fix: add equivalent index for mysql

core/Migrations/Version20240507143125.php

@@ -1,6 +1,7 @@
 <?php
 namespace OC\Migrations;
 
+use Doctrine\DBAL\Platforms\MySqlPlatform;
 use Doctrine\DBAL\Platforms\PostgreSqlPlatform;
 use Doctrine\DBAL\Platforms\SqlitePlatform;
 use OCP\IDBConnection;
@@ -14,6 +15,13 @@ public function sql(IDBConnection $connection) {
  WHERE parent IS NOT NULL AND share_type = 2"];
  }
 
+ if ($dbPlatform instanceof MySqlPlatform) {
+ // This should be preceise enough as mysql ignores nulls in unique indexes
+ return [
+ "CREATE UNIQUE INDEX child_share_unique ON oc_share (item_source, share_with, share_type, parent);"
+ ];
+ }
+
  return [];
  }
 }