一般是负载过高，自动中断

加大这个阈值即可–critical-load Threads_running=400 加到400

pt-online-schema-change –user root
–password xxxxx
–host localhost
D=XXX,t=XXXXX
–execute
–alter “ADD UNIQUE INDEX invite_code (invite_code ASC)”
–nocheck-replication-filters
–charset utf8mb4
–max-load Threads_running=10
–critical-load Threads_running=400
–chunk-time=2.0
–no-check-alter