diff --git a/planner.go b/planner.go
index 4b080d279..9fd141fc8 100644
--- a/planner.go
+++ b/planner.go
@@ -42,6 +42,10 @@ func (p *Planner) PlanStatement(ctx context.Context, stmt sql2.Statement) (*Stmt
 }
 
 func (p *Planner) planStatement(ctx context.Context, stmt sql2.Statement) (StmtNode, error) {
+	if err := p.checkStatement(stmt); err != nil {
+		return nil, err
+	}
+
 	switch stmt := stmt.(type) {
 	case *sql2.SelectStatement:
 		return p.planSelectStatement(ctx, stmt)
@@ -58,21 +62,10 @@ func (p *Planner) planSelectStatement(ctx context.Context, stmt *sql2.SelectStat
 }
 
 func (p *Planner) planAggregateSelectStatement(ctx context.Context, stmt *sql2.SelectStatement) (_ StmtNode, err error) {
-	// Extract table name from source.
-	var source *sql2.QualifiedTableName
-	switch src := stmt.Source.(type) {
-	case *sql2.JoinClause:
-		return nil, fmt.Errorf("cannot use JOIN in aggregate query")
-	case *sql2.ParenSource:
-		return nil, fmt.Errorf("cannot use parenthesized source in aggregate query")
-	case *sql2.QualifiedTableName:
-		source = src
-	case *sql2.SelectStatement:
-		return nil, fmt.Errorf("cannot use sub-select in aggregate query")
-	default:
-		return nil, fmt.Errorf("unexpected source type in aggregate query: %T", source)
+	indexName, err := statementTableName(stmt)
+	if err != nil {
+		return nil, err
 	}
-	indexName := sql2.IdentName(source.Name)
 
 	// Convert WHERE clause.
 	cond, err := p.planExprPQL(ctx, stmt, stmt.WhereExpr)
@@ -83,16 +76,18 @@ func (p *Planner) planAggregateSelectStatement(ctx context.Context, stmt *sql2.S
 	// Extract calls and grouped expressions from column list.
 	// TODO: Recursively traverse all expression trees.
 	var calls []*sql2.Call
-	var aliases []string
-	// var groupByCols []*sql2.Ident // TODO: Convert to QualifiedRef
+	var columns []*StmtColumn
 	for _, c := range stmt.Columns {
-		aliases = append(aliases, c.Name())
+		columns = append(columns, &StmtColumn{
+			Name: c.Name(),
+			Type: sql2.ExprDataType(c.Expr),
+		})
 
 		switch c := c.Expr.(type) {
 		case *sql2.Call:
 			calls = append(calls, c)
-		case *sql2.Ident:
-			// groupByCols = append(groupByCols, c)
+		case *sql2.QualifiedRef:
+			// allowed
 		default:
 			return nil, fmt.Errorf("unsupported expression type in aggregate query: %T", c)
 		}
@@ -107,8 +102,8 @@ func (p *Planner) planAggregateSelectStatement(ctx context.Context, stmt *sql2.S
 	var groupByColNames []string
 	for _, expr := range stmt.GroupByExprs {
 		switch expr := expr.(type) {
-		case *sql2.Ident:
-			groupByColNames = append(groupByColNames, expr.Name)
+		case *sql2.QualifiedRef:
+			groupByColNames = append(groupByColNames, expr.Column.Name)
 		default:
 			return nil, fmt.Errorf("unsupported expression type in GROUP BY clause: %T", expr)
 		}
@@ -119,7 +114,7 @@ func (p *Planner) planAggregateSelectStatement(ctx context.Context, stmt *sql2.S
 	switch callName {
 	case "COUNT":
 		if len(groupByColNames) == 0 {
-			return NewCountNode(p.executor, indexName, aliases[0], cond), nil
+			return NewCountNode(p.executor, indexName, columns[0], cond), nil
 		}
 
 		var aggregate *pql.Call
@@ -127,7 +122,7 @@ func (p *Planner) planAggregateSelectStatement(ctx context.Context, stmt *sql2.S
 			if len(calls[0].Args) != 1 {
 				return nil, fmt.Errorf("distinct count must have exactly one field specified")
 			}
-			field, ok := calls[0].Args[0].(*sql2.Ident)
+			ref, ok := calls[0].Args[0].(*sql2.QualifiedRef)
 			if !ok {
 				return nil, fmt.Errorf("distinct count argument must be a field name")
 			}
@@ -136,28 +131,28 @@ func (p *Planner) planAggregateSelectStatement(ctx context.Context, stmt *sql2.S
 				Name: "Count",
 				Children: []*pql.Call{{
 					Name: "Distinct",
-					Args: map[string]interface{}{"field": field.Name},
+					Args: map[string]interface{}{"field": ref.Column.Name},
 				}},
 			}
 		}
 
-		return NewGroupByNode(p.executor, indexName, groupByColNames, aliases, aggregate, cond), nil
+		return NewGroupByNode(p.executor, indexName, groupByColNames, columns, aggregate, cond), nil
 
 	case "SUM":
 		if len(calls[0].Args) != 1 {
 			return nil, fmt.Errorf("sum must have exactly one field specified")
 		}
-		field, ok := calls[0].Args[0].(*sql2.Ident)
+		ref, ok := calls[0].Args[0].(*sql2.QualifiedRef)
 		if !ok {
 			return nil, fmt.Errorf("sum argument must be a field name")
 		}
 
 		aggregate := &pql.Call{
 			Name: "Sum",
-			Args: map[string]interface{}{"field": field.Name},
+			Args: map[string]interface{}{"field": ref.Column.Name},
 		}
 
-		return NewGroupByNode(p.executor, indexName, groupByColNames, aliases, aggregate, cond), nil
+		return NewGroupByNode(p.executor, indexName, groupByColNames, columns, aggregate, cond), nil
 
 	default:
 		return nil, fmt.Errorf("unsupported call in aggregate query: %s", callName)
@@ -167,21 +162,10 @@ func (p *Planner) planAggregateSelectStatement(ctx context.Context, stmt *sql2.S
 }
 
 func (p *Planner) planNonAggregateSelectStatement(ctx context.Context, stmt *sql2.SelectStatement) (_ StmtNode, err error) {
-	// Extract table name from source.
-	var source *sql2.QualifiedTableName
-	switch src := stmt.Source.(type) {
-	case *sql2.JoinClause:
-		return nil, fmt.Errorf("cannot use JOIN in non-aggregate query")
-	case *sql2.ParenSource:
-		return nil, fmt.Errorf("cannot use parenthesized source in non-aggregate query")
-	case *sql2.QualifiedTableName:
-		source = src
-	case *sql2.SelectStatement:
-		return nil, fmt.Errorf("cannot use sub-select in non-aggregate query")
-	default:
-		return nil, fmt.Errorf("unexpected source type in non-aggregate query: %T", source)
+	indexName, err := statementTableName(stmt)
+	if err != nil {
+		return nil, err
 	}
-	indexName := sql2.IdentName(source.Name)
 
 	// Lookup index.
 	idx := p.executor.Holder.Index(indexName)
@@ -196,57 +180,24 @@ func (p *Planner) planNonAggregateSelectStatement(ctx context.Context, stmt *sql
 	}
 
 	// Build column list.
-	var columnNames, columnAliases []string
+	var srcs []string
+	var columns []*StmtColumn
 	for _, col := range stmt.Columns {
-		// Unqualified wildcard.
-		if col.Star.IsValid() {
-			columnNames = append(columnNames, "_id")
-			columnAliases = append(columnAliases, "_id")
-
-			for _, field := range idx.Fields() {
-				if field.Name() == "_exists" {
-					continue
-				}
-				columnNames = append(columnNames, field.Name())
-				columnAliases = append(columnAliases, field.Name())
-			}
-			continue
-		}
-
 		// Handle expressions and qualified references.
 		switch expr := col.Expr.(type) {
-		case *sql2.Ident:
-			columnNames = append(columnNames, expr.Name)
-			columnAliases = append(columnAliases, col.Name())
-
 		case *sql2.QualifiedRef:
-			if tbl := sql2.IdentName(expr.Table); tbl != "" && tbl != source.TableName() {
-				return nil, fmt.Errorf("no such table: %q", tbl)
-			}
-
-			if expr.Star.IsValid() {
-				columnNames = append(columnNames, "_id")
-				columnAliases = append(columnAliases, "_id")
-
-				for _, field := range idx.Fields() {
-					if field.Name() == "_exists" {
-						continue
-					}
-					columnNames = append(columnNames, field.Name())
-					columnAliases = append(columnAliases, field.Name())
-				}
-
-			} else {
-				columnNames = append(columnNames, sql2.IdentName(expr.Column))
-				columnAliases = append(columnAliases, sql2.IdentName(expr.Column))
-			}
+			srcs = append(srcs, sql2.IdentName(expr.Column))
+			columns = append(columns, &StmtColumn{
+				Name: sql2.IdentName(expr.Column),
+				Type: sql2.ExprDataType(col.Expr),
+			})
 
 		default:
 			return nil, fmt.Errorf("unsupported column expression: %T", expr)
 		}
 	}
 
-	return NewExtractNode(p.executor, indexName, columnNames, columnAliases, cond), nil
+	return NewExtractNode(p.executor, indexName, srcs, columns, cond), nil
 }
 
