Switch most recently downloaded summary query to be faster

Because we only want to show the top 10 for /api/v1/summary, move the
LIMIT within the query for the top downloads rather than joining on
everything and then limiting.

Author: carols10cents

src/controllers/krate/metadata.rs

@@ -16,8 +16,7 @@ use models::krate::ALL_COLUMNS;
 
 /// Handles the `GET /summary` route.
 pub fn summary(req: &mut Request) -> CargoResult<Response> {
-    use diesel::dsl::*;
-    use diesel::sql_types::{BigInt, Nullable};
+    use diesel::sql_query;
     use schema::crates::dsl::*;
 
     let conn = req.db_conn()?;
@@ -57,17 +56,23 @@ pub fn summary(req: &mut Request) -> CargoResult<Response> {
         .limit(10)
         .load(&*conn)?;
 
-    let recent_downloads = sql::<Nullable<BigInt>>("SUM(crate_downloads.downloads)");
-    let most_recently_downloaded = crates
-        .left_join(
-            crate_downloads::table.on(id.eq(crate_downloads::crate_id)
-                .and(crate_downloads::date.gt(date(now - 90.days())))),
-        )
-        .group_by(id)
-        .order(recent_downloads.desc().nulls_last())
-        .limit(10)
-        .select(ALL_COLUMNS)
-        .load::<Crate>(&*conn)?;
+    // This query needs to be structured in this way to have the LIMIT
+    // happen before the joining/sorting for performance reasons.
+    // It needs to use sql_query because Diesel doesn't have a great way
+    // to join on subselects right now :(
+    let most_recently_downloaded = sql_query(
+        "SELECT crates.* \
+         FROM crates \
+         JOIN ( \
+             SELECT crate_downloads.crate_id, SUM(crate_downloads.downloads) \
+             FROM crate_downloads \
+             WHERE crate_downloads.date > date(CURRENT_TIMESTAMP - INTERVAL '90 days') \
+             GROUP BY crate_downloads.crate_id \
+             ORDER BY SUM(crate_downloads.downloads) DESC NULLS LAST \
+             LIMIT 10 \
+         ) cd ON crates.id = cd.crate_id \
+         ORDER BY cd.sum DESC NULLS LAST"
+    ).load::<Crate>(&*conn)?;
 
     let popular_keywords = keywords::table
         .order(keywords::crates_cnt.desc())

src/models/krate.rs

@@ -30,7 +30,8 @@ pub struct CrateDownload {
     pub date: NaiveDate,
 }
 
-#[derive(Debug, Clone, Queryable, Identifiable, Associations, AsChangeset)]
+#[derive(Debug, Clone, Queryable, Identifiable, Associations, AsChangeset, QueryableByName)]
+#[table_name = "crates"]
 pub struct Crate {
     pub id: i32,
     pub name: String,