1 files changed, 53 insertions, 0 deletions
diff --git a/lib/gitlab/sql/set_operator.rb b/lib/gitlab/sql/set_operator.rb
new file mode 100644
index 00000000000..d58a1415493
--- /dev/null
+++ b/lib/gitlab/sql/set_operator.rb
@@ -0,0 +1,53 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module SQL
+    # Class for building SQL set operator statements (UNION, INTERSECT, and
+    # EXCEPT).
+    #
+    # ORDER BYs are dropped from the relations as the final sort order is not
+    # guaranteed any way.
+    #
+    # Example usage:
+    #
+    #     union = Gitlab::SQL::Union.new([user.personal_projects, user.projects])
+    #     sql   = union.to_sql
+    #
+    #     Project.where("id IN (#{sql})")
+    class SetOperator
+      def initialize(relations, remove_duplicates: true)
+        @relations = relations
+        @remove_duplicates = remove_duplicates
+      end
+
+      def self.operator_keyword
+        raise NotImplementedError
+      end
+
+      def to_sql
+        # Some relations may include placeholders for prepared statements, these
+        # aren't incremented properly when joining relations together this way.
+        # By using "unprepared_statements" we remove the usage of placeholders
+        # (thus fixing this problem), at a slight performance cost.
+        fragments = ActiveRecord::Base.connection.unprepared_statement do
+          relations.map { |rel| rel.reorder(nil).to_sql }.reject(&:blank?)
+        end
+
+        if fragments.any?
+          "(" + fragments.join(")\n#{operator_keyword_fragment}\n(") + ")"
+        else
+          'NULL'
+        end
+      end
+
+      # UNION [ALL] | INTERSECT [ALL] | EXCEPT [ALL]
+      def operator_keyword_fragment
+        remove_duplicates ? self.class.operator_keyword : "#{self.class.operator_keyword} ALL"
+      end
+
+      private
+
+      attr_reader :relations, :remove_duplicates
+    end
+  end
+end