⏫ Forwardport of #13033 to 2.3-develop branch

Applied pull request patch https://github.com/magento/magento2/pull/13033.patch (created by @devamitbera) based on commit(s):
  1. 7559783
  2. b853497
  3. be8b258

Fixed GitHub Issues in 2.3-develop branch:
  - #12787: Newsletter\Model\Subscriber::loadByEmail() does not use MySQL index (reported by @schmengler)

app/code/Magento/Newsletter/Setup/UpgradeSchema.php

@@ -0,0 +1,36 @@
+<?php
+/**
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+namespace Magento\Newsletter\Setup;
+
+use Magento\Framework\Setup\ModuleContextInterface;
+use Magento\Framework\Setup\SchemaSetupInterface;
+use Magento\Framework\Setup\UpgradeSchemaInterface;
+
+/**
+ * Upgrade the Newsletter module DB scheme
+ */
+class UpgradeSchema implements UpgradeSchemaInterface
+{
+    /**
+     * {@inheritdoc}
+     */
+    public function upgrade(SchemaSetupInterface $setup, ModuleContextInterface $context)
+    {
+        $setup->startSetup();
+
+        if (version_compare($context->getVersion(), '2.0.1', '<')) {
+            $connection = $setup->getConnection();
+
+            $connection->addIndex(
+                $setup->getTable('newsletter_subscriber'),
+                $setup->getIdxName('newsletter_subscriber', ['subscriber_email']),
+                ['subscriber_email']
+            );
+        }
+
+        $setup->endSetup();
+    }
+}

app/code/Magento/Newsletter/etc/module.xml

@@ -6,7 +6,7 @@
  */
 -->
 <config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Module/etc/module.xsd">
-    <module name="Magento_Newsletter" setup_version="2.0.0">
+    <module name="Magento_Newsletter" setup_version="2.0.1">
         <sequence>
             <module name="Magento_Store"/>
             <module name="Magento_Customer"/>