use an index instead

dev-tools/omdb/src/bin/omdb/db.rs

@@ -4247,21 +4247,16 @@ async fn cmd_db_migrations_list(
         deleted: Option<chrono::DateTime<Utc>>,
     }
 
-    let migrations = datastore
-        .pool_connection_for_tests()
-        .await?
-        .transaction_async(|conn| async move {
-            // Selecting all migration records requires a full table scan
-            conn.batch_execute_async(ALLOW_FULL_TABLE_SCAN_SQL).await?;
-            query
-                .limit(i64::from(u32::from(fetch_opts.fetch_limit)))
-                .order_by(dsl::time_created)
-                .select(Migration::as_select())
-                .load_async(&conn)
-                .await
-                .context("listing migrations")
-        })
-        .await?;
+    let migrations = query
+        .limit(i64::from(u32::from(fetch_opts.fetch_limit)))
+        .order_by(dsl::time_created)
+        // This is just to prove to CRDB that it can use the
+        // migrations-by-time-created index, it doesn't actually do anything.
+        .filter(dsl::time_created.gt(chrono::DateTime::UNIX_EPOCH))
+        .select(Migration::as_select())
+        .load_async(&*datastore.pool_connection_for_tests().await?)
+        .await
+        .context("listing migrations")?;
 
     check_limit(&migrations, fetch_opts.fetch_limit, || "listing migrations");
 

nexus/db-model/src/schema_versions.rs

@@ -17,7 +17,7 @@ use std::collections::BTreeMap;
 ///
 /// This must be updated when you change the database schema.  Refer to
 /// schema/crdb/README.adoc in the root of this repository for details.
-pub const SCHEMA_VERSION: SemverVersion = SemverVersion::new(84, 0, 0);
+pub const SCHEMA_VERSION: SemverVersion = SemverVersion::new(85, 0, 0);
 
 /// List of all past database schema versions, in *reverse* order
 ///
@@ -29,7 +29,8 @@ static KNOWN_VERSIONS: Lazy<Vec<KnownVersion>> = Lazy::new(|| {
         // |  leaving the first copy as an example for the next person.
         // v
         // KnownVersion::new(next_int, "unique-dirname-with-the-sql-files"),
-        KnownVersion::new(84, "region-read-only"),
+        KnownVersion::new(85, "region-read-only"),
+        KnownVersion::new(84, "add-migrations-by-time-created-index"),
         KnownVersion::new(83, "dataset-address-optional"),
         KnownVersion::new(82, "region-port"),
         KnownVersion::new(81, "add-nullable-filesystem-pool"),

schema/crdb/add-migrations-by-time-created-index/up.sql

@@ -0,0 +1,9 @@
+
+/* Migrations by time created.
+ *
+ * Currently, this is only used by OMDB for ordering the `omdb migration list`
+ * output, but it may be used by other UIs in the future...
+*/
+CREATE INDEX IF NOT EXISTS migrations_by_time_created ON omicron.public.migration (
+    time_created
+);

schema/crdb/dbinit.sql

@@ -4131,6 +4131,15 @@ CREATE INDEX IF NOT EXISTS lookup_migrations_by_instance_id ON omicron.public.mi
     instance_id
 );
 
+/* Migrations by time created.
+ *
+ * Currently, this is only used by OMDB for ordering the `omdb migration list`
+ * output, but it may be used by other UIs in the future...
+*/
+CREATE INDEX IF NOT EXISTS migrations_by_time_created ON omicron.public.migration (
+    time_created
+);
+
 /* Lookup region snapshot by snapshot id */
 CREATE INDEX IF NOT EXISTS lookup_region_snapshot_by_snapshot_id on omicron.public.region_snapshot (
     snapshot_id
@@ -4147,7 +4156,7 @@ INSERT INTO omicron.public.db_metadata (
     version,
     target_version
 ) VALUES
-    (TRUE, NOW(), NOW(), '84.0.0', NULL)
+    (TRUE, NOW(), NOW(), '85.0.0', NULL)
 ON CONFLICT DO NOTHING;
 
 COMMIT;