 // planExprPQL returns a PQL call tree for a given expression.
@@ -322,8 +273,8 @@ func (p *Planner) planBinaryExprPQL(ctx context.Context, stmt *sql2.SelectStatem
 	case sql2.EQ, sql2.NE, sql2.LT, sql2.LE, sql2.GT, sql2.GE:
 		// Ensure field reference exists in binary expression.
 		x, y := expr.X, expr.Y
-		xIdent, xOk := x.(*sql2.Ident)
-		yIdent, yOk := y.(*sql2.Ident)
+		xRef, xOk := x.(*sql2.QualifiedRef)
+		yRef, yOk := y.(*sql2.QualifiedRef)
 		if xOk && yOk {
 			return nil, fmt.Errorf("cannot compare fields in a WHERE clause")
 		} else if !xOk && !yOk {
@@ -332,7 +283,7 @@ func (p *Planner) planBinaryExprPQL(ctx context.Context, stmt *sql2.SelectStatem
 
 		// Rewrite expression so field ref is LHS.
 		if !xOk && yOk {
-			xIdent, y = yIdent, x
+			xRef, y = yRef, x
 			switch op {
 			case sql2.LT:
 				op = sql2.GT
@@ -355,7 +306,7 @@ func (p *Planner) planBinaryExprPQL(ctx context.Context, stmt *sql2.SelectStatem
 			return &pql.Call{
 				Name: "Row",
 				Args: map[string]interface{}{
-					sql2.IdentName(xIdent): pqlValue,
+					sql2.IdentName(xRef.Column): pqlValue,
 				},
 			}, nil
 		}
@@ -367,7 +318,7 @@ func (p *Planner) planBinaryExprPQL(ctx context.Context, stmt *sql2.SelectStatem
 		return &pql.Call{
 			Name: "Row",
 			Args: map[string]interface{}{
-				sql2.IdentName(xIdent): &pql.Condition{
+				sql2.IdentName(xRef.Column): &pql.Condition{
 					Op:    pqlOp,
 					Value: pqlValue,
 				},
@@ -426,6 +377,221 @@ func sqlToPQLValue(expr sql2.Expr) (interface{}, error) {
 	}
 }
 
+func (p *Planner) checkStatement(stmt sql2.Statement) error {
+	switch stmt := stmt.(type) {
+	case *sql2.SelectStatement:
+		return p.checkSelectStatement(stmt)
+	default:
+		return nil
+	}
+}
+
+func (p *Planner) checkSelectStatement(stmt *sql2.SelectStatement) error {
+	indexName, err := statementTableName(stmt)
+	if err != nil {
+		return err
+	}
+
+	// Look up index.
+	idx := p.executor.Holder.Index(indexName)
+	if idx == nil {
+		return newNotFoundError(ErrIndexNotFound, indexName)
+	}
+
+	// Replace wildcards with column references.
+	columns := make([]*sql2.ResultColumn, 0, len(stmt.Columns))
+	for _, col := range stmt.Columns {
+		// Unqualified wildcard.
+		isWildcard := col.Star.IsValid()
+		if ref, ok := col.Expr.(*sql2.QualifiedRef); ok && ref.Star.IsValid() {
+			if ref.Table.Name != indexName {
+				return fmt.Errorf("no such table: %q", ref.Table.Name)
+			}
+			isWildcard = true
+		}
+
+		// Simply add column as-is if it is not a wildcard.
+		if !isWildcard {
+			columns = append(columns, col)
+			continue
+		}
+
+		// Add identifier field first.
+		columns = append(columns, &sql2.ResultColumn{
+			Expr: &sql2.QualifiedRef{
+				Table:  &sql2.Ident{Name: idx.Name()},
+				Column: &sql2.Ident{Name: "_id"},
+			},
+		})
+
+		// Then add all fields besides the existence bit.
+		for _, field := range idx.Fields() {
+			if field.Name() == "_exists" {
+				continue
+			}
+			columns = append(columns, &sql2.ResultColumn{
+				Expr: &sql2.QualifiedRef{
+					Table:  &sql2.Ident{Name: idx.Name()},
+					Column: &sql2.Ident{Name: field.Name()},
+				},
+			})
+		}
+	}
+	stmt.Columns = columns
+
+	// Type check expressions in statement.
+	for _, col := range stmt.Columns {
+		if err := p.checkExpr(&col.Expr, stmt); err != nil {
+			return err
+		}
+	}
+
+	if err := p.checkExpr(&stmt.WhereExpr, stmt); err != nil {
+		return err
+	}
+
+	for i := range stmt.GroupByExprs {
+		if err := p.checkExpr(&stmt.GroupByExprs[i], stmt); err != nil {
+			return err
+		}
+	}
+
+	if err := p.checkExpr(&stmt.HavingExpr, stmt); err != nil {
+		return err
+	}
+
+	for _, term := range stmt.OrderingTerms {
+		if err := p.checkExpr(&term.X, stmt); err != nil {
+			return err
+		}
+	}
+
+	if err := p.checkExpr(&stmt.LimitExpr, stmt); err != nil {
+		return err
+	}
+
+	if err := p.checkExpr(&stmt.OffsetExpr, stmt); err != nil {
+		return err
+	}
+
+	return nil
+}
+
+func (p *Planner) checkExpr(expr *sql2.Expr, stmt sql2.Statement) error {
+	if e, err := sql2.Walk(&sqlExprTypeChecker{
+		holder: p.executor.Holder,
+		stmt:   stmt,
+	}, *expr); err != nil {
+		return err
+	} else if e != nil {
+		*expr = e.(sql2.Expr)
+	} else {
+		*expr = nil
+	}
+	return nil
+}
+
+// sqlExprTypeChecker recursively performs type checking within an expression.
+// Called by sqlTypeChecker. Implements sql2.Visitor.
+type sqlExprTypeChecker struct {
+	holder *Holder
+	stmt   sql2.Statement // scope
+}
+
+var _ sql2.Visitor = (*sqlExprTypeChecker)(nil)
+
+func (v *sqlExprTypeChecker) Visit(node sql2.Node) (_ sql2.Visitor, _ sql2.Node, err error) {
+	switch n := node.(type) {
+	case *sql2.Call:
+		for i := range n.Args {
+			if err := v.checkExpr(&n.Args[i]); err != nil {
+				return nil, nil, err
+			}
+		}
+		return nil, node, nil // skip
+	case *sql2.Ident:
+		if node, err = v.visitIdent(n); err != nil {
+			return nil, nil, err
+		}
+		return nil, node, nil
+	case *sql2.QualifiedRef:
+		if node, err = v.visitQualifiedRef(n); err != nil {
+			return nil, nil, err
+		}
+		return nil, node, nil
+	default:
+		return v, node, nil
+	}
+}
+
+func (v *sqlExprTypeChecker) visitIdent(ident *sql2.Ident) (sql2.Node, error) {
+	indexName, err := statementTableName(v.stmt)
+	if err != nil {
+		return nil, err
+	}
+
+	// Convert to a table qualified reference and validate through ref visit function.
+	return v.visitQualifiedRef(&sql2.QualifiedRef{
+		Table:  &sql2.Ident{Name: indexName},
+		Column: &sql2.Ident{Name: ident.Name},
+	})
+}
+
+func (v *sqlExprTypeChecker) visitQualifiedRef(ref *sql2.QualifiedRef) (sql2.Node, error) {
+	idx := v.holder.Index(ref.Table.Name)
+	if idx == nil {
+		return nil, newNotFoundError(ErrIndexNotFound, ref.Table.Name)
+	}
+
+	switch name := ref.Column.Name; name {
+	case "_id":
+		ref.DataType = sql2.DataTypeInt
+	default:
+		field := idx.Field(ref.Column.Name)
+		if field == nil {
+			return nil, newNotFoundError(ErrFieldNotFound, ref.Column.Name)
+		}
+		ref.DataType = fieldSQLDataType(field)
+	}
+
+	return ref, nil
+}
+
+func (v *sqlExprTypeChecker) checkExpr(node *sql2.Expr) error {
+	if expr, err := sql2.Walk(&sqlExprTypeChecker{
+		holder: v.holder,
+		stmt:   v.stmt,
+	}, *node); err != nil {
+		return err
+	} else if expr != nil {
+		*node = expr.(sql2.Expr)
+	} else {
+		*node = nil
+	}
+	return nil
+}
+
+func (v *sqlExprTypeChecker) VisitEnd(node sql2.Node) (sql2.Node, error) { return node, nil }
+
+func fieldSQLDataType(f *Field) string {
+	if f.Keys() {
+		return sql2.DataTypeText
+	}
+
+	switch f.Type() {
+	case FieldTypeInt, FieldTypeMutex, FieldTypeSet:
+		return sql2.DataTypeInt
+	case FieldTypeBool:
+		return sql2.DataTypeBool
+	case FieldTypeDecimal:
+		return sql2.DataTypeDecimal
+	case FieldTypeTime, FieldTypeTimestamp:
+		return sql2.DataTypeTimestamp
+	default:
+		return ""
+	}
+}
+
 type Stmt struct {
 	node StmtNode
 }
@@ -473,7 +639,7 @@ func (rs *StmtRows) Err() error {
 	return nil
 }
 
