Added some docs, fixed a test

Author: zachmu

sql/core.go

@@ -45,6 +45,8 @@ type Expression interface {
 	WithChildren(children ...Expression) (Expression, error)
 }
 
+// RowIterExpression is an Expression that returns a RowIter rather than a scalar, used to implement functions that 
+// return sets.
 type RowIterExpression interface {
 	Expression
 	// EvalRowIter evaluates the expression, which must be a RowIter

sql/rowexec/rel_iters.go

@@ -245,12 +245,15 @@ func (i *ProjectIter) ProjectRowWithNestedIters(
 	return i.ProjectRowWithNestedIters(ctx)
 }
 
+// RowIterEvaluator is an expression that returns the next value from a sql.RowIter each time Eval is called.
 type RowIterEvaluator struct {
 	iter     sql.RowIter
 	typ      sql.Type
 	finished bool
 }
 
+var _ sql.Expression = (*RowIterEvaluator)(nil)
+
 func (r RowIterEvaluator) Resolved() bool {
 	return true
 }
@@ -296,8 +299,6 @@ func (r RowIterEvaluator) WithChildren(children ...sql.Expression) (sql.Expressi
 	return &r, nil
 }
 
-var _ sql.Expression = (*RowIterEvaluator)(nil)
-
 // ProjectRow evaluates a set of projections.
 func ProjectRow(
 	ctx *sql.Context,

sql/types/conversion_test.go

@@ -207,7 +207,7 @@ func TestGeneralizeTypes(t *testing.T) {
 		{Time, Date, DatetimeMaxPrecision},
 		{Date, Date, Date},
 		{Date, Timestamp, DatetimeMaxPrecision},
-		{Timestamp, Timestamp, Timestamp},
+		{Timestamp, Timestamp, TimestampMaxPrecision},
 		{Timestamp, TimestampMaxPrecision, TimestampMaxPrecision},
 		{Timestamp, Datetime, DatetimeMaxPrecision},
 		{Null, Int64, Int64},