What is the ID rule when a query plan is presented as a tree? What is the execution order for this tree?

No rule exists for these IDs but the IDs are unique. When IDs are generated, a counter works and adds one when one plan is generated. The execution order has nothing to do with the ID. The whole query plan is a tree and the execution process starts from the root node and the data is returned to the upper level continuously. For details about the query plan, see Understanding the TiDB Query Execution Plan.