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25 lines
1.1 KiB
Markdown
25 lines
1.1 KiB
Markdown
# Perform SQL Explain With ActiveRecord
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Want to check out the performance characteristics of some SQL query from
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within a Pry session? `ActiveRecord` allows you to perform a SQL `explain`
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on any `ActiveRecord::Relation` object. After chaining some Arel functions
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together, add an `#explain`.
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Here is an example:
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```ruby
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Recipe.all.joins(:ingredient_amounts).explain
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Recipe Load (0.9ms) SELECT "recipes".* FROM "recipes" INNER JOIN "ingredient_amounts" ON "ingredient_amounts"."recipe_id" = "recipes"."id"
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=> EXPLAIN for: SELECT "recipes".* FROM "recipes" INNER JOIN "ingredient_amounts" ON "ingredient_amounts"."recipe_id" = "recipes"."id"
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QUERY PLAN
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----------------------------------------------------------------------------
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Hash Join (cost=1.09..26.43 rows=22 width=148)
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Hash Cond: (ingredient_amounts.recipe_id = recipes.id)
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-> Seq Scan on ingredient_amounts (cost=0.00..21.00 rows=1100 width=4)
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-> Hash (cost=1.04..1.04 rows=4 width=148)
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-> Seq Scan on recipes (cost=0.00..1.04 rows=4 width=148)
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(5 rows)
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```
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[source](https://robots.thoughtbot.com/why-postgres-wont-always-use-an-index)
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