fix: classify SELECT DATABASE() as non-pure data query in isPureDataQ…

…uery

executor.go

@@ -264,6 +264,8 @@ func isPureDataQuery(n sql.Node) bool {
 		switch tn.Database().Name() {
 		case "mysql", "information_schema", "performance_schema", "sys":
 			return false
+		case "":
+			return false
 		}
 	}
 	return true

executor_test.go

@@ -6,9 +6,12 @@ import (
 	"testing"
 
 	"github.com/DATA-DOG/go-sqlmock"
+	"github.com/dolthub/go-mysql-server/enginetest"
+	"github.com/dolthub/go-mysql-server/enginetest/scriptgen/setup"
 	"github.com/dolthub/go-mysql-server/memory"
 	"github.com/dolthub/go-mysql-server/sql"
 	"github.com/dolthub/go-mysql-server/sql/plan"
+	"github.com/dolthub/go-mysql-server/sql/planbuilder"
 	"github.com/dolthub/go-mysql-server/sql/types"
 	"github.com/dolthub/vitess/go/sqltypes"
 	"github.com/stretchr/testify/assert"
@@ -98,3 +101,56 @@ func TestDuckBuilder_Select(t *testing.T) {
 
 	assert.NoError(t, mock.ExpectationsWereMet())
 }
+
+func TestIsPureDataQuery(t *testing.T) {
+	harness := NewDefaultDuckHarness()
+	harness.Setup(
+		setup.MydbData,
+		[]setup.SetupScript{
+			{
+				"CREATE TABLE users (id INT PRIMARY KEY, name VARCHAR(255))",
+			},
+		})
+	ctx := enginetest.NewContext(harness)
+	engine, err := harness.NewEngine(t)
+	require.NoError(t, err)
+	tests := []struct {
+		name     string
+		query    string
+		expected bool
+	}{
+		{
+			name:     "Simple SELECT query",
+			query:    "SELECT * FROM users",
+			expected: true,
+		},
+		{
+			name:     "Query from mysql system table",
+			query:    "SELECT * FROM mysql.user",
+			expected: false,
+		},
+		{
+			name:     "Query with system function",
+			query:    "SELECT DATABASE()",
+			expected: false,
+		},
+		// {
+		// 	name:     "Query with subquery from system table",
+		// 	query:    "SELECT u.name, (SELECT COUNT(*) FROM mysql.user) FROM users u",
+		// 	expected: false,
+		// },
+		{
+			name:     "Query from information_schema",
+			query:    "SELECT * FROM information_schema.tables",
+			expected: false,
+		},
+	}
+	for _, tt := range tests {
+		parsed, _, err := planbuilder.Parse(ctx, engine.EngineAnalyzer().Catalog, tt.query)
+		require.NoError(t, err)
+
+		result := isPureDataQuery(parsed)
+		assert.Equal(t, tt.expected, result, "isPureDataQuery() for query '%s'", tt.query)
+	}
+
+}