-func (rs *StmtRows) Columns() []string {
+func (rs *StmtRows) Columns() []*StmtColumn {
 	return rs.node.Columns()
 }
 
@@ -574,6 +740,11 @@ func (r *StmtRow) Err() error {
 	return r.err
 }
 
+type StmtColumn struct {
+	Name string
+	Type string
+}
+
 type StmtNode interface {
 	// Initializes the node to its start.
 	First(ctx context.Context) error
@@ -585,7 +756,7 @@ type StmtNode interface {
 	Row() []interface{}
 
 	// Returns column definitions for the node.
-	Columns() []string
+	Columns() []*StmtColumn
 
 	// Returns a reference to the value register for a named column.
 	// Lookup(table, column string) (interface{}, error)
@@ -597,39 +768,38 @@ var _ StmtNode = (*ExtractNode)(nil)
 type ExtractNode struct {
 	executor  *executor
 	indexName string
-	columns   []string
-	aliases   []string
+	srcs      []string
+	columns   []*StmtColumn
 	cond      *pql.Call
 
 	result []ExtractedTableColumn
 	row    []interface{}
 }
 
-func NewExtractNode(executor *executor, indexName string, columns, aliases []string, cond *pql.Call) *ExtractNode {
+func NewExtractNode(executor *executor, indexName string, srcs []string, columns []*StmtColumn, cond *pql.Call) *ExtractNode {
 	if cond == nil {
 		cond = &pql.Call{Name: "All"}
 	}
 
 	// Ensure ID column is always the first column.
-	if len(columns) > 0 && columns[0] != "_id" {
-		columns = append([]string{"_id"}, columns...)
-		aliases = append([]string{"_id"}, aliases...)
+	// TODO(benbjohnson): Don't require id first.
+	if len(srcs) > 0 && srcs[0] != "_id" {
+		srcs = append([]string{"_id"}, srcs...)
+		columns = append([]*StmtColumn{{Name: "_id", Type: sql2.DataTypeInt}}, columns...)
 	}
 
-	// TODO: Move "_id" column to the first position if it is specified later on in column list.
-
 	return &ExtractNode{
 		executor:  executor,
 		indexName: indexName,
-		columns:   columns, // source column names
-		aliases:   aliases, // external column alias
+		srcs:      srcs,    // source column names
+		columns:   columns, // external column alias
 		cond:      cond,
-		row:       make([]interface{}, len(columns)),
+		row:       make([]interface{}, len(srcs)),
 	}
 }
 
-func (n *ExtractNode) Columns() []string {
-	return n.aliases
+func (n *ExtractNode) Columns() []*StmtColumn {
+	return n.columns
 }
 
 func (n *ExtractNode) First(ctx context.Context) error {
@@ -674,11 +844,11 @@ func (n *ExtractNode) init(ctx context.Context) error {
 	// Generate PQL query with all specified rows.
 	// Skip first column as it is the ID column.
 	call := &pql.Call{Name: "Extract", Children: []*pql.Call{n.cond}}
-	for _, column := range n.columns[1:] {
+	for _, src := range n.srcs[1:] {
 		call.Children = append(call.Children,
 			&pql.Call{
 				Name: "Rows",
-				Args: map[string]interface{}{"field": column},
+				Args: map[string]interface{}{"field": src},
 			},
 		)
 	}
@@ -709,28 +879,28 @@ var _ StmtNode = (*CountNode)(nil)
 
 // CountNode executes a COUNT(*) against a FeatureBase index and returns a single row.
 type CountNode struct {
-	executor   *executor
-	indexName  string
-	columnName string
-	cond       *pql.Call // conditional
+	executor  *executor
+	indexName string
+	column    *StmtColumn
+	cond      *pql.Call // conditional
 
 	row []interface{}
 }
 
-func NewCountNode(executor *executor, indexName string, columnName string, cond *pql.Call) *CountNode {
+func NewCountNode(executor *executor, indexName string, column *StmtColumn, cond *pql.Call) *CountNode {
 	if cond == nil {
 		cond = &pql.Call{Name: "All"}
 	}
 	return &CountNode{
-		executor:   executor,
-		indexName:  indexName,
-		columnName: columnName,
-		cond:       cond,
+		executor:  executor,
+		indexName: indexName,
+		column:    column,
+		cond:      cond,
 	}
 }
 
-func (n *CountNode) Columns() []string {
-	return []string{n.columnName}
+func (n *CountNode) Columns() []*StmtColumn {
+	return []*StmtColumn{n.column}
 }
 
 func (n *CountNode) First(ctx context.Context) error {
@@ -764,8 +934,8 @@ func (n *CountNode) Row() []interface{} { return n.row }
 type GroupByNode struct {
 	executor  *executor
 	indexName string
-	columns   []string
-	aliases   []string
+	srcs      []string
+	columns   []*StmtColumn
 	aggregate *pql.Call
 	cond      *pql.Call
 
@@ -775,20 +945,20 @@ type GroupByNode struct {
 	row []interface{}
 }
 
-func NewGroupByNode(executor *executor, indexName string, columns, aliases []string, aggregate, cond *pql.Call) *GroupByNode {
+func NewGroupByNode(executor *executor, indexName string, srcs []string, columns []*StmtColumn, aggregate, cond *pql.Call) *GroupByNode {
 	return &GroupByNode{
 		executor:  executor,
 		indexName: indexName,
+		srcs:      srcs,
 		columns:   columns,
-		aliases:   aliases,
 		aggregate: aggregate,
 		cond:      cond,
-		row:       make([]interface{}, len(columns)+1),
+		row:       make([]interface{}, len(srcs)+1),
 	}
 }
 
-func (n *GroupByNode) Columns() []string {
-	return append([]string{"_aggregate"}, n.columns...)
+func (n *GroupByNode) Columns() []*StmtColumn {
+	return n.columns
 }
 
 func (n *GroupByNode) First(ctx context.Context) error {
@@ -840,9 +1010,9 @@ func (n *GroupByNode) fetch(ctx context.Context) (*GroupCounts, error) {
 	}
 
 	// Choose fields to group by.
-	for _, col := range n.columns {
+	for _, src := range n.srcs {
 		call.Children = append(call.Children, &pql.Call{
-			Name: "Rows", Args: map[string]interface{}{"_field": col},
+			Name: "Rows", Args: map[string]interface{}{"_field": src},
 		})
 	}
 
@@ -862,3 +1032,30 @@ func (n *GroupByNode) fetch(ctx context.Context) (*GroupCounts, error) {
 }
 
 func (n *GroupByNode) Row() []interface{} { return n.row }
+
+// statementTableName returns the table name for a single table SELECT statement.
+//
+// NOTE: This function is only temporary until we support more source types.
+func statementTableName(stmt sql2.Statement) (string, error) {
+	switch stmt := stmt.(type) {
+	case *sql2.SelectStatement:
+		return sourceTableName(stmt.Source)
+	default:
+		return "", fmt.Errorf("statement not currently supported")
+	}
+}
+
+func sourceTableName(source sql2.Source) (string, error) {
+	switch source := source.(type) {
+	case *sql2.JoinClause:
+		return "", fmt.Errorf("joins are not currently supported")
+	case *sql2.ParenSource:
+		return "", fmt.Errorf("parenthesized source is not currently supported")
+	case *sql2.QualifiedTableName:
+		return sql2.IdentName(source.Name), nil
+	case *sql2.SelectStatement:
+		return "", fmt.Errorf("sub-selects are not currently supported")
+	default:
+		return "", fmt.Errorf("unexpected source type: %T", source)
+	}
+}
diff --git a/planner_test.go b/planner_test.go
index 4748046b6..e92bd77a6 100644
--- a/planner_test.go
+++ b/planner_test.go
@@ -210,54 +210,80 @@ func TestPlanner_Select(t *testing.T) {
 	}
 
 	t.Run("UnqualifiedColumns", func(t *testing.T) {
-		results := mustQueryRows(t, c.GetNode(0).Server, `SELECT _id, a, b FROM i0`)
-		if diff := cmp.Diff(results, [][]interface{}{
+		results, columns := mustQueryRows(t, c.GetNode(0).Server, `SELECT _id, a, b FROM i0`)
+		if diff := cmp.Diff([][]interface{}{
 			{int64(1), int64(10), int64(100)},
 			{int64(2), int64(20), int64(200)},
-		}); diff != "" {
+		}, results); diff != "" {
+			t.Fatal(diff)
+		}
+
+		if diff := cmp.Diff([]*pilosa.StmtColumn{
+			{Name: "_id", Type: "INT"},
+			{Name: "a", Type: "INT"},
+			{Name: "b", Type: "INT"},
+		}, columns); diff != "" {
 			t.Fatal(diff)
 		}
 	})
 
 	t.Run("QualifiedColumns", func(t *testing.T) {
-		results := mustQueryRows(t, c.GetNode(0).Server, `SELECT i0._id, i0.a, i0.b FROM i0`)
-		if diff := cmp.Diff(results, [][]interface{}{
+		results, columns := mustQueryRows(t, c.GetNode(0).Server, `SELECT i0._id, i0.a, i0.b FROM i0`)
+		if diff := cmp.Diff([][]interface{}{
 			{int64(1), int64(10), int64(100)},
 			{int64(2), int64(20), int64(200)},
-		}); diff != "" {
+		}, results); diff != "" {
+			t.Fatal(diff)
+		}
+
+		if diff := cmp.Diff([]*pilosa.StmtColumn{
+			{Name: "_id", Type: "INT"},
+			{Name: "a", Type: "INT"},
+			{Name: "b", Type: "INT"},
+		}, columns); diff != "" {
 			t.Fatal(diff)
 		}
 	})
 
