if the pipeline produce temporary table , the reduce number of the temporary table whose input table is temporary table may become very small in some cases, since temporary table has no content .
And, I may found the root cause in my case：
PColletionImpl.materializeAt(sourceTarget) this method will be invoked when node splits to create temporary table, source sourceTarget binds with the new temporary table whose size is 0, since its path was just created, the this.size will be 0. After that, when getSize() was invoked by setting reduce number, since the size is 0, it will just return 0, which makes reduce number too small.
So i think the code of materializeAt() should check sourceTarget's size, like below: