Simpler migration strategy for MySQL.

MySQL does not return the number of inserted rows (or at least not
expose them through cmd_tuples). Let's do one-shot INSERT instead.

db/post_migrate/20180223124427_build_user_contributed_projects_table.rb

@@ -6,7 +6,24 @@ class BuildUserContributedProjectsTable < ActiveRecord::Migration
 
   disable_ddl_transaction!
 
-  BATCH_SIZE = 100000
+  def up
+    if Gitlab::Database.postgresql?
+      PostgresStrategy.new
+    else
+      MysqlStrategy.new
+    end.up
+  end
+
+  def down
+    execute "TRUNCATE user_contributed_projects"
+  end
+
+  private
+
+  class PostgresStrategy < ActiveRecord::Migration
+    include Gitlab::Database::MigrationHelpers
+
+    BATCH_SIZE = 100_000
     SLEEP_TIME = 30
 
     def up
@@ -30,11 +47,7 @@ class BuildUserContributedProjectsTable < ActiveRecord::Migration
         end while result.cmd_tuples > 0
       end
 
-    execute "ANALYZE user_contributed_projects" if Gitlab::Database.postgresql?
-  end
-
-  def down
-    execute "TRUNCATE user_contributed_projects"
+      execute "ANALYZE user_contributed_projects"
     end
 
     private
@@ -45,4 +58,20 @@ class BuildUserContributedProjectsTable < ActiveRecord::Migration
     ensure
       remove_concurrent_index(*args) if index_exists?(*args)
     end
+  end
+
+  class MysqlStrategy < ActiveRecord::Migration
+    include Gitlab::Database::MigrationHelpers
+
+    def up
+      execute <<~SQL
+        INSERT INTO user_contributed_projects (user_id, project_id)
+        SELECT e.user_id, e.project_id
+        FROM (SELECT DISTINCT author_id AS user_id, project_id FROM events WHERE project_id IS NOT NULL) AS e
+        LEFT JOIN user_contributed_projects ucp USING (user_id, project_id)
+        WHERE ucp.user_id IS NULL
+      SQL
+    end
+  end
+
 end