142 lines
3.9 KiB
Markdown
142 lines
3.9 KiB
Markdown
# 优化查询（解决N+1问题）
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您是否注意到某些GraphQL查询需要执行数百个数据库查询，这些查询通常包含重复的数据，让我们来看看为什么以及如何修复它。
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## 查询解析
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想象一下，如果您有一个简单的查询，例如：
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```graphql
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query { todos { users { name } } }
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```
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实现`User`的resolver代码如下：
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```rust
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struct User {
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id: u64,
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}
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#[Object]
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impl User {
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async fn name(&self, ctx: &Context<'_>) -> Result<String> {
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let pool = ctx.data_unchecked::<Pool<Postgres>>();
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let (name,): (String,) = sqlx::query_as("SELECT name FROM user WHERE id = $1")
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.bind(self.id)
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.fetch_one(pool)
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.await?;
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Ok(name)
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}
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}
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```
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执行查询将调用`Todos`的resolver，该resolver执行`SELECT * FROM todo`并返回N个`Todo`对象。然后对每个`Todo`对象同时调用`User`的
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resolver执行`SELECT name FROM user where id = $1`。
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例如：
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```sql
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SELECT id, todo, user_id FROM todo
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SELECT name FROM user WHERE id = $1
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SELECT name FROM user WHERE id = $1
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SELECT name FROM user WHERE id = $1
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SELECT name FROM user WHERE id = $1
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SELECT name FROM user WHERE id = $1
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SELECT name FROM user WHERE id = $1
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SELECT name FROM user WHERE id = $1
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SELECT name FROM user WHERE id = $1
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SELECT name FROM user WHERE id = $1
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SELECT name FROM user WHERE id = $1
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SELECT name FROM user WHERE id = $1
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SELECT name FROM user WHERE id = $1
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SELECT name FROM user WHERE id = $1
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SELECT name FROM user WHERE id = $1
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SELECT name FROM user WHERE id = $1
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SELECT name FROM user WHERE id = $1
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```
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执行了多次`SELECT name FROM user WHERE id = $1`，并且，大多数`Todo`对象都属于同一个用户，我们需要优化这些代码！
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## Dataloader
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我们需要对查询分组，并且排除重复的查询。`Dataloader`就能完成这个工作，[facebook](https://github.com/facebook/dataloader) 给出了一个请求范围的批处理和缓存解决方案。
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下面是使用`DataLoader`来优化查询请求的例子：
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```rust
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use async_graphql::*;
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use async_graphql::dataloader::*;
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use itertools::Itertools;
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use std::sync::Arc;
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struct UserNameLoader {
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pool: sqlx::Pool<Postgres>,
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}
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#[async_trait::async_trait]
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impl Loader<u64> for UserNameLoader {
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type Value = String;
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type Error = Arc<sqlx::Error>;
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async fn load(&self, keys: &[u64]) -> Result<HashMap<u64, Self::Value>, Self::Error> {
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let pool = ctx.data_unchecked::<Pool<Postgres>>();
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let query = format!("SELECT name FROM user WHERE id IN ({})", keys.iter().join(","));
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Ok(sqlx::query_as(query)
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.fetch(&self.pool)
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.map_ok(|name: String| name)
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.map_err(Arc::new)
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.try_collect().await?)
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}
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}
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struct User {
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id: u64,
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}
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#[Object]
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impl User {
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async fn name(&self, ctx: &Context<'_>) -> Result<String> {
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let loader = ctx.data_unchecked::<DataLoader<UserNameLoader>>();
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let name: Option<String> = loader.load_one(self.id).await?;
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name.ok_or_else(|| "Not found".into())
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}
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}
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```
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最终只需要两个查询语句，就查询出了我们想要的结果！
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```sql
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SELECT id, todo, user_id FROM todo
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SELECT name FROM user WHERE id IN (1, 2, 3, 4)
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```
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## 同一个Loader支持多种数据类型
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你可以为同一个`Loader`实现多种数据类型，就像下面这样：
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```rust
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struct PostgresLoader {
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pool: sqlx::Pool<Postgres>,
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}
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#[async_trait::async_trait]
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impl Loader<UserId> for PostgresLoader {
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type Value = User;
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type Error = Arc<sqlx::Error>;
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async fn load(&self, keys: &[UserId]) -> Result<HashMap<UserId, Self::Value>, Self::Error> {
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// 从数据库中加载User
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}
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}
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#[async_trait::async_trait]
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impl Loader<TodoId> for PostgresLoader {
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type Value = Todo;
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type Error = sqlx::Error;
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async fn load(&self, keys: &[TodoId]) -> Result<HashMap<TodoId, Self::Value>, Self::Error> {
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// 从数据库中加载Todo
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}
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}
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```
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