projection: Return correct collation information (#15801)

Signed-off-by: Dirkjan Bussink <d.bussink@gmail.com>

go/vt/vtgate/engine/projection.go

@@ -21,6 +21,7 @@ import (
 	"sync"
 
 	"vitess.io/vitess/go/mysql"
+	"vitess.io/vitess/go/mysql/collations"
 	"vitess.io/vitess/go/sqltypes"
 	querypb "vitess.io/vitess/go/vt/proto/query"
 	"vitess.io/vitess/go/vt/sqlparser"
@@ -75,7 +76,7 @@ func (p *Projection) TryExecute(ctx context.Context, vcursor VCursor, bindVars m
 		resultRows = append(resultRows, resultRow)
 	}
 	if wantfields {
-		result.Fields, err = p.evalFields(env, result.Fields)
+		result.Fields, err = p.evalFields(env, result.Fields, vcursor.ConnCollation())
 		if err != nil {
 			return nil, err
 		}
@@ -96,7 +97,7 @@ func (p *Projection) TryStreamExecute(ctx context.Context, vcursor VCursor, bind
 		defer mu.Unlock()
 		if wantfields {
 			once.Do(func() {
-				fields, err = p.evalFields(env, qr.Fields)
+				fields, err = p.evalFields(env, qr.Fields, vcursor.ConnCollation())
 				if err != nil {
 					return
 				}
@@ -135,14 +136,14 @@ func (p *Projection) GetFields(ctx context.Context, vcursor VCursor, bindVars ma
 		return nil, err
 	}
 	env := evalengine.NewExpressionEnv(ctx, bindVars, vcursor)
-	qr.Fields, err = p.evalFields(env, qr.Fields)
+	qr.Fields, err = p.evalFields(env, qr.Fields, vcursor.ConnCollation())
 	if err != nil {
 		return nil, err
 	}
 	return qr, nil
 }
 
-func (p *Projection) evalFields(env *evalengine.ExpressionEnv, infields []*querypb.Field) ([]*querypb.Field, error) {
+func (p *Projection) evalFields(env *evalengine.ExpressionEnv, infields []*querypb.Field, coll collations.ID) ([]*querypb.Field, error) {
 	// TODO: once the evalengine becomes smart enough, we should be able to remove the
 	// dependency on these fields altogether
 	env.Fields = infields
@@ -157,10 +158,15 @@ func (p *Projection) evalFields(env *evalengine.ExpressionEnv, infields []*query
 		if !sqltypes.IsNull(typ.Type()) && !typ.Nullable() {
 			fl |= uint32(querypb.MySqlFlag_NOT_NULL_FLAG)
 		}
+		typCol := typ.Collation()
+		if sqltypes.IsTextOrBinary(typ.Type()) && typCol != collations.CollationBinaryID {
+			typCol = coll
+		}
+
 		fields = append(fields, &querypb.Field{
 			Name:         col,
 			Type:         typ.Type(),
-			Charset:      uint32(typ.Collation()),
+			Charset:      uint32(typCol),
 			ColumnLength: uint32(typ.Size()),
 			Decimals:     uint32(typ.Scale()),
 			Flags:        fl,

go/vt/vtgate/engine/projection_test.go

@@ -228,3 +228,41 @@ func TestFields(t *testing.T) {
 		})
 	}
 }
+
+func TestFieldConversion(t *testing.T) {
+	var testCases = []struct {
+		name      string
+		expr      string
+		typ       querypb.Type
+		collation collations.ID
+	}{
+		{
+			name:      `convert different charset`,
+			expr:      `_latin1 0xFF`,
+			typ:       sqltypes.VarChar,
+			collation: collations.MySQL8().DefaultConnectionCharset(),
+		},
+	}
+
+	for _, testCase := range testCases {
+		t.Run(testCase.name, func(t *testing.T) {
+			arg, err := sqlparser.NewTestParser().ParseExpr(testCase.expr)
+			require.NoError(t, err)
+			bindExpr, err := evalengine.Translate(arg, &evalengine.Config{
+				Environment: vtenv.NewTestEnv(),
+				Collation:   collations.MySQL8().DefaultConnectionCharset(),
+			})
+			require.NoError(t, err)
+			proj := &Projection{
+				Cols:       []string{"col"},
+				Exprs:      []evalengine.Expr{bindExpr},
+				Input:      &SingleRow{},
+				noTxNeeded: noTxNeeded{},
+			}
+			qr, err := proj.TryExecute(context.Background(), &noopVCursor{}, nil, true)
+			require.NoError(t, err)
+			assert.Equal(t, testCase.typ, qr.Fields[0].Type)
+			assert.Equal(t, testCase.collation, collations.ID(qr.Fields[0].Charset))
+		})
+	}
+}