Enabling tracing
 
 
- Per handle
- 
$h->{TraceLevel} = $level;
 
 $h->trace($level);
 $h->trace($level, $filename); # $filename used for all handles
 
- Trace level only affects that handle and any new child handles created from it
- Child handles get trace level of parent in effect at time of creation
- Can be set via DSN:   "dbi:Driver(TraceLevel=2):…"
- Global (internal to application)
- Sets effective global default minimum trace level
- Global (external to application)
- 
- Enabled using DBI_TRACE environment variable
 DBI_TRACE=digits		  same as DBI->trace(digits);
 DBI_TRACE=digits=filename	  same as DBI->trace(digits, filename);