELECT t1.*
+          FROM (#{union.to_sql}) AS t1
+          JOIN (
+            SELECT
+              COALESCE(user_id, -1) AS user_id,
+              COALESCE(invite_email, 'NULL') AS invite_email,
+              MIN(CASE WHEN type = 'ProjectMember' THEN 1 WHEN type = 'GroupMember' THEN 2 ELSE 3 END) AS type_number
+            FROM
+            (#{union.to_sql}) AS t3
+            GROUP BY COALESCE(user_id, -1), COALESCE(invite_email, 'NULL')
+          ) AS t2 ON COALESCE(t1.user_id, -1) = t2.user_id
+                 AND COALESCE(t1.invite_email, 'NULL') = t2.invite_email
+                 AND CASE WHEN t1.type = 'ProjectMember' THEN 1 WHEN t1.type = 'GroupMember' THEN 2 ELSE 3 END = t2.type_number
+      SQL
+    end
+  end
 end