How to reset node id to a lower number - Drupal 7 I have loaded thousand of nodes in different tests. How can I safely delete all the extra nodes and obly the database table to increment the node nid after the next valid (not deleted) nid? I.e., now I have 50 valid nodes, and the index starts at 250.000. How can I force it to start at nid = 51 without making the whole database crash ? 

A secure way, please. Not low level, but with a module or something that respects referential integrity of the whole database.
The node id is the primary key of the {node} database table. You need to reset the "auto_increment" property of that table to 1 (or 51 in your case). You can do that with PHPMyAdmin, or other MySQL client.