# Optimizing N+1 queries
Have you noticed some GraphQL queries end can make hundreds of database queries, often with mostly repeated data? Lets take a look why and how to fix it.
## Query Resolution
Imagine if you have a simple query like this:
```graphql
query {
todos {
users {
name
}
}
}
```
and `User` resolver is like this:
```rust,ignore
struct User {
id: u64,
}
#[Object]
impl User {
async fn name(&self, ctx: &Context<'_>) -> Result<String> {
let pool = ctx.data_unchecked::<Pool<Postgres>>();
let (name,): (String,) = sqlx::query_as("SELECT name FROM user WHERE id = $1")
.bind(self.id)
.fetch_one(pool)
.await?;
Ok(name)
}
}
```
The query executor will call the `Todos` resolver which does a `select * from todo and return N todos`. Then for each
of the todos, concurrently, call the `User` resolver, `SELECT from USER where id = todo.user_id`.
eg：
```sql
SELECT id, todo, user_id FROM todo
SELECT name FROM user WHERE id = $1
SELECT name FROM user WHERE id = $1
SELECT name FROM user WHERE id = $1
SELECT name FROM user WHERE id = $1
SELECT name FROM user WHERE id = $1
SELECT name FROM user WHERE id = $1
SELECT name FROM user WHERE id = $1
SELECT name FROM user WHERE id = $1
SELECT name FROM user WHERE id = $1
SELECT name FROM user WHERE id = $1
SELECT name FROM user WHERE id = $1
SELECT name FROM user WHERE id = $1
SELECT name FROM user WHERE id = $1
SELECT name FROM user WHERE id = $1
SELECT name FROM user WHERE id = $1
SELECT name FROM user WHERE id = $1
```
After executing `SELECT name FROM user WHERE id = $1` many times, and most `Todo` objects belong to the same user, we
need to optimize these codes!
## Dataloader
We need to group queries and exclude duplicate queries. `Dataloader` can do this.
[facebook](https://github.com/facebook/dataloader) gives a request-scope batch and caching solution.
The following is an example of using `DataLoader` to optimize queries::
```rust,ignore
use async_graphql::*;
use async_graphql::dataloader::*;
use itertools::Itertools;
use std::sync::Arc;
struct UserNameLoader {
pool: sqlx::Pool<Postgres>,
}
#[async_trait::async_trait]
impl Loader<u64> for UserNameLoader {
type Value = String;
type Error = Arc<sqlx::Error>;
async fn load(&self, keys: &[u64]) -> Result<HashMap<u64, Self::Value>, Self::Error> {
let query = format!("SELECT name FROM user WHERE id IN ({})", keys.iter().join(","));
Ok(sqlx::query_as(query)
.fetch(&self.pool)
.map_ok(|name: String| name)
.map_err(Arc::new)
.try_collect().await?)
}
}
#[derive(SimpleObject)]
#[graphql(complex)]
struct User {
id: u64,
}
#[ComplexObject]
impl User {
async fn name(&self, ctx: &Context<'_>) -> Result<String> {
let loader = ctx.data_unchecked::<DataLoader<UserNameLoader>>();
let name: Option<String> = loader.load_one(self.id).await?;
name.ok_or_else(|| "Not found".into())
}
}
```
In the end, only two SQLs are needed to query the results we want!
```sql
SELECT id, todo, user_id FROM todo
SELECT name FROM user WHERE id IN (1, 2, 3, 4)
```
## Implement multiple data types
You can implement multiple data types for the same `Loader`, like this:
```rust,ignore
# extern crate async_graphql;
# use async_graphql::*;
struct PostgresLoader {
pool: sqlx::Pool<Postgres>,
}
#[async_trait::async_trait]
impl Loader<UserId> for PostgresLoader {
type Value = User;
type Error = Arc<sqlx::Error>;
async fn load(&self, keys: &[UserId]) -> Result<HashMap<UserId, Self::Value>, Self::Error> {
// Load users from database
}
}
#[async_trait::async_trait]
impl Loader<TodoId> for PostgresLoader {
type Value = Todo;
type Error = sqlx::Error;
async fn load(&self, keys: &[TodoId]) -> Result<HashMap<TodoId, Self::Value>, Self::Error> {
// Load todos from database
}
}
```