apache/spark — pull request #2155 by chenghao-intel (2014-08-27 05:56:31+00:00)
Title: [SPARK-3198] [SQL] Remove the TreeNode.id

Thus id property of the TreeNode API does save time in a faster way to compare 2 TreeNodes, it is kind of performance bottleneck during the expression object creation in a multi-threading env (because of the memory barrier).
Fortunately, the tree node comparison only happen once in master, so even we remove it, the entire performance will not be affected.