Index: src/wp-admin/includes/upgrade.php
===================================================================
--- src/wp-admin/includes/upgrade.php	(リビジョン 41019)
+++ src/wp-admin/includes/upgrade.php	(作業コピー)
@@ -2856,6 +2856,11 @@
 			maybe_convert_table_to_utf8mb4( $wpdb->termmeta );
 		}
 	}
+
+	// Upgrade version prior to 4.9.
+	if ( $wp_current_db_version < 38591 ) {
+		$wpdb->query( "ALTER TABLE $wpdb->postmeta ADD INDEX meta_key_meta_value(meta_key(191), meta_value(64))" );
+	}
 }
 
 if ( !function_exists( 'install_global_terms' ) ) :
Index: src/wp-includes/version.php
===================================================================
--- src/wp-includes/version.php	(リビジョン 41019)
+++ src/wp-includes/version.php	(作業コピー)
@@ -11,7 +11,7 @@
  *
  * @global int $wp_db_version
  */
-$wp_db_version = 38590;
+$wp_db_version = 38591;
 
 /**
  * Holds the TinyMCE version