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perform-sql-explain-with-activerecord.md

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Perform SQL Explain With ActiveRecord

Want to check out the performance characteristics of some SQL query from within a Pry session? ActiveRecord allows you to perform a SQL explain on any ActiveRecord::Relation object. After chaining some Arel functions together, add an #explain.

Here is an example:

Recipe.all.joins(:ingredient_amounts).explain
  Recipe Load (0.9ms)  SELECT "recipes".* FROM "recipes" INNER JOIN "ingredient_amounts" ON "ingredient_amounts"."recipe_id" = "recipes"."id"
=> EXPLAIN for: SELECT "recipes".* FROM "recipes" INNER JOIN "ingredient_amounts" ON "ingredient_amounts"."recipe_id" = "recipes"."id"
                                 QUERY PLAN
----------------------------------------------------------------------------
 Hash Join  (cost=1.09..26.43 rows=22 width=148)
   Hash Cond: (ingredient_amounts.recipe_id = recipes.id)
   ->  Seq Scan on ingredient_amounts  (cost=0.00..21.00 rows=1100 width=4)
   ->  Hash  (cost=1.04..1.04 rows=4 width=148)
         ->  Seq Scan on recipes  (cost=0.00..1.04 rows=4 width=148)
(5 rows)

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