429 lines
24 KiB
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429 lines
24 KiB
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<main>
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<h1 id="look-ahead"><a class="header" href="#look-ahead">Look-ahead</a></h1>
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<blockquote>
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<p>In backtracking algorithms, <strong>look ahead</strong> is the generic term for a subprocedure that attempts to foresee the effects of choosing a branching variable to evaluate one of its values. The two main aims of look-ahead are to choose a variable to evaluate next and to choose the order of values to assign to it.</p>
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</blockquote>
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<p>In <a href="https://graphql.org">GraphQL</a>, look-ahead machinery allows us to introspect the currently <a href="https://spec.graphql.org/October2021#sec-Execution">executed</a> <a href="https://spec.graphql.org/October2021#sec-Language.Operations%5C">GraphQL operation</a> to see which <a href="https://spec.graphql.org/October2021#sec-Language.Fields">fields</a> has been actually selected by it.</p>
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<p>In <a href="https://docs.rs/juniper">Juniper</a>, it's represented by the <a href="https://docs.rs/juniper/0.16.1/juniper/executor/struct.Executor.html#method.look_ahead"><code>Executor::look_ahead()</code></a> method.</p>
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<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span><span class="boring">extern crate juniper;
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</span><span class="boring">use juniper::{graphql_object, Executor, GraphQLObject, ScalarValue};
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</span><span class="boring">
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</span><span class="boring">type UserId = i32;
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</span><span class="boring">
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</span>#[derive(GraphQLObject)]
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struct Person {
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id: UserId,
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name: String,
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}
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struct Query;
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#[graphql_object]
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// NOTICE: Specifying `ScalarValue` as custom named type parameter,
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// so its name is similar to the one used in methods.
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#[graphql(scalar = S: ScalarValue)]
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impl Query {
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fn persons<S: ScalarValue>(executor: &Executor<'_, '_, (), S>) -> Vec<Person> {
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// Let's see which `Person`'s fields were selected in the client query.
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for field_name in executor.look_ahead().children().names() {
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dbg!(field_name);
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}
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// ...
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<span class="boring"> unimplemented!()
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</span> }
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}
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<span class="boring">}</span></code></pre></pre>
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<blockquote>
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<p><strong>TIP</strong>: <code>S: ScalarValue</code> type parameter on the method is required here to keep the <a href="https://docs.rs/juniper/0.16.1/juniper/executor/struct.Executor.html"><code>Executor</code></a> being generic over <a href="https://docs.rs/juniper/0.16.1/juniper/trait.ScalarValue.html"><code>ScalarValue</code></a> types. We, instead, could have used the <a href="https://docs.rs/juniper/0.16.1/juniper/enum.DefaultScalarValue.html"><code>DefaultScalarValue</code></a>, which is the default <a href="https://docs.rs/juniper/0.16.1/juniper/trait.ScalarValue.html"><code>ScalarValue</code></a> type for the <a href="https://docs.rs/juniper/0.16.1/juniper/executor/struct.Executor.html"><code>Executor</code></a>, and make our code more ergonomic, but less flexible and generic.</p>
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<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span><span class="boring">extern crate juniper;
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</span><span class="boring">use juniper::{graphql_object, DefaultScalarValue, Executor, GraphQLObject};
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</span><span class="boring">
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</span><span class="boring">type UserId = i32;
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</span><span class="boring">
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</span><span class="boring">#[derive(GraphQLObject)]
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</span><span class="boring">struct Person {
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</span><span class="boring"> id: UserId,
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</span><span class="boring"> name: String,
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</span><span class="boring">}
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</span><span class="boring">
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</span><span class="boring">struct Query;
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</span><span class="boring">
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</span>#[graphql_object]
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#[graphql(scalar = DefaultScalarValue)]
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impl Query {
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fn persons(executor: &Executor<'_, '_, ()>) -> Vec<Person> {
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for field_name in executor.look_ahead().children().names() {
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dbg!(field_name);
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}
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// ...
