Warn about subqueries when replacing pluck with pick

[thiago-gitlab]

The RuboCop::Cop::Rails::Pick cop "Enforces the use of ‘pickoverpluck(…).first`."

When using this cop on an existing code base, it's not possible for the cop to detect whether the relation already has a limit applied.

In these cases, following the advice will cause a subquery to be added to the resulting query.

Model.limit(100).pluck(:a).first is not equivalent to Model.limit(100).pick(:a). The former generates a SQL query with a LIMIT 100. The latter generates a query with LIMIT 1 and subquery with LIMIT 100.

This proposed documentation update highlights the scenario so the developer can better judge when to obey the cop.

Example: https://gitlab.com/gitlab-org/gitlab/-/issues/455116

Note: I haven't updated docs/modules/ROOT/pages/cops_rails.adoc as I believe this is done when a new version of the gem is cut.

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Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.

[koic]

Thanks!