 	t.Run("UnqualifiedStar", func(t *testing.T) {
-		results := mustQueryRows(t, c.GetNode(0).Server, `SELECT * FROM i0`)
-		if diff := cmp.Diff(results, [][]interface{}{
+		results, columns := mustQueryRows(t, c.GetNode(0).Server, `SELECT * FROM i0`)
+		if diff := cmp.Diff([][]interface{}{
 			{int64(1), int64(10), int64(100)},
 			{int64(2), int64(20), int64(200)},
-		}); diff != "" {
+		}, results); diff != "" {
+			t.Fatal(diff)
+		}
+
+		if diff := cmp.Diff([]*pilosa.StmtColumn{
+			{Name: "_id", Type: "INT"},
+			{Name: "a", Type: "INT"},
+			{Name: "b", Type: "INT"},
+		}, columns); diff != "" {
 			t.Fatal(diff)
 		}
 	})
 
 	t.Run("QualifiedStar", func(t *testing.T) {
-		results := mustQueryRows(t, c.GetNode(0).Server, `SELECT i0.* FROM i0`)
-		if diff := cmp.Diff(results, [][]interface{}{
+		results, columns := mustQueryRows(t, c.GetNode(0).Server, `SELECT i0.* FROM i0`)
+		if diff := cmp.Diff([][]interface{}{
 			{int64(1), int64(10), int64(100)},
 			{int64(2), int64(20), int64(200)},
-		}); diff != "" {
+		}, results); diff != "" {
+			t.Fatal(diff)
+		}
+
+		if diff := cmp.Diff([]*pilosa.StmtColumn{
+			{Name: "_id", Type: "INT"},
+			{Name: "a", Type: "INT"},
+			{Name: "b", Type: "INT"},
+		}, columns); diff != "" {
 			t.Fatal(diff)
 		}
 	})
 
 	t.Run("ErrFieldNotFound", func(t *testing.T) {
-		stmt, err := c.GetNode(0).Server.PlanSQL(context.Background(), `SELECT xyz FROM i0`)
-		if err != nil {
-			t.Fatal(err)
-		}
-		defer stmt.Close()
-
-		var xyz interface{}
-		if err := stmt.QueryRowContext(context.Background()).Scan(&xyz); err == nil || !strings.Contains(err.Error(), `xyz: field not found`) {
+		_, err := c.GetNode(0).Server.PlanSQL(context.Background(), `SELECT xyz FROM i0`)
+		if err == nil || !strings.Contains(err.Error(), `xyz: field not found`) {
 			t.Fatalf("unexpected error: %v", err)
 		}
 	})
@@ -301,37 +327,58 @@ func TestPlanner_GroupBy(t *testing.T) {
 	}
 
 	t.Run("Count", func(t *testing.T) {
-		results := mustQueryRows(t, c.GetNode(0).Server, `SELECT COUNT(*), x FROM i0 GROUP BY x`)
+		results, columns := mustQueryRows(t, c.GetNode(0).Server, `SELECT COUNT(*), x FROM i0 GROUP BY x`)
 		if diff := cmp.Diff([][]interface{}{
 			{int64(2), int64(10)},
 			{int64(2), int64(20)},
 		}, results); diff != "" {
 			t.Fatal(diff)
 		}
+
+		if diff := cmp.Diff([]*pilosa.StmtColumn{
+			{Name: "count", Type: "INT"},
+			{Name: "x", Type: "INT"},
+		}, columns); diff != "" {
+			t.Fatal(diff)
+		}
 	})
 
 	t.Run("DistinctCount", func(t *testing.T) {
-		results := mustQueryRows(t, c.GetNode(0).Server, `SELECT COUNT(DISTINCT z), x FROM i0 GROUP BY x`)
+		results, columns := mustQueryRows(t, c.GetNode(0).Server, `SELECT COUNT(DISTINCT z), x FROM i0 GROUP BY x`)
 		if diff := cmp.Diff([][]interface{}{
 			{int64(1), int64(10)},
 			{int64(2), int64(20)},
 		}, results); diff != "" {
 			t.Fatal(diff)
 		}
+
+		if diff := cmp.Diff([]*pilosa.StmtColumn{
+			{Name: "count", Type: "INT"},
+			{Name: "x", Type: "INT"},
+		}, columns); diff != "" {
+			t.Fatal(diff)
+		}
 	})
 
 	t.Run("Sum", func(t *testing.T) {
-		results := mustQueryRows(t, c.GetNode(0).Server, `SELECT sum(y), x FROM i0 GROUP BY x`)
+		results, columns := mustQueryRows(t, c.GetNode(0).Server, `SELECT sum(y), x FROM i0 GROUP BY x`)
 		if diff := cmp.Diff([][]interface{}{
 			{int64(300), int64(10)},
 			{int64(100), int64(20)},
 		}, results); diff != "" {
 			t.Fatal(diff)
 		}
+
+		if diff := cmp.Diff([]*pilosa.StmtColumn{
+			{Name: "sum", Type: "INT"},
+			{Name: "x", Type: "INT"},
+		}, columns); diff != "" {
+			t.Fatal(diff)
+		}
 	})
 }
 
-func mustQueryRows(tb testing.TB, svr *pilosa.Server, q string) [][]interface{} {
+func mustQueryRows(tb testing.TB, svr *pilosa.Server, q string) (results [][]interface{}, columns []*pilosa.StmtColumn) {
 	tb.Helper()
 
 	stmt, err := svr.PlanSQL(context.Background(), q)
@@ -345,7 +392,7 @@ func mustQueryRows(tb testing.TB, svr *pilosa.Server, q string) [][]interface{}
 		tb.Fatal(err)
 	}
 
-	results := make([][]interface{}, 0)
+	results = make([][]interface{}, 0)
 	for rows.Next() {
 		result := make([]interface{}, len(rows.Columns()))
 
@@ -365,5 +412,5 @@ func mustQueryRows(tb testing.TB, svr *pilosa.Server, q string) [][]interface{}
 		tb.Fatal(err)
 	}
 
-	return results
+	return results, rows.Columns()
 }
diff --git a/sql2/ast.go b/sql2/ast.go
index ce231781f..929d1f945 100644
--- a/sql2/ast.go
+++ b/sql2/ast.go
@@ -194,6 +194,26 @@ func StatementSource(stmt Statement) Source {
 	}
 }
 
+// Data types
+const (
+	DataTypeBool      = "BOOL"
+	DataTypeDecimal   = "DECIMAL"
+	DataTypeInt       = "INT"
+	DataTypeSet       = "SET"
+	DataTypeText      = "TEXT"
+	DataTypeTimestamp = "TIMESTAMP"
+)
+
+// IsDataTypeValid returns true if typ is a valid data type.
+func IsDataTypeValid(typ string) bool {
+	switch typ {
+	case DataTypeBool, DataTypeInt, DataTypeDecimal, DataTypeText:
+		return true
+	default:
+		return false
+	}
+}
+
 type Expr interface {
 	Node
 	expr()
@@ -279,6 +299,49 @@ func cloneExprs(a []Expr) []Expr {
 	return other
 }
 
+// ExprDataType returns the data type for an expression.
+func ExprDataType(expr Expr) string {
+	if expr == nil {
+		return ""
+	}
+
+	switch expr := expr.(type) {
+	// Simple type assertions
+	case *BindExpr, *ExprList, *Ident, *NullLit, *Raise:
+		return ""
+	case *BlobLit, *StringLit:
+		return DataTypeText
+	case *BoolLit, *Exists, *Range:
+		return DataTypeBool
+	case *NumberLit:
+		return DataTypeInt
+
+	// Complex type assertions
+	case *BinaryExpr:
+		return ExprDataType(expr.X)
+	case *Call:
+		return DataTypeInt // TODO: May be different for some aggregations
+	case *CaseExpr:
+		if len(expr.Blocks) > 0 {
+			return ExprDataType(expr.Blocks[0].Body)
+		} else if expr.ElseExpr != nil {
+			return ExprDataType(expr.ElseExpr)
+		}
+		return ""
+	case *CastExpr:
+		return "" // TODO: Inspect expr.Type.Name
+	case *ParenExpr:
+		return ExprDataType(expr.X)
+	case *QualifiedRef:
+		return expr.DataType
+	case *UnaryExpr:
+		return ExprDataType(expr.X)
+
+	default:
+		panic(fmt.Sprintf("invalid expr type: %T", expr))
+	}
+}
+
 // ExprString returns the string representation of expr.
 // Returns a blank string if expr is nil.
 func ExprString(expr Expr) string {
@@ -1811,6 +1874,9 @@ type QualifiedRef struct {
 	Dot    Pos    // position of dot
 	Star   Pos    // position of * (result column only)
 	Column *Ident // column name
+
+	// Set by the planner; not at parse-time
+	DataType string
 }
 
