connect: reduce allocations when building SET command (#1111)

* connect: reduce allocations when building SET command * handleParams: use strings.Builder instead of direct []byte

Author: dolmen

connection.go

@@ -47,7 +47,7 @@ type mysqlConn struct{
 
 // Handles parameters set in DSN after the connection is established
 func (mc*mysqlConn) handleParams() (errerror){
-varparams []string
+varcmdSet strings.Builder
 forparam, val:=rangemc.cfg.Params{
 switchparam{
 // Charset: character_set_connection, character_set_client, character_set_results
@@ -64,14 +64,23 @@ func (mc *mysqlConn) handleParams() (err error){
 return
  }
 
-// Other system vars
+// Other system vars accumulated in a single SET command
 default:
-params=append(params, param+"="+val)
+ifcmdSet.Len() ==0{
+// Heuristic: 29 chars for each other key=value to reduce reallocations
+cmdSet.Grow(4+len(param) +1+len(val) +30*(len(mc.cfg.Params)-1))
+cmdSet.WriteString("SET ")
+ } else{
+cmdSet.WriteByte(',')
+ }
+cmdSet.WriteString(param)
+cmdSet.WriteByte('=')
+cmdSet.WriteString(val)
  }
  }
 
-iflen(params) >0{
-err=mc.exec("SET "+strings.Join(params, ","))
+ifcmdSet.Len() >0{
+err=mc.exec(cmdSet.String())
 iferr!=nil{
 return
  }