expression: refactor builtin function RAND to be compatible with MySQL#9387

Merged

zz-jason merged 4 commits intopingcap:masterfrom XuHuaiyu:rand_data_race

Feb 22, 2019

+33-15

Commits on Feb 21, 2019

executor, expression: 1. make rand more compatiable with MySQL
XuHuaiyu
committed

Commits on Feb 22, 2019