Add support for 'traceQueries' DSN configuration parameter

[gm42]

Description

This is a proposal for a way to trace each and every SQL query being executed by the driver. I do not expect it to be merged in its current form but rather please give me some feedback about how to achieve the purpose:

• being able to trace the SQL connections executed by a single goroutine

Right now I am printing them to errLog, although I'd possibly use a separate logger (as these are not really errors). It's also a common desire to be able to turn on/off the feature at runtime, which would require yet another mutex (I am not particularly happy about the one introduced here either).

Checklist

• Code compiles correctly
• Created tests which fail without the change
• All tests passing
• Extended the README / documentation, if necessary
• Added myself / the copyright holder to the AUTHORS file

Edit: renamed the parameter from debugQueries to traceQueries

[methane]

Why do you want to do this in driver level instead of higher level?

[gm42]

@methane I am using packages that wrap the driver and I am using the driver itself; by printing the information I need just before it's actually being executed I have a convenient central location from where to collect it; but I think that's obvious already?

[methane]

For example, gorp have logging feature.
You can also custom (hand made) query through gorp. https://godoc.org/github.com/coopernurse/gorp#DbMap.Exec

Since DSN limitation, it's difficult to configure logging. (format, logger, etc...)
I think database/sql package or package wraps it should provide more flexible logging support.

[gm42]

I agree, this should better be at database/sql level. Here in this proposal I was piggybacking on the existing logging feature of this driver.

I am looking at a general solution that allows me to capture all interactions of my Go application with the MySQL server.

Alternatively, one could write a shim package for go-sql-driver/mysql and wrap the methods there.

[methane]

FYI: This pull request doesn't log query executed without prepared statement.

[gm42]

@methane I will fix that, thanks for reporting

[gm42]

FYI I have just re-based & squashed this PR

[bgaifullin]

@gm42 https://github.com/mailru/dbr has instruments to trace queries and it is driver agnostic.

[dolmen]

Use https://github.com/luna-duclos/instrumentedsql which is able to wrap any driver for logging or other use cases.