While doing profiling of one tool I found what function Rows.Scan() is suffering from
lock contention. The reason is what function convertAssign() use fmt.Sprintf() to
convert rows data to desired type.
So if you read a lot of data from database and use num processors > 1 you can get
result like in attached before.svg. I see there 2 problems:
1. Lock contention in function fmt.newPrinter()/free()
2. A lot of time spend inside fmt.doPrintf()

This issue is not about fmt package itself, my proposal is to use functions from strconv
package instead of calling fmt.Sprintf("%v", src). As you can see on the
second profiling, after replacing fmt.Sprintf() to
strconv.FormatBool/FormatFloat/FormatInt performance of Scanf increased about 2 times. 

I can submit my patch for this improvement if you would like to accept this issue.

Environment: 
OS: RHEL 6.5
Go version: 1.2 and development
24 CPU's

Attachments:

1. before.svg (98258 bytes)
2. after.svg (105460 bytes)