Remove N+1 queries in your Ruby on Rails app

The post says N+1 queries slow pages because the app keeps asking the database for associated records inside a loop. It then shows the usual Rails tools for avoiding that pattern, plus a few ways to collapse repeated counts into fewer queries.

Reading notes

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• N+1 happens when an app makes extra database calls to fetch the same associated data over and over.
• In the example, iterating through items and calling each item’s category causes one query for the items and more queries for categories.
• preload loads the main records and the associated records in separate queries, and it does not work with a where clause on the associated table.
• includes can choose between separate queries and a join, depending on the query shape.
• references can force includes to use a LEFT OUTER JOIN.
• eager_load behaves like includes plus references and uses one query with a LEFT OUTER JOIN.
• Repeated count queries with a dynamic date condition can be replaced with one grouped query and a hash lookup.
• Some sets of counts can be combined into one SQL query with selected aliases.
• The bullet gem helps find N+1 queries during development by showing browser notifications.