Add is_sticky, last_posted_at index to improve performance (#23)

This PR introduces an additional index [is_sticky, last_posted_at] to the discussions table. We discovered that when viewing discussions in a tag with a large number of discussions (approx 1.4M in our case), performance was poor when using the default sort criteria (sort by latest).

extensions/sticky/migrations/2021_01_13_000000_add_discussion_last_posted_at_indices.php

@@ -0,0 +1,25 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * For detailed copyright and license information, please view the
+ * LICENSE file that was distributed with this source code.
+ */
+
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Database\Schema\Builder;
+
+return [
+    'up' => function (Builder $schema) {
+        $schema->table('discussions', function (Blueprint $table) {
+            $table->index(['is_sticky', 'last_posted_at']);
+        });
+    },
+
+    'down' => function (Builder $schema) {
+        $schema->table('discussions', function (Blueprint $table) {
+            $table->dropIndex(['is_sticky', 'last_posted_at']);
+        });
+    }
+];