๐ What is it?
Simply put, a lot of SQL query executes in loop
๐ For example
There are two tables in database
โผ country table
| id | name |
|---|---|
| 1 | Japan |
| 2 | Poland |
โผ food table
| id | country_id | name |
|---|---|---|
| 1 | 1 | sushi |
| 2 | 1 | ramen |
| 3 | 1 | udon |
| 4 | 2 | pierogi |
| 5 | 2 | kotlet |
| 6 | 2 | ziemniaki |
Get country name
(I'm going to use ruby code)
# SELECT * FROM country;
Country.all.each do |country|
put country.name
# Japan
# Poland
end
Call another table in loop
# SELECT * FROM country;
Country.all.each do |country|
put country.name
# Japan
# Poland
# SELECT * FROM food WHERE country_id=1;
# SELECT * FROM food WHERE country_id=2;
# ๐ซ ๐ญ if country table had 200 records, 200 query would be executed!!! OMG
country.foods.each do |food|
put food.name
# sushi
# ramen
# udon
# pierogi
# kotlet
# ziemniaki
end
end
๐ How to solve it?
- JOIN
- Eager Load
JOIN
Integrate tables !
# SELECT * FROM country INNER JOIN food ON country.id=food.country_id
# ๐ ๐ only 1 query !
Country.join(:food).all.each do |country|
country.foods.each do |food|
end
end
Eager Load
Not integrate tables completely like JOIN, but executes SELECT at first !
# SELECT * FROM country;
# SELECT * FROM food WHERE country_id IN(1,2)
# ๐ ๐ only 2 queries !
Country.includes(:food).all.each do |country|
country.foods.each do |food|
end
end
โญ if you want to use eager load in Ruby, just use includes
๐ Disadvantage
Memory leak because Join/Eager load need to execute huge SELECT at first
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