*: address comment

executor/prepared.go

@@ -104,6 +104,8 @@ func (e *PrepareExec) Close() error {
 func (e *PrepareExec) DoPrepare() {
 	vars := variable.GetSessionVars(e.Ctx)
 	if e.ID != 0 {
+		// Must be the case when we retry a prepare.
+		// Make sure it is idempotent.
 		_, ok := vars.PreparedStmts[e.ID]
 		if ok {
 			return
@@ -122,6 +124,10 @@ func (e *PrepareExec) DoPrepare() {
 	stmt := l.Stmts()[0]
 	var extractor paramMarkerExtractor
 	stmt.Accept(&extractor)
+
+	// The parameter markers are appended in visiting order, which may not
+	// be the same as the position order in the query string. We need to
+	// sort it by position.
 	sorter := &paramMarkerSorter{markers: extractor.markers}
 	sort.Sort(sorter)
 	e.ParamCount = len(sorter.markers)

sessionctx/variable/session.go

@@ -79,6 +79,11 @@ func GetSessionVars(ctx context.Context) *SessionVars {
 	return v
 }
 
+const (
+	characterSetConnection = "character_set_connection"
+	collationConnection    = "collation_connection"
+)
+
 // GetCharsetInfo gets charset and collation for current context.
 // What character set should the server translate a statement to after receiving it?
 // For this, the server uses the character_set_connection and collation_connection system variables.
@@ -90,8 +95,8 @@ func GetSessionVars(ctx context.Context) *SessionVars {
 // See: https://dev.mysql.com/doc/refman/5.7/en/charset-connection.html
 func GetCharsetInfo(ctx context.Context) (charset, collation string) {
 	sessionVars := GetSessionVars(ctx)
-	charset = sessionVars.Systems["character_set_connection"]
-	collation = sessionVars.Systems["collation_connection"]
+	charset = sessionVars.Systems[characterSetConnection]
+	collation = sessionVars.Systems[collationConnection]
 	return
 }