High Performance PostgreSQL for Rails: Explanation regarding the use of includes with a limit (Page 124)

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Dear Andrew, @andatki

I found the explanation regarding the use of .includes() with a limit in the context of vehicle reservations to be quite insightful. However, I believe the discussion on performance implications could benefit from further clarification, particularly regarding the difference in behavior with and without a limit.

Here’s a suggested rewrite to enhance clarity:

When using .includes() to load vehicle reservations, the query is split into two parts. First, a query retrieves a limited number of vehicles (e.g., 2 vehicles). Then, another query fetches the associated reservations for these vehicles using an IN clause with the vehicle_ids of the retrieved vehicles. Keep in mind that if the IN clause contains too many values, it can lead to performance issues.

Performance Implications

  • Without limit: If there are many vehicles, the IN clause can become very large, which may degrade performance.
  • With limit: The IN clause remains small, improving performance for the secondary query.

Additionally, this explanation can move before LEFT OUTER JOIN part, in order to avoid a context switch.

I hope this suggestion helps make the performance implications clearer for the readers.

Best regards,

Viktor

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Thank you @viktorianer. Your description makes sense. If we revise the book in the future, I’ll pay extra attention to this section, as eager loading is of course a very common pattern.

In my experience, I’ve seen poor performance from big lists of values in an IN clause on big tables. I wrote up some thoughts about this in a blog post here: Big Problems From Big IN lists with Ruby on Rails and PostgreSQL | Software Engineer, Author, High Performance PostgreSQL for Rails

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