When started with the --log-update=file_name switch mysqld
makes a log file with all SQL command that update data. It results in a
file with name of file_name.# where # is a number that is
increased for each refresh. If you do not give a file name the current
hostname is used.
The logging is smart since it only writes statements that really update
data. So an UPDATE or a DELETE with a WHERE that finds no
rows is not written to the log. It even skips UPDATEs that updates
a column to the value it had before.
If you want to update a database according from a update log you could do the following:
cat file-name.* | mysql
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