fix count distinct with decimals

enginetest/queries/queries.go

@@ -8892,6 +8892,18 @@ from typestable`,
 			{"abc"},
 		},
 	},
+	{
+		Query: "select count(distinct cast(i as decimal)) from mytable;",
+		Expected: []sql.Row{
+			{3},
+		},
+	},
+	{
+		Query: "select count(distinct null);",
+		Expected: []sql.Row{
+			{0},
+		},
+	},
 }
 
 var KeylessQueries = []QueryTest{

enginetest/queries/script_queries.go

@@ -5167,6 +5167,27 @@ CREATE TABLE tab3 (
 			},
 		},
 	},
+	{
+		Name: "count distinct decimals",
+		SetUpScript: []string{
+			"create table t (i int, j int)",
+			"insert into t values (1, 11), (11, 1)",
+		},
+		Assertions: []ScriptTestAssertion{
+			{
+				Query: "select count(distinct i, j) from t;",
+				Expected: []sql.Row{
+					{2},
+				},
+			},
+			{
+				Query: "select count(distinct cast(i as decimal), cast(j as decimal)) from t;",
+				Expected: []sql.Row{
+					{2},
+				},
+			},
+		},
+	},
 }
 
 var SpatialScriptTests = []ScriptTest{

sql/expression/function/aggregation/count_test.go

@@ -100,7 +100,7 @@ func TestCountDistinctEvalStar(t *testing.T) {
 	require.NoError(b.Update(ctx, sql.NewRow(1)))
 	require.NoError(b.Update(ctx, sql.NewRow(nil)))
 	require.NoError(b.Update(ctx, sql.NewRow(1, 2, 3)))
-	require.Equal(int64(5), evalBuffer(t, b))
+	require.Equal(int64(4), evalBuffer(t, b))
 }
 
 func TestCountDistinctEvalString(t *testing.T) {

sql/expression/function/aggregation/unary_agg_buffers.go

@@ -4,7 +4,7 @@ import (
 	"fmt"
 	"reflect"
 
-	"github.com/mitchellh/hashstructure"
+	"github.com/cespare/xxhash/v2"
 	"github.com/shopspring/decimal"
 
 	"github.com/dolthub/go-mysql-server/sql"
@@ -402,7 +402,7 @@ func (c *countDistinctBuffer) Update(ctx *sql.Context, row sql.Row) error {
 	if _, ok := c.exprs[0].(*expression.Star); ok {
 		value = row
 	} else {
-		val := make([]interface{}, len(c.exprs))
+		val := make(sql.Row, len(c.exprs))
 		for i, expr := range c.exprs {
 			v, err := expr.Eval(ctx, row)
 			if err != nil {
@@ -417,12 +417,30 @@ func (c *countDistinctBuffer) Update(ctx *sql.Context, row sql.Row) error {
 		value = val
 	}
 
-	hash, err := hashstructure.Hash(value, nil)
-	if err != nil {
-		return fmt.Errorf("count distinct unable to hash value: %s", err)
+	var str string
+	for _, val := range value.(sql.Row) {
+		// skip nil values
+		if val == nil {
+			return nil
+		}
+		v, _, err := types.Text.Convert(val)
+		if err != nil {
+			return err
+		}
+		vv, ok := v.(string)
+		if !ok {
+			return fmt.Errorf("count distinct unable to hash value: %s", err)
+		}
+		str += vv + ","
 	}
 
-	c.seen[hash] = struct{}{}
+	hash := xxhash.New()
+	_, err := hash.WriteString(str)
+	if err != nil {
+		return err
+	}
+	h := hash.Sum64()
+	c.seen[h] = struct{}{}
 
 	return nil
 }