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So today I started to implement a number of complex regex-based rules and templates to get rsyslog to parse ISC DHCPd logs in realtime. Unfortunately one of my templates must have been wrong, because I started seeing some errors about a field that cannot be blank in /var/log/messages. Unfortunately, rsyslog doesn’t log the query that raised the error, or the name of the template, or anything else useful - just that there was a database error. With over a dozen new templates, this didn’t help much. But the following technique did:

MySQLd has a General Query Log that can be activated by addling a line like:

log=/tmp/query.log`

to /etc/my.cnf under the [mysqld] section. This will log *all* queries to the specified log file, even if they resulted in an error or did not manipulate data.

I couldn’t find documentation on the log file format, but I observed that each line appeared to begin with some whitespace, then a number (perhaps a connection or section number, or maybe some sort of query ordering number), then the word “Query”, then the query. The following awk expression prints everything from the third column on, dropping the first two columns (the number and “Query”):

awk '{ for (i=2; i<=NF; i++) printf "%s ", $i; printf "\n"; }'

For easy analysis, the output of that can be piped into sort and then uniq.


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