 // IsAggregate returns false.
@@ -3061,10 +3127,12 @@ func (c *ResultColumn) Name() string {
 	}
 
 	switch expr := c.Expr.(type) {
+	case *Call:
+		return strings.ToLower(IdentName(expr.Name))
 	case *Ident:
 		return IdentName(expr)
 	case *QualifiedRef:
-		return expr.String()
+		return IdentName(expr.Column)
 	default:
 		return ""
 	}
diff --git a/sql2/ast_test.go b/sql2/ast_test.go
index 8d3c1fed5..25cf95ac9 100644
--- a/sql2/ast_test.go
+++ b/sql2/ast_test.go
@@ -1143,14 +1143,14 @@ func AssertNodeStringerPanic(tb testing.TB, node sql.Node, msg string) {
 func StripPos(root sql.Node) sql.Node {
 	zero := reflect.ValueOf(sql.Pos{})
 
-	_ = sql.Walk(sql.VisitFunc(func(node sql.Node) error {
+	_, _ = sql.Walk(sql.VisitFunc(func(node sql.Node) (sql.Node, error) {
 		value := reflect.Indirect(reflect.ValueOf(node))
 		for i := 0; i < value.NumField(); i++ {
 			if field := value.Field(i); field.Type() == zero.Type() {
 				field.Set(zero)
 			}
 		}
-		return nil
+		return node, nil
 	}), root)
 	return root
 }
diff --git a/sql2/walk.go b/sql2/walk.go
index e8592a7b6..c04ca76ba 100644
--- a/sql2/walk.go
+++ b/sql2/walk.go
@@ -18,8 +18,8 @@ package sql2
 // If the result visitor w is not nil, Walk visits each of the children
 // of node with the visitor w, followed by a call of w.Visit(nil).
 type Visitor interface {
-	Visit(node Node) (w Visitor, err error)
-	VisitEnd(node Node) error
+	Visit(node Node) (w Visitor, n Node, err error)
+	VisitEnd(node Node) (Node, error)
 }
 
 // Walk traverses an AST in depth-first order: It starts by calling
@@ -27,606 +27,774 @@ type Visitor interface {
 // v.Visit(node) is not nil, Walk is invoked recursively with visitor
 // w for each of the non-nil children of node, followed by a call of
 // w.Visit(nil).
-func Walk(v Visitor, node Node) error {
+func Walk(v Visitor, node Node) (Node, error) {
 	return walk(v, node)
 }
 
-func walk(v Visitor, node Node) (err error) {
+func walk(v Visitor, node Node) (_ Node, err error) {
 	// Visit the node itself
-	if v, err = v.Visit(node); err != nil {
-		return err
+	if v, node, err = v.Visit(node); err != nil {
+		return node, err
 	} else if v == nil {
-		return nil
+		return node, nil
 	}
 
 	// Visit node's children.
 	switch n := node.(type) {
 	case *Assignment:
 		if err := walkIdentList(v, n.Columns); err != nil {
-			return err
+			return node, err
 		}
-		if err := walkExpr(v, n.Expr); err != nil {
-			return err
+		if err := walkExpr(v, &n.Expr); err != nil {
+			return node, err
 		}
 
 	case *ExplainStatement:
 		if n.Stmt != nil {
-			if err := walk(v, n.Stmt); err != nil {
-				return err
+			if stmt, err := walk(v, n.Stmt); err != nil {
+				return node, err
+			} else if stmt == nil {
+				n.Stmt = nil
+			} else {
+				n.Stmt = stmt.(Statement)
 			}
 		}
 
 	case *RollbackStatement:
-		if err := walkIdent(v, n.SavepointName); err != nil {
-			return err
+		if err := walkIdent(v, &n.SavepointName); err != nil {
+			return node, err
 		}
 
 	case *SavepointStatement:
-		if err := walkIdent(v, n.Name); err != nil {
-			return err
+		if err := walkIdent(v, &n.Name); err != nil {
+			return node, err
 		}
 
 	case *ReleaseStatement:
-		if err := walkIdent(v, n.Name); err != nil {
-			return err
+		if err := walkIdent(v, &n.Name); err != nil {
+			return node, err
 		}
 
 	case *CreateTableStatement:
-		if err := walkIdent(v, n.Name); err != nil {
-			return err
+		if err := walkIdent(v, &n.Name); err != nil {
+			return node, err
 		}
 		if err := walkColumnDefinitionList(v, n.Columns); err != nil {
-			return err
+			return node, err
 		}
 		if err := walkConstraintList(v, n.Constraints); err != nil {
-			return err
+			return node, err
 		}
 		if n.Select != nil {
-			if err := walk(v, n.Select); err != nil {
-				return err
+			if sel, err := walk(v, n.Select); err != nil {
+				return node, err
+			} else if sel != nil {
+				n.Select = sel.(*SelectStatement)
+			} else {
+				n.Select = nil
 			}
 		}
 
 	case *AlterTableStatement:
-		if err := walkIdent(v, n.Name); err != nil {
-			return err
+		if err := walkIdent(v, &n.Name); err != nil {
+			return node, err
 		}
-		if err := walkIdent(v, n.NewName); err != nil {
-			return err
+		if err := walkIdent(v, &n.NewName); err != nil {
+			return node, err
 		}
-		if err := walkIdent(v, n.ColumnName); err != nil {
-			return err
+		if err := walkIdent(v, &n.ColumnName); err != nil {
+			return node, err
 		}
-		if err := walkIdent(v, n.NewColumnName); err != nil {
-			return err
+		if err := walkIdent(v, &n.NewColumnName); err != nil {
+			return node, err
 		}
 		if n.ColumnDef != nil {
-			if err := walk(v, n.ColumnDef); err != nil {
-				return err
+			if def, err := walk(v, n.ColumnDef); err != nil {
+				return node, err
+			} else if def != nil {
+				n.ColumnDef = def.(*ColumnDefinition)
+			} else {
+				n.ColumnDef = nil
 			}
 		}
 
 	case *AnalyzeStatement:
-		if err := walkIdent(v, n.Name); err != nil {
-			return err
+		if err := walkIdent(v, &n.Name); err != nil {
+			return node, err
 		}
 
 	case *CreateViewStatement:
-		if err := walkIdent(v, n.Name); err != nil {
-			return err
+		if err := walkIdent(v, &n.Name); err != nil {
+			return node, err
 		}
 		if err := walkIdentList(v, n.Columns); err != nil {
-			return err
+			return node, err
 		}
 		if n.Select != nil {
-			if err := walk(v, n.Select); err != nil {
-				return err
+			if sel, err := walk(v, n.Select); err != nil {
+				return node, err
+			} else if sel != nil {
+				n.Select = sel.(*SelectStatement)
+			} else {
+				n.Select = nil
 			}
 		}
 
 	case *DropTableStatement:
-		if err := walkIdent(v, n.Name); err != nil {
-			return err
+		if err := walkIdent(v, &n.Name); err != nil {
+			return node, err
 		}
 
 	case *DropViewStatement:
-		if err := walkIdent(v, n.Name); err != nil {
-			return err
+		if err := walkIdent(v, &n.Name); err != nil {
+			return node, err
 		}
 
 	case *DropIndexStatement:
-		if err := walkIdent(v, n.Name); err != nil {
-			return err
+		if err := walkIdent(v, &n.Name); err != nil {
+			return node, err
 		}
 
 	case *DropTriggerStatement:
-		if err := walkIdent(v, n.Name); err != nil {
-			return err
+		if err := walkIdent(v, &n.Name); err != nil {
+			return node, err
 		}
 
 	case *CreateIndexStatement:
-		if err := walkIdent(v, n.Name); err != nil {
-			return err
+		if err := walkIdent(v, &n.Name); err != nil {
+			return node, err
 		}
-		if err := walkIdent(v, n.Table); err != nil {
-			return err
+		if err := walkIdent(v, &n.Table); err != nil {
+			return node, err
 		}
 		if err := walkIndexedColumnList(v, n.Columns); err != nil {
-			return err
+			return node, err
 		}
-		if err := walkExpr(v, n.WhereExpr); err != nil {
-			return err
+		if err := walkExpr(v, &n.WhereExpr); err != nil {
+			return node, err
 		}
 
 	case *CreateTriggerStatement:
-		if err := walkIdent(v, n.Name); err != nil {
-			return err
+		if err := walkIdent(v, &n.Name); err != nil {
+			return node, err
 		}
 		if err := walkIdentList(v, n.UpdateOfColumns); err != nil {
-			return err
+			return node, err
 		}
-		if err := walkIdent(v, n.Table); err != nil {
-			return err
+		if err := walkIdent(v, &n.Table); err != nil {
+			return node, err
 		}
-		if err := walkExpr(v, n.WhenExpr); err != nil {
-			return err
+		if err := walkExpr(v, &n.WhenExpr); err != nil {
+			return node, err
 		}
-		for _, x := range n.Body {
-			if err := walk(v, x); err != nil {
-				return err
+		for i := range n.Body {
+			if body, err := walk(v, n.Body[i]); err != nil {
+				return node, err
+			} else if body != nil {
+				n.Body[i] = body.(Statement)
+			} else {
+				n.Body[i] = nil
 			}
 		}
 
