perf(tags): add index on mail_message_tags.tag_id

lib/Migration/Version5100Date20260619101500.php

@@ -0,0 +1,46 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace OCA\Mail\Migration;
+
+use Closure;
+use OCP\DB\ISchemaWrapper;
+use OCP\Migration\IOutput;
+use OCP\Migration\SimpleMigrationStep;
+
+/**
+ * @psalm-api
+ */
+class Version5100Date20260619101500 extends SimpleMigrationStep {
+
+	/**
+	 * @param IOutput $output
+	 * @param Closure(): ISchemaWrapper $schemaClosure
+	 * @param array $options
+	 * @return null|ISchemaWrapper
+	 */
+	#[\Override]
+	public function changeSchema(IOutput $output, Closure $schemaClosure, array $options): ?ISchemaWrapper {
+		$schema = $schemaClosure();
+
+		if (!$schema->hasTable('mail_message_tags')) {
+			return null;
+		}
+
+		$table = $schema->getTable('mail_message_tags');
+
+		if ($table->hasIndex('mail_msg_tags_tag_id_idx')) {
+			return null;
+		}
+
+		$table->addIndex(['tag_id'], 'mail_msg_tags_tag_id_idx');
+
+		return $schema;
+	}
+}