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<span class="boring"> unimplemented!()
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</span> }
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}
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<span class="boring">}</span></code></pre></pre>
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</blockquote>
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<h2 id="n1-problem"><a class="header" href="#n1-problem">N+1 problem</a></h2>
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<p>Naturally, look-ahead machinery allows us to solve <a href="n_plus_1.html">the N+1 problem</a> by introspecting the requested fields and performing loading in batches eagerly, before actual resolving of those fields:</p>
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<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span><span class="boring">extern crate anyhow;
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</span><span class="boring">extern crate juniper;
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</span><span class="boring">use std::collections::HashMap;
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</span><span class="boring">use anyhow::anyhow;
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</span><span class="boring">use juniper::{graphql_object, Executor, GraphQLObject, ScalarValue};
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</span><span class="boring">
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</span><span class="boring">type CultId = i32;
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</span><span class="boring">type UserId = i32;
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</span><span class="boring">
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</span><span class="boring">struct Repository;
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</span><span class="boring">
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</span><span class="boring">impl juniper::Context for Repository {}
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</span><span class="boring">
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</span><span class="boring">impl Repository {
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</span><span class="boring"> async fn load_cult_by_id(&self, cult_id: CultId) -> anyhow::Result<Option<Cult>> { unimplemented!() }
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</span><span class="boring"> async fn load_cults_by_ids(&self, cult_ids: &[CultId]) -> anyhow::Result<HashMap<CultId, Cult>> { unimplemented!() }
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</span><span class="boring"> async fn load_all_persons(&self) -> anyhow::Result<Vec<Person>> { unimplemented!() }
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</span><span class="boring">}
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</span><span class="boring">
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</span><span class="boring">enum Either<L, R> {
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</span><span class="boring"> Absent(L),
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</span><span class="boring"> Loaded(R),
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</span><span class="boring">}
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</span><span class="boring">
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</span>#[derive(Clone, GraphQLObject)]
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struct Cult {
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id: CultId,
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name: String,
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}
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struct Person {
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id: UserId,
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name: String,
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cult: Either<CultId, Cult>,
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}
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#[graphql_object]
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#[graphql(context = Repository)]
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impl Person {
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fn id(&self) -> CultId {
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self.id
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}
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fn name(&self) -> &str {
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self.name.as_str()
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}
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async fn cult(&self, #[graphql(ctx)] repo: &Repository) -> anyhow::Result<Cult> {
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match &self.cult {
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Either::Loaded(cult) => Ok(cult.clone()),
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Either::Absent(cult_id) => {
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// Effectively performs the following SQL query:
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// SELECT id, name FROM cults WHERE id = ${cult_id} LIMIT 1
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repo.load_cult_by_id(*cult_id)
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.await?
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.ok_or_else(|| anyhow!("No cult exists for ID `{cult_id}`"))
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}
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}
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}
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}
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struct Query;
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#[graphql_object]
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#[graphql(context = Repository, scalar = S: ScalarValue)]
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impl Query {
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async fn persons<S: ScalarValue>(
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#[graphql(ctx)] repo: &Repository,
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executor: &Executor<'_, '_, Repository, S>,
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) -> anyhow::Result<Vec<Person>> {
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// Effectively performs the following SQL query:
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// SELECT id, name, cult_id FROM persons
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let mut persons = repo.load_all_persons().await?;
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// If the `Person.cult` field has been requested.
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if executor.look_ahead()
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.children()
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.iter()
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.any(|sel| sel.field_original_name() == "cult")
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{
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// Gather `Cult.id`s to load eagerly.
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let cult_ids = persons
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.iter()
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.filter_map(|p| {
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match &p.cult {
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Either::Absent(cult_id) => Some(*cult_id),
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// If for some reason a `Cult` is already loaded,
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// then just skip it.
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Either::Loaded(_) => None,
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}
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})
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.collect::<Vec<_>>();
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// Load the necessary `Cult`s eagerly.
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// Effectively performs the following SQL query:
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// SELECT id, name FROM cults WHERE id IN (${cult_id1}, ${cult_id2}, ...)
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let cults = repo.load_cults_by_ids(&cult_ids).await?;
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// Populate `persons` with the loaded `Cult`s, so they do not perform
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// any SQL queries on resolving.
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for p in &mut persons {
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let Either::Absent(cult_id) = &p.cult else { continue; };
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p.cult = Either::Loaded(
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cults.get(cult_id)
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.ok_or_else(|| anyhow!("No cult exists for ID `{cult_id}`"))?
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.clone(),
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);
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}
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}
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Ok(persons)
|
|
}
|
|
}
|
|
<span class="boring">}</span></code></pre></pre>
|
|
<p>And so, performing a <a href="n_plus_1.html">GraphQL query which lead to N+1 problem</a></p>
|
|
<pre><code class="language-graphql">query {
|
|
persons {
|
|
id
|
|
name
|
|
cult {
|
|
id
|
|
name
|
|
}
|
|
}
|
|
}
|
|
</code></pre>
|
|
<p>will lead to efficient <a href="https://en.wikipedia.org/wiki/SQL">SQL</a> queries, just as expected:</p>
|
|
<pre><code class="language-sql">SELECT id, name, cult_id FROM persons;
|
|
SELECT id, name FROM cults WHERE id IN (1, 2, 3, 4);
|
|
</code></pre>
|
|
<h2 id="more-features"><a class="header" href="#more-features">More features</a></h2>
|
|
<p>See more available look-ahead features in the API docs of the <a href="https://docs.rs/juniper/0.16.1/juniper/executor/struct.LookAheadSelection.html"><code>LookAheadSelection</code></a> and the <a href="https://docs.rs/juniper/0.16.1/juniper/executor/struct.LookAheadChildren.html"><code>LookAheadChildren</code></a>.</p>
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