 	case *SelectStatement:
 		if n.WithClause != nil {
-			if err := walk(v, n.WithClause); err != nil {
-				return err
+			if clause, err := walk(v, n.WithClause); err != nil {
+				return node, err
+			} else if clause != nil {
+				n.WithClause = clause.(*WithClause)
+			} else {
+				n.WithClause = nil
 			}
 		}
-		for _, x := range n.ValueLists {
-			if err := walk(v, x); err != nil {
-				return err
+		for i := range n.ValueLists {
+			if list, err := walk(v, n.ValueLists[i]); err != nil {
+				return node, err
+			} else if list != nil {
+				n.ValueLists[i] = list.(*ExprList)
+			} else {
+				n.ValueLists[i] = nil
 			}
 		}
-		for _, x := range n.Columns {
-			if err := walk(v, x); err != nil {
-				return err
+		for i := range n.Columns {
+			if col, err := walk(v, n.Columns[i]); err != nil {
+				return node, err
+			} else if col != nil {
+				n.Columns[i] = col.(*ResultColumn)
+			} else {
+				n.Columns[i] = nil
 			}
 		}
 		if n.Source != nil {
-			if err := walk(v, n.Source); err != nil {
-				return err
+			if src, err := walk(v, n.Source); err != nil {
+				return node, err
+			} else if src != nil {
+				n.Source = n.Source.(Source)
+			} else {
+				n.Source = nil
 			}
 		}
-		if err := walkExpr(v, n.WhereExpr); err != nil {
-			return err
+		if err := walkExpr(v, &n.WhereExpr); err != nil {
+			return node, err
 		}
 		if err := walkExprList(v, n.GroupByExprs); err != nil {
-			return err
+			return node, err
 		}
-		if err := walkExpr(v, n.HavingExpr); err != nil {
-			return err
+		if err := walkExpr(v, &n.HavingExpr); err != nil {
+			return node, err
 		}
-		for _, x := range n.Windows {
-			if err := walk(v, x); err != nil {
-				return err
+		for i := range n.Windows {
+			if w, err := walk(v, n.Windows[i]); err != nil {
+				return node, err
+			} else if w != nil {
+				n.Windows[i] = w.(*Window)
+			} else {
+				n.Windows[i] = nil
 			}
 		}
 		if n.Compound != nil {
-			if err := walk(v, n.Compound); err != nil {
-				return err
+			if stmt, err := walk(v, n.Compound); err != nil {
+				return node, err
+			} else if stmt != nil {
+				n.Compound = stmt.(*SelectStatement)
+			} else {
+				n.Compound = nil
 			}
 		}
-		for _, x := range n.OrderingTerms {
-			if err := walk(v, x); err != nil {
-				return err
+		for i := range n.OrderingTerms {
+			if term, err := walk(v, n.OrderingTerms[i]); err != nil {
+				return node, err
+			} else if term != nil {
+				n.OrderingTerms[i] = term.(*OrderingTerm)
+			} else {
+				n.OrderingTerms[i] = nil
 			}
 		}
-		if err := walkExpr(v, n.LimitExpr); err != nil {
-			return err
+		if err := walkExpr(v, &n.LimitExpr); err != nil {
+			return node, err
 		}
-		if err := walkExpr(v, n.OffsetExpr); err != nil {
-			return err
+		if err := walkExpr(v, &n.OffsetExpr); err != nil {
+			return node, err
 		}
 
 	case *InsertStatement:
 		if n.WithClause != nil {
-			if err := walk(v, n.WithClause); err != nil {
-				return err
+			if clause, err := walk(v, n.WithClause); err != nil {
+				return node, err
+			} else if clause != nil {
+				n.WithClause = clause.(*WithClause)
+			} else {
+				n.WithClause = nil
 			}
 		}
-		if err := walkIdent(v, n.Table); err != nil {
-			return err
+		if err := walkIdent(v, &n.Table); err != nil {
+			return node, err
 		}
-		if err := walkIdent(v, n.Alias); err != nil {
-			return err
+		if err := walkIdent(v, &n.Alias); err != nil {
+			return node, err
 		}
 		if err := walkIdentList(v, n.Columns); err != nil {
-			return err
+			return node, err
 		}
-		for _, x := range n.ValueLists {
-			if err := walk(v, x); err != nil {
-				return err
+		for i := range n.ValueLists {
+			if list, err := walk(v, n.ValueLists[i]); err != nil {
+				return node, err
+			} else if list != nil {
+				n.ValueLists[i] = list.(*ExprList)
+			} else {
+				n.ValueLists[i] = nil
 			}
 		}
 		if n.Select != nil {
-			if err := walk(v, n.Select); err != nil {
-				return err
+			if sel, err := walk(v, n.Select); err != nil {
+				return node, err
+			} else if sel != nil {
+				n.Select = sel.(*SelectStatement)
+			} else {
+				n.Select = nil
 			}
 		}
 		if n.UpsertClause != nil {
-			if err := walk(v, n.UpsertClause); err != nil {
-				return err
+			if clause, err := walk(v, n.UpsertClause); err != nil {
+				return node, err
+			} else if clause != nil {
+				n.UpsertClause = clause.(*UpsertClause)
+			} else {
+				n.UpsertClause = nil
 			}
 		}
 
 	case *UpdateStatement:
 		if n.WithClause != nil {
-			if err := walk(v, n.WithClause); err != nil {
-				return err
+			if clause, err := walk(v, n.WithClause); err != nil {
+				return node, err
+			} else if clause != nil {
+				n.WithClause = clause.(*WithClause)
+			} else {
+				n.WithClause = nil
 			}
 		}
 		if n.Table != nil {
-			if err := walk(v, n.Table); err != nil {
-				return err
+			if tbl, err := walk(v, n.Table); err != nil {
+				return node, err
+			} else if tbl != nil {
+				n.Table = tbl.(*QualifiedTableName)
+			} else {
+				n.Table = nil
 			}
 		}
-		for _, x := range n.Assignments {
-			if err := walk(v, x); err != nil {
-				return err
+		for i := range n.Assignments {
+			if assign, err := walk(v, n.Assignments[i]); err != nil {
+				return node, err
+			} else if assign != nil {
+				n.Assignments[i] = assign.(*Assignment)
+			} else {
+				n.Assignments[i] = nil
 			}
 		}
-		if err := walkExpr(v, n.WhereExpr); err != nil {
-			return err
+		if err := walkExpr(v, &n.WhereExpr); err != nil {
+			return node, err
 		}
 
 	case *UpsertClause:
 		if err := walkIndexedColumnList(v, n.Columns); err != nil {
-			return err
+			return node, err
 		}
-		if err := walkExpr(v, n.WhereExpr); err != nil {
-			return err
+		if err := walkExpr(v, &n.WhereExpr); err != nil {
+			return node, err
 		}
-		for _, x := range n.Assignments {
-			if err := walk(v, x); err != nil {
-				return err
+		for i := range n.Assignments {
+			if assign, err := walk(v, n.Assignments[i]); err != nil {
+				return node, err
+			} else if assign != nil {
+				n.Assignments[i] = assign.(*Assignment)
+			} else {
+				n.Assignments[i] = nil
 			}
 		}
-		if err := walkExpr(v, n.UpdateWhereExpr); err != nil {
-			return err
+		if err := walkExpr(v, &n.UpdateWhereExpr); err != nil {
+			return node, err
 		}
 
 	case *DeleteStatement:
 		if n.WithClause != nil {
-			if err := walk(v, n.WithClause); err != nil {
-				return err
+			if clause, err := walk(v, n.WithClause); err != nil {
+				return node, err
+			} else if clause != nil {
+				n.WithClause = clause.(*WithClause)
+			} else {
+				n.WithClause = nil
 			}
 		}
 		if n.Table != nil {
-			if err := walk(v, n.Table); err != nil {
-				return err
+			if tbl, err := walk(v, n.Table); err != nil {
+				return node, err
+			} else if tbl != nil {
+				n.Table = tbl.(*QualifiedTableName)
+			} else {
+				n.Table = nil
 			}
 		}
-		if err := walkExpr(v, n.WhereExpr); err != nil {
-			return err
+		if err := walkExpr(v, &n.WhereExpr); err != nil {
+			return node, err
 		}
-		for _, x := range n.OrderingTerms {
-			if err := walk(v, x); err != nil {
-				return err
+		for i := range n.OrderingTerms {
+			if term, err := walk(v, n.OrderingTerms[i]); err != nil {
+				return node, err
+			} else if term != nil {
+				n.OrderingTerms[i] = term.(*OrderingTerm)
+			} else {
+				n.OrderingTerms[i] = nil
 			}
 		}
-		if err := walkExpr(v, n.LimitExpr); err != nil {
-			return err
+		if err := walkExpr(v, &n.LimitExpr); err != nil {
+			return node, err
 		}
-		if err := walkExpr(v, n.OffsetExpr); err != nil {
-			return err
+		if err := walkExpr(v, &n.OffsetExpr); err != nil {
+			return node, err
 		}
 
 	case *PrimaryKeyConstraint:
-		if err := walkIdent(v, n.Name); err != nil {
-			return err
+		if err := walkIdent(v, &n.Name); err != nil {
+			return node, err
 		}
 		if err := walkIdentList(v, n.Columns); err != nil {
-			return err
+			return node, err
 		}
 
