Re: MySQL gets long semaphore locks on dict0dict.cc - but on DML operations

[Rick James]

"Don't queue it, just do it".

If it often better to launch a process to do something, rather than put things in a MySQL table that serves as a Queue.

I suspect the 17K rows is bogus; have you checked SELECT COUNT(*) FROM mytable;

Was the table created with innodb_file_per_table=ON? If so, run OPTIMIZE TABLE mytable; maybe that will clean things up.

Was the 99GB of free space caused by failing to delete rows for a long time?

If you are having performance issues, suggest compressing (in the client) the JSON string and putting it into `MEDIUMBLOB` instead of `LONGTEXT`.

I realize my answer does not directly address your problem, but it may make it into remission.