How to update multiple records in mysql

Question

I want to calculate interest over millions of record in mysql. So I am using scheduler

create event cal_interest 
on every 1 day 
do 
update 
userTable
set interest=(money*rate)/100 

My question are:
1. Is it possible millions of record update simultaneously?
2. Any possibility some record updated and some are failure.
3. If it wrong way to calculate interest over multiple record so please suggest me how can do this ?

Answer 1

1. In general, updating millions of rows at once isn't a very good idea. especially if you have database cluster (there'll be replication delays almost for sure). Better strategy is to split the update into batches.

2. Yes. There is always a possibility of failure

3. See 1 :) Split your table into batches of N records (N from 100 to 1000) and update them batch-by-batch. One of the strategies is to make a client job that initiates and monitors these batch updates. (one possible way: add an indexed field to store the date of the last update, then choose N rows which have last_update_date < current_date)

Comment: by "splitting the table" I didn't mean physically splitting, just the following:

• add the field where you keep the date of the last sync (and make it indexed) (e.g. last_sync_date);

• when the job starts, within the cycle do the following:

  • retrieve ID's of the next N records (e.g. N=500) with last_sync_date < curdate():

  • if you didn't get anything, you are done, exit the cycle;

  • otherwise, set interest=(money*rate)/100, last_sync_date = curdate() for the records with these IDs .

  I would rather do it as a job written outside of MySQL and scheduled via e.g. cron (because then it's easier to monitor how the job runs and kill it if necessary ), but you can, in theory, do it in MySQL too, for example (untested) something like that (I assume that your records have unique IDs stored in the field id):

delimiter |

create event cal_interest 
on every 1 day 
do
    create temporary table if not exists temp_ids(id int) engine=memory;
    declare keep_sync int default 1;
    begin
        repeat
            truncate temp_ids;
            insert into temp_ids(id) select id from userTable where last_sync_date < curdate() limit 500;
            select count(1) from temp_ids into keep_sync;
            update userTable set interest=(money*rate)/100, last_sync_date = curdate() where id in (select id from temp_ids) ids;
        until keep_sync>0;
        drop table temp_ids;
    end |

delimiter ;