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Add Perform SQL Explain With ActiveRecord as a rails til

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jbranchaud
2016-05-01 17:15:44 -05:00
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rails/perform-sql-explain-with-activerecord.md Normal file
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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:
```ruby
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)
```
[source](https://robots.thoughtbot.com/why-postgres-wont-always-use-an-index)