rows=0 minimum required=50 out of=1000
*** 2015-06-18 16:35:58.344
qerhjFetch: max probe row length (mpl=0)
*** RowSrcId: 2, qerhjFreeSpace(): free hash-join memory
kxhfRemoveChunk: remove chunk 0 from slot table
对比两个实验，你会发现，当Maximum number of rows in a bucket低的时候效率会高很多，当SQL中出现Maximum number of rows in a bucket高的情况，要想办法把它降低，打散。