Improved cart/checkout performance optimizing coupon code check query (…

…#1110)

Co-authored-by: Hannes Drittler <h.drittler@lemundo.de>
Co-authored-by: Fabrizio Balliano <fabrizio.balliano@gmail.com>

app/code/core/Mage/SalesRule/Model/Resource/Rule/Collection.php

@@ -85,7 +85,7 @@ public function setValidationFilter($websiteId, $customerGroupId, $couponCode =
                     $connection->quoteInto(
                         'main_table.rule_id = rule_coupons.rule_id AND main_table.coupon_type != ?',
                         Mage_SalesRule_Model_Rule::COUPON_TYPE_NO_COUPON
-                    ),
+                    ) . $connection->quoteInto(' AND rule_coupons.code = ?', $couponCode),
                     ['code']
                 );