 	case *NotNullConstraint:
-		if err := walkIdent(v, n.Name); err != nil {
-			return err
+		if err := walkIdent(v, &n.Name); err != nil {
+			return node, err
 		}
 
 	case *UniqueConstraint:
-		if err := walkIdent(v, n.Name); err != nil {
-			return err
+		if err := walkIdent(v, &n.Name); err != nil {
+			return node, err
 		}
 		if err := walkIdentList(v, n.Columns); err != nil {
-			return err
+			return node, err
 		}
 
 	case *CheckConstraint:
-		if err := walkIdent(v, n.Name); err != nil {
-			return err
+		if err := walkIdent(v, &n.Name); err != nil {
+			return node, err
 		}
-		if err := walkExpr(v, n.Expr); err != nil {
-			return err
+		if err := walkExpr(v, &n.Expr); err != nil {
+			return node, err
 		}
 
 	case *DefaultConstraint:
-		if err := walkIdent(v, n.Name); err != nil {
-			return err
+		if err := walkIdent(v, &n.Name); err != nil {
+			return node, err
 		}
-		if err := walkExpr(v, n.Expr); err != nil {
-			return err
+		if err := walkExpr(v, &n.Expr); err != nil {
+			return node, err
 		}
 
 	case *ForeignKeyConstraint:
-		if err := walkIdent(v, n.Name); err != nil {
-			return err
+		if err := walkIdent(v, &n.Name); err != nil {
+			return node, err
 		}
 		if err := walkIdentList(v, n.Columns); err != nil {
-			return err
+			return node, err
 		}
-		if err := walkIdent(v, n.ForeignTable); err != nil {
-			return err
+		if err := walkIdent(v, &n.ForeignTable); err != nil {
+			return node, err
 		}
 		if err := walkIdentList(v, n.ForeignColumns); err != nil {
-			return err
+			return node, err
 		}
-		for _, x := range n.Args {
-			if err := walk(v, x); err != nil {
-				return err
+		for i := range n.Args {
+			if arg, err := walk(v, n.Args[i]); err != nil {
+				return node, err
+			} else if arg != nil {
+				n.Args[i] = arg.(*ForeignKeyArg)
+			} else {
+				n.Args[i] = nil
 			}
 		}
 
 	case *ParenExpr:
-		if err := walkExpr(v, n.X); err != nil {
-			return err
+		if err := walkExpr(v, &n.X); err != nil {
+			return node, err
 		}
 
 	case *UnaryExpr:
-		if err := walkExpr(v, n.X); err != nil {
-			return err
+		if err := walkExpr(v, &n.X); err != nil {
+			return node, err
 		}
 
 	case *BinaryExpr:
-		if err := walkExpr(v, n.X); err != nil {
-			return err
+		if err := walkExpr(v, &n.X); err != nil {
+			return node, err
 		}
-		if err := walkExpr(v, n.Y); err != nil {
-			return err
+		if err := walkExpr(v, &n.Y); err != nil {
+			return node, err
 		}
 
 	case *CastExpr:
-		if err := walkExpr(v, n.X); err != nil {
-			return err
+		if err := walkExpr(v, &n.X); err != nil {
+			return node, err
 		}
 		if n.Type != nil {
-			if err := walk(v, n.Type); err != nil {
-				return err
+			if typ, err := walk(v, n.Type); err != nil {
+				return node, err
+			} else if typ != nil {
+				n.Type = typ.(*Type)
+			} else {
+				n.Type = nil
 			}
 		}
 
 	case *CaseBlock:
-		if err := walkExpr(v, n.Condition); err != nil {
-			return err
+		if err := walkExpr(v, &n.Condition); err != nil {
+			return node, err
 		}
-		if err := walkExpr(v, n.Body); err != nil {
-			return err
+		if err := walkExpr(v, &n.Body); err != nil {
+			return node, err
 		}
 
 	case *CaseExpr:
-		if err := walkExpr(v, n.Operand); err != nil {
-			return err
+		if err := walkExpr(v, &n.Operand); err != nil {
+			return node, err
 		}
-		for _, x := range n.Blocks {
-			if err := walk(v, x); err != nil {
-				return err
+		for i := range n.Blocks {
+			if blk, err := walk(v, n.Blocks[i]); err != nil {
+				return node, err
+			} else if blk != nil {
+				n.Blocks[i] = blk.(*CaseBlock)
+			} else {
+				n.Blocks[i] = nil
 			}
 		}
-		if err := walkExpr(v, n.ElseExpr); err != nil {
-			return err
+		if err := walkExpr(v, &n.ElseExpr); err != nil {
+			return node, err
 		}
 
 	case *ExprList:
 		if err := walkExprList(v, n.Exprs); err != nil {
-			return err
+			return node, err
 		}
 
 	case *QualifiedRef:
-		if err := walkIdent(v, n.Table); err != nil {
-			return err
+		if err := walkIdent(v, &n.Table); err != nil {
+			return node, err
 		}
-		if err := walkIdent(v, n.Column); err != nil {
-			return err
+		if err := walkIdent(v, &n.Column); err != nil {
+			return node, err
 		}
 
 	case *Call:
-		if err := walkIdent(v, n.Name); err != nil {
-			return err
+		if err := walkIdent(v, &n.Name); err != nil {
+			return node, err
 		}
 		if err := walkExprList(v, n.Args); err != nil {
-			return err
+			return node, err
 		}
 		if n.Filter != nil {
-			if err := walk(v, n.Filter); err != nil {
-				return err
+			if filter, err := walk(v, n.Filter); err != nil {
+				return node, err
+			} else if filter != nil {
+				n.Filter = filter.(*FilterClause)
+			} else {
+				n.Filter = nil
 			}
 		}
 		if n.Over != nil {
-			if err := walk(v, n.Over); err != nil {
-				return err
+			if over, err := walk(v, n.Over); err != nil {
+				return node, err
+			} else if over != nil {
+				n.Over = over.(*OverClause)
+			} else {
+				n.Over = nil
 			}
 		}
 
 	case *FilterClause:
-		if err := walkExpr(v, n.X); err != nil {
-			return err
+		if err := walkExpr(v, &n.X); err != nil {
+			return node, err
 		}
 
 	case *OverClause:
-		if err := walkIdent(v, n.Name); err != nil {
-			return err
+		if err := walkIdent(v, &n.Name); err != nil {
+			return node, err
 		}
 		if n.Definition != nil {
-			if err := walk(v, n.Definition); err != nil {
-				return err
+			if def, err := walk(v, n.Definition); err != nil {
+				return node, err
+			} else if def != nil {
+				n.Definition = def.(*WindowDefinition)
+			} else {
+				n.Definition = nil
 			}
 		}
 
 	case *OrderingTerm:
-		if err := walkExpr(v, n.X); err != nil {
-			return err
+		if err := walkExpr(v, &n.X); err != nil {
+			return node, err
 		}
 
 	case *FrameSpec:
-		if err := walkExpr(v, n.X); err != nil {
-			return err
+		if err := walkExpr(v, &n.X); err != nil {
+			return node, err
 		}
-		if err := walkExpr(v, n.Y); err != nil {
-			return err
+		if err := walkExpr(v, &n.Y); err != nil {
+			return node, err
 		}
 
 	case *Range:
-		if err := walkExpr(v, n.X); err != nil {
-			return err
+		if err := walkExpr(v, &n.X); err != nil {
+			return node, err
 		}
-		if err := walkExpr(v, n.Y); err != nil {
-			return err
+		if err := walkExpr(v, &n.Y); err != nil {
+			return node, err
 		}
 
 	case *Raise:
 		if n.Error != nil {
-			if err := walk(v, n.Error); err != nil {
-				return err
+			if e, err := walk(v, n.Error); err != nil {
+				return node, err
+			} else if e != nil {
+				n.Error = e.(*StringLit)
+			} else {
+				n.Error = nil
 			}
 		}
 
 	case *Exists:
 		if n.Select != nil {
-			if err := walk(v, n.Select); err != nil {
-				return err
+			if sel, err := walk(v, n.Select); err != nil {
+				return node, err
+			} else if sel != nil {
+				n.Select = sel.(*SelectStatement)
+			} else {
+				n.Select = nil
 			}
 		}
 
 	case *ParenSource:
 		if n.X != nil {
-			if err := walk(v, n.X); err != nil {
-				return err
+			if x, err := walk(v, n.X); err != nil {
+				return node, err
+			} else if x != nil {
+				n.X = x.(Source)
+			} else {
+				n.X = nil
 			}
 		}
-		if err := walkIdent(v, n.Alias); err != nil {
-			return err
+		if err := walkIdent(v, &n.Alias); err != nil {
+			return node, err
 		}
 
 	case *QualifiedTableName:
-		if err := walkIdent(v, n.Name); err != nil {
-			return err
+		if err := walkIdent(v, &n.Name); err != nil {
+			return node, err
 		}
-		if err := walkIdent(v, n.Alias); err != nil {
-			return err
+		if err := walkIdent(v, &n.Alias); err != nil {
+			return node, err
 		}
-		if err := walkIdent(v, n.Index); err != nil {
-			return err
+		if err := walkIdent(v, &n.Index); err != nil {
+			return node, err
 		}
 
