[MySQL] Deleting/Updating Rows Common To 2 Tables – Speed And Slave Lag Considerations
Introduction
A question I recently saw on Stack Overflow titled Faster way to delete matching [database] rows? prompted me to organize my thoughts and observations on the subject and quickly jot them down here.
Here is the brief description of the task: say, you have 2 MySQL tables a and b. The tables contain the same type of data, for example log entries. Now you want to delete all or a subset of the entries in table a that exist in table b.
Solutions Suggested By Others
DELETE FROM a WHERE EXISTS (SELECT b.id FROM b WHERE b.id = a.id);
DELETE a FROM a INNER JOIN b on a.id=b.id;
MySQL Slave Lag (Delay) Explained And 7 Ways To Battle It
Updated: August 8th, 2010
Slave delay can be a nightmare. I battle it every day and know plenty of people who curse the serialization problem of replication. For those who are not familiar with it, replication on MySQL slaves runs commands in series – one by one, while the master may run them in parallel. This fact usually causes bottlenecks. Consider these 2 examples:
- Between 1 and 100 UPDATE queries are constantly running on the master in parallel. If the slave IO is only fast enough to handle 50 of them without lagging, as soon as 51 start running, the slaves starts to lag.
- A more common problem is when one query takes an hour to run (let's say, it's an UPDATE with a
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