大量消息收件箱完成第一层性能优化

src/Jiaowu.Api/Controllers/NotificationsController.cs

@@ -58,6 +58,7 @@ public sealed class NotificationsController(
         var total = await source.CountAsync(cancellationToken);
         var items = await source
             .OrderByDescending(x => x.CreatedAt)
+            .ThenByDescending(x => x.Id)
             .Skip((page - 1) * pageSize)
             .Take(pageSize)
             .Select(x => new

src/Jiaowu.Api/Infrastructure/Persistence/AppDbContext.cs

@@ -1475,8 +1475,11 @@ public sealed class AppDbContext(DbContextOptions<AppDbContext> options)
             entity.Property(x => x.Title).HasMaxLength(200);
             entity.Property(x => x.Content).HasColumnType("longtext");
             entity.Property(x => x.LinkUrl).HasMaxLength(300);
-            entity.HasIndex(x => new { x.UserId, x.IsRead });
-            entity.HasIndex(x => new { x.UserId, x.Category, x.CreatedAt });
+            // Each inbox query starts with its recipient.  Keep the selected
+            // sort fields in the index so large inboxes do not need a filesort.
+            entity.HasIndex(x => new { x.UserId, x.CreatedAt, x.Id });
+            entity.HasIndex(x => new { x.UserId, x.IsRead, x.CreatedAt, x.Id });
+            entity.HasIndex(x => new { x.UserId, x.Category, x.CreatedAt, x.Id });
             entity.HasIndex(x => x.MessageDispatchId);
             entity.HasIndex(x => x.CreatedAt);
             entity.HasOne(x => x.MessageDispatch)

src/Jiaowu.Api/Infrastructure/Persistence/DevelopmentSqliteMigrator.cs

@@ -102,6 +102,8 @@ public sealed class DevelopmentSqliteMigrator(
         "20260809_53_course_grade_statistics_refresh_settings";
     private const string ExperimentCourseGradesMigration =
         "20260809_54_experiment_course_grades";
+    private const string NotificationInboxIndexesMigration =
+        "20260810_55_notification_inbox_indexes";
 
     public async Task MigrateAsync(CancellationToken cancellationToken = default)
     {
@@ -731,6 +733,10 @@ public sealed class DevelopmentSqliteMigrator(
             ExperimentCourseGradesMigration,
             experimentCourseGradesExist ? [] : ExperimentCourseGradesStatements,
             cancellationToken);
+        await ApplyMigrationAsync(
+            NotificationInboxIndexesMigration,
+            NotificationInboxIndexesStatements,
+            cancellationToken);
     }
 
     private async Task ApplyMigrationAsync(
@@ -2999,6 +3005,15 @@ public sealed class DevelopmentSqliteMigrator(
         """CREATE UNIQUE INDEX IF NOT EXISTS "IX_CourseGradeStatisticsRefreshSettings_Key" ON "CourseGradeStatisticsRefreshSettings" ("Key");"""
     ];
 
+    private static readonly string[] NotificationInboxIndexesStatements =
+    [
+        """DROP INDEX "IX_Notifications_UserId_IsRead";""",
+        """DROP INDEX "IX_Notifications_UserId_Category_CreatedAt";""",
+        """CREATE INDEX "IX_Notifications_UserId_CreatedAt_Id" ON "Notifications" ("UserId", "CreatedAt", "Id");""",
+        """CREATE INDEX "IX_Notifications_UserId_IsRead_CreatedAt_Id" ON "Notifications" ("UserId", "IsRead", "CreatedAt", "Id");""",
+        """CREATE INDEX "IX_Notifications_UserId_Category_CreatedAt_Id" ON "Notifications" ("UserId", "Category", "CreatedAt", "Id");"""
+    ];
+
     private static readonly string[] ExperimentCourseGradesStatements =
     [
         """

src/Jiaowu.Api/Infrastructure/Persistence/Migrations/MySql/20260810090831_OptimizeNotificationInboxIndexes.cs

@@ -0,0 +1,63 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace Jiaowu.Api.Infrastructure.Persistence.Migrations.MySql
+{
+    /// <inheritdoc />
+    public partial class OptimizeNotificationInboxIndexes : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropIndex(
+                name: "IX_Notifications_UserId_Category_CreatedAt",
+                table: "Notifications");
+
+            migrationBuilder.DropIndex(
+                name: "IX_Notifications_UserId_IsRead",
+                table: "Notifications");
+
+            migrationBuilder.CreateIndex(
+                name: "IX_Notifications_UserId_Category_CreatedAt_Id",
+                table: "Notifications",
+                columns: new[] { "UserId", "Category", "CreatedAt", "Id" });
+
+            migrationBuilder.CreateIndex(
+                name: "IX_Notifications_UserId_CreatedAt_Id",
+                table: "Notifications",
+                columns: new[] { "UserId", "CreatedAt", "Id" });
+
+            migrationBuilder.CreateIndex(
+                name: "IX_Notifications_UserId_IsRead_CreatedAt_Id",
+                table: "Notifications",
+                columns: new[] { "UserId", "IsRead", "CreatedAt", "Id" });
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropIndex(
+                name: "IX_Notifications_UserId_Category_CreatedAt_Id",
+                table: "Notifications");
+
+            migrationBuilder.DropIndex(
+                name: "IX_Notifications_UserId_CreatedAt_Id",
+                table: "Notifications");
+
+            migrationBuilder.DropIndex(
+                name: "IX_Notifications_UserId_IsRead_CreatedAt_Id",
+                table: "Notifications");
+
+            migrationBuilder.CreateIndex(
+                name: "IX_Notifications_UserId_Category_CreatedAt",
+                table: "Notifications",
+                columns: new[] { "UserId", "Category", "CreatedAt" });
+
+            migrationBuilder.CreateIndex(
+                name: "IX_Notifications_UserId_IsRead",
+                table: "Notifications",
+                columns: new[] { "UserId", "IsRead" });
+        }
+    }
+}

src/Jiaowu.Api/Infrastructure/Persistence/Migrations/MySql/AppDbContextModelSnapshot.cs

@@ -3417,9 +3417,11 @@ namespace Jiaowu.Api.Infrastructure.Persistence.Migrations.MySql
 
                     b.HasIndex("MessageDispatchId");
 
-                    b.HasIndex("UserId", "IsRead");
+                    b.HasIndex("UserId", "CreatedAt", "Id");
 
-                    b.HasIndex("UserId", "Category", "CreatedAt");
+                    b.HasIndex("UserId", "Category", "CreatedAt", "Id");
+
+                    b.HasIndex("UserId", "IsRead", "CreatedAt", "Id");
 
                     b.ToTable("Notifications");
                 });