 	case *JoinClause:
 		if n.X != nil {
-			if err := walk(v, n.X); err != nil {
-				return err
+			if x, err := walk(v, n.X); err != nil {
+				return node, err
+			} else if x != nil {
+				n.X = x.(Source)
+			} else {
+				n.X = nil
 			}
 		}
 		if n.Operator != nil {
-			if err := walk(v, n.Operator); err != nil {
-				return err
+			if op, err := walk(v, n.Operator); err != nil {
+				return node, err
+			} else if op != nil {
+				n.Operator = op.(*JoinOperator)
+			} else {
+				n.Operator = nil
 			}
 		}
 		if n.Y != nil {
-			if err := walk(v, n.Y); err != nil {
-				return err
+			if y, err := walk(v, n.Y); err != nil {
+				return node, err
+			} else if y != nil {
+				n.Y = y.(Source)
+			} else {
+				n.Y = nil
 			}
 		}
 		if n.Constraint != nil {
-			if err := walk(v, n.Constraint); err != nil {
-				return err
+			if cons, err := walk(v, n.Constraint); err != nil {
+				return node, err
+			} else if cons != nil {
+				n.Constraint = cons.(JoinConstraint)
+			} else {
+				n.Constraint = nil
 			}
 		}
 
 	case *OnConstraint:
-		if err := walkExpr(v, n.X); err != nil {
-			return err
+		if err := walkExpr(v, &n.X); err != nil {
+			return node, err
 		}
 
 	case *UsingConstraint:
 		if err := walkIdentList(v, n.Columns); err != nil {
-			return err
+			return node, err
 		}
 
 	case *ColumnDefinition:
-		if err := walkIdent(v, n.Name); err != nil {
-			return err
+		if err := walkIdent(v, &n.Name); err != nil {
+			return node, err
 		}
 		if n.Type != nil {
-			if err := walk(v, n.Type); err != nil {
-				return err
+			if typ, err := walk(v, n.Type); err != nil {
+				return node, err
+			} else if typ != nil {
+				n.Type = typ.(*Type)
+			} else {
+				n.Type = nil
 			}
 		}
 		if err := walkConstraintList(v, n.Constraints); err != nil {
-			return err
+			return node, err
 		}
 
 	case *ResultColumn:
-		if err := walkExpr(v, n.Expr); err != nil {
-			return err
+		if err := walkExpr(v, &n.Expr); err != nil {
+			return node, err
 		}
-		if err := walkIdent(v, n.Alias); err != nil {
-			return err
+		if err := walkIdent(v, &n.Alias); err != nil {
+			return node, err
 		}
 
 	case *IndexedColumn:
-		if err := walkExpr(v, n.X); err != nil {
-			return err
+		if err := walkExpr(v, &n.X); err != nil {
+			return node, err
 		}
 
 	case *Window:
-		if err := walkIdent(v, n.Name); err != nil {
-			return err
+		if err := walkIdent(v, &n.Name); err != nil {
+			return node, err
 		}
 		if n.Definition != nil {
-			if err := walk(v, n.Definition); err != nil {
-				return err
+			if def, err := walk(v, n.Definition); err != nil {
+				return node, err
+			} else if def != nil {
+				n.Definition = def.(*WindowDefinition)
+			} else {
+				n.Definition = nil
 			}
 		}
 
 	case *WindowDefinition:
-		if err := walkIdent(v, n.Base); err != nil {
-			return err
+		if err := walkIdent(v, &n.Base); err != nil {
+			return node, err
 		}
 		if err := walkExprList(v, n.Partitions); err != nil {
-			return err
+			return node, err
 		}
-		for _, x := range n.OrderingTerms {
-			if err := walk(v, x); err != nil {
-				return err
+		for i := range n.OrderingTerms {
+			if term, err := walk(v, n.OrderingTerms[i]); err != nil {
+				return node, err
+			} else if term != nil {
+				n.OrderingTerms[i] = term.(*OrderingTerm)
+			} else {
+				n.OrderingTerms[i] = nil
 			}
 		}
 		if n.Frame != nil {
-			if err := walk(v, n.Frame); err != nil {
-				return err
+			if frame, err := walk(v, n.Frame); err != nil {
+				return node, err
+			} else if frame != nil {
+				n.Frame = frame.(*FrameSpec)
+			} else {
+				n.Frame = nil
 			}
 		}
 
 	case *Type:
-		if err := walkIdent(v, n.Name); err != nil {
-			return err
+		if err := walkIdent(v, &n.Name); err != nil {
+			return node, err
 		}
 		if n.Precision != nil {
-			if err := walk(v, n.Precision); err != nil {
-				return err
+			if p, err := walk(v, n.Precision); err != nil {
+				return node, err
+			} else if p != nil {
+				n.Precision = p.(*NumberLit)
+			} else {
+				n.Precision = nil
 			}
 		}
 		if n.Scale != nil {
-			if err := walk(v, n.Scale); err != nil {
-				return err
+			if scale, err := walk(v, n.Scale); err != nil {
+				return node, err
+			} else if scale != nil {
+				n.Scale = scale.(*NumberLit)
+			} else {
+				n.Scale = nil
 			}
 		}
 	}
@@ -637,59 +805,72 @@ func walk(v Visitor, node Node) (err error) {
 
 // VisitFunc represents a function type that implements Visitor.
 // Only executes on node entry.
-type VisitFunc func(Node) error
+type VisitFunc func(Node) (Node, error)
 
 // Visit executes fn. Walk visits node children if fn returns true.
-func (fn VisitFunc) Visit(node Node) (Visitor, error) {
-	if err := fn(node); err != nil {
-		return nil, err
+func (fn VisitFunc) Visit(node Node) (Visitor, Node, error) {
+	node, err := fn(node)
+	if err != nil {
+		return nil, nil, err
 	}
-	return fn, nil
+	return fn, node, nil
 }
 
 // VisitEnd is a no-op.
-func (fn VisitFunc) VisitEnd(node Node) error { return nil }
+func (fn VisitFunc) VisitEnd(node Node) (Node, error) { return node, nil }
 
 // VisitEndFunc represents a function type that implements Visitor.
 // Only executes on node exit.
-type VisitEndFunc func(Node) error
+type VisitEndFunc func(Node) (Node, error)
 
 // Visit is a no-op.
-func (fn VisitEndFunc) Visit(node Node) (Visitor, error) { return fn, nil }
+func (fn VisitEndFunc) Visit(node Node) (Visitor, Node, error) { return fn, node, nil }
 
 // VisitEnd executes fn.
-func (fn VisitEndFunc) VisitEnd(node Node) error { return fn(node) }
+func (fn VisitEndFunc) VisitEnd(node Node) (Node, error) { return fn(node) }
 
-func walkIdent(v Visitor, x *Ident) error {
-	if x != nil {
-		if err := walk(v, x); err != nil {
-			return err
-		}
+func walkIdent(v Visitor, x **Ident) error {
+	if *x == nil {
+		return nil
+	}
+
+	ident, err := walk(v, *x)
+	if err != nil {
+		return err
+	} else if ident != nil {
+		*x = ident.(*Ident)
+	} else {
+		*x = nil
 	}
 	return nil
 }
 
 func walkIdentList(v Visitor, a []*Ident) error {
-	for _, x := range a {
-		if err := walk(v, x); err != nil {
+	for i := range a {
+		if err := walkIdent(v, &a[i]); err != nil {
 			return err
 		}
 	}
 	return nil
 }
 
-func walkExpr(v Visitor, x Expr) error {
-	if x != nil {
-		if err := walk(v, x); err != nil {
-			return err
-		}
+func walkExpr(v Visitor, x *Expr) error {
+	if *x == nil {
+		return nil
+	}
+	if other, err := walk(v, *x); err != nil {
+		return err
+	} else if other != nil {
+		*x = other.(Expr)
+	} else {
+		*x = nil
 	}
 	return nil
 }
 
 func walkExprList(v Visitor, a []Expr) error {
-	for _, x := range a {
-		if err := walk(v, x); err != nil {
+	for i := range a {
+		if err := walkExpr(v, &a[i]); err != nil {
 			return err
 		}
 	}
@@ -697,27 +878,39 @@ func walkExprList(v Visitor, a []Expr) error {
 }
 
 func walkConstraintList(v Visitor, a []Constraint) error {
-	for _, x := range a {
-		if err := walk(v, x); err != nil {
+	for i := range a {
+		if cons, err := walk(v, a[i]); err != nil {
 			return err
+		} else if cons != nil {
+			a[i] = cons.(Constraint)
+		} else {
+			a[i] = nil
 		}
 	}
 	return nil
 }
 
 func walkIndexedColumnList(v Visitor, a []*IndexedColumn) error {
-	for _, x := range a {
-		if err := walk(v, x); err != nil {
+	for i := range a {
+		if col, err := walk(v, a[i]); err != nil {
 			return err
+		} else if col != nil {
+			a[i] = col.(*IndexedColumn)
+		} else {
+			a[i] = nil
 		}
 	}
 	return nil
 }
 
 func walkColumnDefinitionList(v Visitor, a []*ColumnDefinition) error {
-	for _, x := range a {
-		if err := walk(v, x); err != nil {
+	for i := range a {
+		if def, err := walk(v, a[i]); err != nil {
 			return err
+		} else if def != nil {
+			a[i] = def.(*ColumnDefinition)
+		} else {
+			a[i] = nil
 		}
 	}
 	return nil