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

@@ -6,31 +6,12 @@ class BuildUserContributedProjectsTable < ActiveRecord::Migration
 
   disable_ddl_transaction!
 
-  BATCH_SIZE = 100000
-  SLEEP_TIME = 30
-
   def up
-    with_index(:events, [:author_id, :project_id], name: 'events_user_contributions_temp', where: 'project_id IS NOT NULL') do
-      iteration = 0
-      records = 0
-      begin
-        Rails.logger.info "Building user_contributed_projects table, batch ##{iteration}"
-        result = 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
-          LIMIT #{BATCH_SIZE}
-        SQL
-        iteration += 1
-        records += result.cmd_tuples
-        Rails.logger.info "Building user_contributed_projects table, batch ##{iteration} complete, created #{records} overall"
-        Kernel.sleep(SLEEP_TIME) if result.cmd_tuples > 0
-      end while result.cmd_tuples > 0
-    end
-
-    execute "ANALYZE user_contributed_projects" if Gitlab::Database.postgresql?
+    if Gitlab::Database.postgresql?
+      PostgresStrategy.new
+    else
+      MysqlStrategy.new
+    end.up
   end
 
   def down
@@ -39,10 +20,58 @@ class BuildUserContributedProjectsTable < ActiveRecord::Migration
 
   private
 
-  def with_index(*args)
-    add_concurrent_index(*args) unless index_exists?(*args)
-    yield
-  ensure
-    remove_concurrent_index(*args) if index_exists?(*args)
+  class PostgresStrategy < ActiveRecord::Migration
+    include Gitlab::Database::MigrationHelpers
+
+    BATCH_SIZE = 100_000
+    SLEEP_TIME = 30
+
+    def up
+      with_index(:events, [:author_id, :project_id], name: 'events_user_contributions_temp', where: 'project_id IS NOT NULL') do
+        iteration = 0
+        records = 0
+        begin
+          Rails.logger.info "Building user_contributed_projects table, batch ##{iteration}"
+          result = 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
+            LIMIT #{BATCH_SIZE}
+          SQL
+          iteration += 1
+          records += result.cmd_tuples
+          Rails.logger.info "Building user_contributed_projects table, batch ##{iteration} complete, created #{records} overall"
+          Kernel.sleep(SLEEP_TIME) if result.cmd_tuples > 0
+        end while result.cmd_tuples > 0
+      end
+
+      execute "ANALYZE user_contributed_projects"
+    end
+
+    private
+
+    def with_index(*args)
+      add_concurrent_index(*args) unless index_exists?(*args)
+      yield
+    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