8a72f5c427
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
51 lines
1.6 KiB
Ruby
51 lines
1.6 KiB
Ruby
# frozen_string_literal: true
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module FromUnion
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extend ActiveSupport::Concern
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class_methods do
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# Produces a query that uses a FROM to select data using a UNION.
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#
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# Using a FROM for a UNION has in the past lead to better query plans. As
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# such, we generally recommend this pattern instead of using a WHERE IN.
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#
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# Example:
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# users = User.from_union([User.where(id: 1), User.where(id: 2)])
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#
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# This would produce the following SQL query:
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#
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# SELECT *
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# FROM (
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# SELECT *
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# FROM users
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# WHERE id = 1
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#
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# UNION
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#
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# SELECT *
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# FROM users
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# WHERE id = 2
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# ) users;
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#
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# members - An Array of ActiveRecord::Relation objects to use in the UNION.
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#
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# remove_duplicates - A boolean indicating if duplicate entries should be
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# removed. Defaults to true.
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#
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# alias_as - The alias to use for the sub query. Defaults to the name of the
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# table of the current model.
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# rubocop: disable Gitlab/Union
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def from_union(members, remove_duplicates: true, alias_as: table_name)
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union = Gitlab::SQL::Union
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.new(members, remove_duplicates: remove_duplicates)
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.to_sql
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# This pattern is necessary as a bug in Rails 4 can cause the use of
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# `from("string here").includes(:foo)` to break ActiveRecord. This is
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# fixed in https://github.com/rails/rails/pull/25374, which is released as
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# part of Rails 5.
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from([Arel.sql("(#{union}) #{alias_as}")])
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end
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# rubocop: enable Gitlab/Union
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end
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end
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