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< h1 id = "n1-problem" > < a class = "header" href = "#n1-problem" > N+1 problem< / a > < / h1 >
< p > A common issue with < a href = "https://graphql.org" > GraphQL< / a > server implementations is how the < a href = "https://spec.graphql.org/October2021#sec-Executing-Fields" > resolvers< / a > query their datasource. With a naive and straightforward approach we quickly run into the N+1 problem, resulting in a large number of unnecessary database queries or < a href = "https://en.wikipedia.org/wiki/HTTP" > HTTP< / a > requests.< / p >
< pre > < pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > < span class = "boring" > extern crate anyhow;
< / span > < span class = "boring" > extern crate juniper;
< / span > < span class = "boring" > use anyhow::anyhow;
< / span > < span class = "boring" > use juniper::{graphql_object, GraphQLObject};
< / span > < span class = "boring" >
< / span > < span class = "boring" > type CultId = i32;
< / span > < span class = "boring" > type UserId = i32;
< / span > < span class = "boring" >
< / span > < span class = "boring" > struct Repository;
< / span > < span class = "boring" >
< / span > < span class = "boring" > impl juniper::Context for Repository {}
< / span > < span class = "boring" >
< / span > < span class = "boring" > impl Repository {
< / span > < span class = "boring" > async fn load_cult_by_id(& self, cult_id: CultId) -> anyhow::Result< Option< Cult> > { unimplemented!() }
< / span > < span class = "boring" > async fn load_all_persons(& self) -> anyhow::Result< Vec< Person> > { unimplemented!() }
< / span > < span class = "boring" > }
< / span > < span class = "boring" >
< / span > #[derive(GraphQLObject)]
struct Cult {
id: CultId,
name: String,
}
struct Person {
id: UserId,
name: String,
cult_id: CultId,
}
#[graphql_object]
#[graphql(context = Repository)]
impl Person {
fn id(& self) -> CultId {
self.id
}
fn name(& self) -> & str {
self.name.as_str()
}
async fn cult(& self, #[graphql(ctx)] repo: & Repository) -> anyhow::Result< Cult> {
// Effectively performs the following SQL query:
// SELECT id, name FROM cults WHERE id = ${cult_id} LIMIT 1
repo.load_cult_by_id(self.cult_id)
.await?
.ok_or_else(|| anyhow!("No cult exists for ID `{}`", self.cult_id))
}
}
struct Query;
#[graphql_object]
#[graphql(context = Repository)]
impl Query {
async fn persons(#[graphql(ctx)] repo: & Repository) -> anyhow::Result< Vec< Person> > {
// Effectively performs the following SQL query:
// SELECT id, name, cult_id FROM persons
repo.load_all_persons().await
}
}
< span class = "boring" > }< / span > < / code > < / pre > < / pre >
< p > Let's say we want to list a bunch of < code > cult< / code > s < code > persons< / code > were in:< / p >
< pre > < code class = "language-graphql" > query {
persons {
id
name
cult {
id
name
}
}
}
< / code > < / pre >
< p > Once the < code > persons< / code > < a href = "https://spec.graphql.org/October2021#sec-List" > list< / a > has been < a href = "https://spec.graphql.org/October2021#sec-Executing-Fields" > resolved< / a > , a separate < a href = "https://en.wikipedia.org/wiki/SQL" > SQL< / a > query is run to find the < code > cult< / code > of each < code > Person< / code > . We can see how this could quickly become a problem.< / p >
< pre > < code class = "language-sql" > SELECT id, name, cult_id FROM persons;
SELECT id, name FROM cults WHERE id = 1;
SELECT id, name FROM cults WHERE id = 2;
SELECT id, name FROM cults WHERE id = 1;
SELECT id, name FROM cults WHERE id = 3;
SELECT id, name FROM cults WHERE id = 4;
SELECT id, name FROM cults WHERE id = 1;
SELECT id, name FROM cults WHERE id = 2;
-- and so on...
< / code > < / pre >
< p > There are several ways how this problem may be resolved in < a href = "https://docs.rs/juniper" > Juniper< / a > . The most common ones are:< / p >
< ul >
< li > < a href = "dataloader.html" > DataLoader< / a > < / li >
< li > < a href = "lookahead.html" > Look-ahead machinery< / a >
< ul >
< li > < a href = "eager_loading.html" > Eager loading< / a > < / li >
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< / li >
< / ul >
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