The full documentation of the DBUG library is in files dbug/user.* in the MySQL source tree. Here are some of the DBUG tags we now use:

Arguments to the function.

Results from the function.

Something that may be interesting.

When something doesn't go the usual route or may be wrong.

When something went wrong.

Write in a loop, that is probably only useful when debugging the loop. These should normally be deleted when you are satisfied with the code and it has been in real use for a while.

Some tags specific to mysqld, because we want to watch these carefully:

Starting/stopping transactions.

info when mysqld is preparing to die.

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