Make RowsAffected faster

The current implementation uses two loops, first one to find out
where digits start and second one to loop over the digits. The
modified implementation starts at the end and loops till digits
end. It is a bit faster than baseline.

Author: achanda

pgconn/pgconn.go

@@ -777,25 +777,20 @@ func NewCommandTag(s string) CommandTag {
 // RowsAffected returns the number of rows affected. If the CommandTag was not
 // for a row affecting command (e.g. "CREATE TABLE") then it returns 0.
 func (ct CommandTag) RowsAffected() int64 {
-	// Find last non-digit
-	idx := -1
+	// Parse the number from the end in a single pass.
+	var n int64
+	var mult int64 = 1
+
 	for i := len(ct.s) - 1; i >= 0; i-- {
-		if ct.s[i] >= '0' && ct.s[i] <= '9' {
-			idx = i
+		c := ct.s[i]
+		if c >= '0' && c <= '9' {
+			n += int64(c-'0') * mult
+			mult *= 10
 		} else {
 			break
 		}
 	}
 
-	if idx == -1 {
-		return 0
-	}
-
-	var n int64
-	for _, b := range ct.s[idx:] {
-		n = n*10 + int64(b-'0')
-	}
-
 	return n
 }