Issue #2843595 by hussainweb, heddn, jonhattan: Add indexes to migrate_map_* tables

core/modules/migrate/src/Plugin/migrate/id_map/Sql.php

@@ -313,8 +313,10 @@ protected function ensureTables() {
       // and map from the source field names to the map/msg field names.
       $count = 1;
       $source_id_schema = array();
+      $indexes = [];
       foreach ($this->migration->getSourcePlugin()->getIds() as $id_definition) {
         $mapkey = 'sourceid' . $count++;
+        $indexes['source'][] = $mapkey;
         $source_id_schema[$mapkey] = $this->getFieldSchema($id_definition);
         $source_id_schema[$mapkey]['not null'] = TRUE;
       }
@@ -369,6 +371,7 @@ protected function ensureTables() {
         'description' => 'Mappings from source identifier value(s) to destination identifier value(s).',
         'fields' => $fields,
         'primary key' => array(static::SOURCE_IDS_HASH),
+        'indexes' => $indexes,
       );
       $this->getDatabase()->schema()->createTable($this->mapTableName, $schema);
 

core/modules/migrate/tests/src/Unit/MigrateSqlIdMapEnsureTablesTest.php

@@ -34,6 +34,10 @@ public function testEnsureTablesNotExist() {
       'type' => 'int',
       'not null' => TRUE,
     );
+    $fields['sourceid2'] = array(
+      'type' => 'int',
+      'not null' => TRUE,
+    );
     $fields['destid1'] = array(
       'type' => 'varchar',
       'length' => 255,
@@ -72,6 +76,9 @@ public function testEnsureTablesNotExist() {
       'description' => 'Mappings from source identifier value(s) to destination identifier value(s).',
       'fields' => $fields,
       'primary key' => array('source_ids_hash'),
+      'indexes' => [
+        'source' => ['sourceid1', 'sourceid2'],
+      ],
     );
     $schema = $this->getMockBuilder('Drupal\Core\Database\Schema')
       ->disableOriginalConstructor()
@@ -205,6 +212,9 @@ protected function runEnsureTablesTest($schema) {
       'source_id_property' => array(
         'type' => 'integer',
       ),
+      'source_id_property_2' => array(
+        'type' => 'integer',
+      ),
     ));
     $migration->expects($this->any())
       ->method('getSourcePlugin')