As someone has post before, you can not set SQL_BIG_SELECTS on my.cnf at server startup. This kind of variable does not support that.
I had a same problem with a Symfony application showing this annoying error:
The SELECT would examine more than MAX_JOIN_SIZE rows; check your WHERE and use SET SQL_BIG_SELECTS=1 or SET SQL_MAX_JOIN_SIZE=# if the SELECT is okay
But you can increase the number of varialbe
max_join_size which is related to sql_big_selects
I was able to fix it executing a command as privileged mysql user:
# SET GLOBAL max_join_size=18446744073709551615;
Or you can include it in my.cnf because max_join_size is allowed to set up in configuration file
Well, I hope this can help someone else.