add unique index 注意事项

给业务添加唯一索引之前，必须核对当前的数据是否违反唯一索引约束：

检测：

select   count( distinct app_id,count_date,cluster,priority) from events  ;   

select   count( *) from events  ;   


select   app_id,count_date,cluster,priority ,count(*)  from events group by  app_id,count_date,cluster,priority  having count(*) >1 limit 20;   

违反约束：

alter table events  add unique index uk_app(app_id,count_date,cluster,priority);       

ERROR 1062 (23000): Duplicate entry '2882303761517233197-2020-09-21-ALL-1' for key 'uk_id'