Drop the unused id_country index on address when upgrading

upgrade/php/ps_915_drop_address_country_index.php

@@ -0,0 +1,23 @@
+<?php
+/**
+ * For the full copyright and license information, please view the
+ * LICENSE.md file that was distributed with this source code.
+ */
+
+use PrestaShop\Module\AutoUpgrade\Database\DbWrapper;
+
+function ps_915_drop_address_country_index(): bool
+{
+    // Shops that ran into the slow orders grid were told to drop this index by hand, so it can
+    // already be gone. DROP INDEX on a missing index is an error and MySQL has no IF EXISTS for
+    // indexes, so check before dropping rather than fail the upgrade on exactly those shops.
+    $existingIndex = DbWrapper::executeS(
+        'SHOW INDEX FROM `' . _DB_PREFIX_ . 'address` WHERE `Key_name` = \'id_country\''
+    );
+
+    if (empty($existingIndex)) {
+        return true;
+    }
+
+    return DbWrapper::execute('ALTER TABLE `' . _DB_PREFIX_ . 'address` DROP INDEX `id_country`');
+}

upgrade/sql/9.1.5.sql

@@ -0,0 +1,8 @@
+SET SESSION sql_mode='';
+SET NAMES 'utf8mb4';
+
+-- https://github.com/PrestaShop/PrestaShop/pull/42162
+-- Drop the unused id_country index on address. No query reads it - every core reference joins
+-- country on its primary key - and it misleads the optimizer on the orders grid into driving from
+-- country_lang instead of orders, turning a 50 row page into a scan with a filesort.
+/* PHP:ps_915_drop_address_country_index(); */;