Added FromUnion to easily select from a UNION

This commit adds the module `FromUnion`, which provides the class method
`from_union`. This simplifies the process of selecting data from the
result of a UNION, and reduces the likelihood of making mistakes. As a
result, instead of this:

    union = Gitlab::SQL::Union.new([foo, bar])

    Foo.from("(#{union.to_sql}) #{Foo.table_name}")

We can now write this instead:

    Foo.from_union([foo, bar])

This commit also includes some changes to make this new setup work
properly. For example, a bug in Rails 4
(https://github.com/rails/rails/issues/24193) would break the use of
`from("sub-query-here").includes(:relation)` in certain cases. There was
also a CI query which appeared to repeat a lot of conditions from an
outer query on an inner query, which isn't necessary.

Finally, we include a RuboCop cop to ensure developers use this new
module, instead of using Gitlab::SQL::Union directly.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/51307

1 files changed, 51 insertions, 0 deletions

diff --git a/app/models/concerns/from_union.rb b/app/models/concerns/from_union.rb
new file mode 100644
index 00000000000..9b8595b1211
--- /dev/null
+++ b/app/models/concerns/from_union.rb
@@ -0,0 +1,51 @@
+# frozen_string_literal: true
+
+module FromUnion
+  extend ActiveSupport::Concern
+
+  class_methods do
+    # Produces a query that uses a FROM to select data using a UNION.
+    #
+    # Using a FROM for a UNION has in the past lead to better query plans. As
+    # such, we generally recommend this pattern instead of using a WHERE IN.
+    #
+    # Example:
+    #     users = User.from_union([User.where(id: 1), User.where(id: 2)])
+    #
+    # This would produce the following SQL query:
+    #
+    #     SELECT *
+    #     FROM (
+    #       SELECT *
+    #       FROM users
+    #       WHERE id = 1
+    #
+    #       UNION
+    #
+    #       SELECT *
+    #       FROM users
+    #       WHERE id = 2
+    #     ) users;
+    #
+    # members - An Array of ActiveRecord::Relation objects to use in the UNION.
+    #
+    # remove_duplicates - A boolean indicating if duplicate entries should be
+    #                     removed. Defaults to true.
+    #
+    # alias_as - The alias to use for the sub query. Defaults to the name of the
+    #            table of the current model.
+    # rubocop: disable Gitlab/Union
+    def from_union(members, remove_duplicates: true, alias_as: table_name)
+      union = Gitlab::SQL::Union
+        .new(members, remove_duplicates: remove_duplicates)
+        .to_sql
+
+      # This pattern is necessary as a bug in Rails 4 can cause the use of
+      # `from("string here").includes(:foo)` to break ActiveRecord. This is
+      # fixed in https://github.com/rails/rails/pull/25374, which is released as
+      # part of Rails 5.
+      from([Arel.sql("(#{union}) #{alias_as}")])
+    end
+    # rubocop: enable Gitlab/Union
+  end
+end