mysql

Are you planning on storing blobs in Innodb tables? If yes, think again. According to the manual,

InnoDB stores the first 768 bytes of a VARBINARY, VARCHAR, BLOB, or TEXT column in the row, and the rest into separate pages.

This translates into slower retrieval for blobs. However, InnoDB can skip reading blobs if they are not in select column list.

One solution for this is to keep blobs in a separate myisam table and do a join when you need the blob column.

If you are a heavy user of MySQL, I think you will appreciate this list of saved bug searches. For example, check out "p1_bugs_in_5.1" or "ActiveInnoDBBugs" These saved searches are also available as rss feeds.

Thanks to Artem Russakovskii for this nice explanation of what Seconds_Behind_Master means

In order to see if a slave is lagging, execute the 'show slave status' command and look for the Seconds_Behind_Master value. The way this value is calculated can be slightly ambiguous and unclear, so I'll explain. It is simply a difference between the 2 timestamps the time of the last received (and queued up in the relay log) query that already executed on the master and the time of the currently executing query on the slave.

As he says in the post, this value is not a precise indication of the replication delay.

Links:

• MySQL Slave Delay Explained And 7 Ways To Battle It
• Other variables in Show Slave Status