From c0d84b7ef7fe639f8ac95a47e31b151784e7300a Mon Sep 17 00:00:00 2001
From: Max Hoffman <maximilian.wolfgang1@gmail.com>
Date: Wed, 18 Dec 2024 15:20:57 -0800
Subject: [PATCH 1/2] cache session charset

---
 sql/base_session.go | 20 ++++++++++++--------
 1 file changed, 12 insertions(+), 8 deletions(-)

diff --git a/sql/base_session.go b/sql/base_session.go
index e08e642325..b6ef2b648f 100644
--- a/sql/base_session.go
+++ b/sql/base_session.go
@@ -47,6 +47,7 @@ type BaseSession struct {
 	lastQueryInfo    map[string]*atomic.Value
 	tx               Transaction
 	ignoreAutocommit bool
+	charset          CharacterSetID
 
 	// When the MySQL database updates any tables related to privileges, it increments its counter. We then update our
 	// privilege set if our counter doesn't equal the database's counter.
@@ -263,15 +264,18 @@ func (s *BaseSession) GetCharacterSet() CharacterSetID {
 
 // GetCharacterSetResults returns the result character set for this session (defined by the system variable `character_set_results`).
 func (s *BaseSession) GetCharacterSetResults() CharacterSetID {
-	sysVar, _ := s.systemVars[characterSetResultsSysVarName]
-	if sysVar.Val == nil {
-		return CharacterSet_Unspecified
-	}
-	charSet, err := ParseCharacterSet(sysVar.Val.(string))
-	if err != nil {
-		panic(err) // shouldn't happen
+	if s.charset == CharacterSet_Unspecified {
+		sysVar, _ := s.systemVars[characterSetResultsSysVarName]
+		if sysVar.Val == nil {
+			return CharacterSet_Unspecified
+		}
+		var err error
+		s.charset, err = ParseCharacterSet(sysVar.Val.(string))
+		if err != nil {
+			panic(err) // shouldn't happen
+		}
 	}
-	return charSet
+	return s.charset
 }
 
 // GetCollation returns the collation for this session (defined by the system variable `collation_connection`).

From fb510d95a7421e53579f3c797b75df9f44eb6f06 Mon Sep 17 00:00:00 2001
From: Max Hoffman <maximilian.wolfgang1@gmail.com>
Date: Thu, 19 Dec 2024 09:36:14 -0800
Subject: [PATCH 2/2] add cache invalidation

---
 sql/base_session.go | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/sql/base_session.go b/sql/base_session.go
index b6ef2b648f..d5998dc6fe 100644
--- a/sql/base_session.go
+++ b/sql/base_session.go
@@ -178,6 +178,9 @@ func (s *BaseSession) setSessVar(ctx *Context, sysVar SystemVariable, value inte
 	}
 	sysVarName := strings.ToLower(sysVar.GetName())
 	s.systemVars[sysVarName] = svv
+	if sysVarName == characterSetResultsSysVarName {
+		s.charset = CharacterSet_Unspecified
+	}
 	return nil
 }