diff --git a/executor.go b/executor.go
index f83c0cbf3..7c23ac6e4 100644
--- a/executor.go
+++ b/executor.go
@@ -2006,6 +2006,8 @@ func (e *executor) translateCall(index string, idx *Index, c *pql.Call) error {
 	} else if c.Name == "Rows" {
 		fieldName = callArgString(c, "field")
 		rowKey = "previous"
+	} else if c.Name == "GroupBy" {
+		return errors.Wrap(e.translateGroupByCall(index, idx, c), "translating GroupBy")
 	} else {
 		colKey = "col"
 		fieldName = callArgString(c, "field")
@@ -2067,6 +2069,61 @@ func (e *executor) translateCall(index string, idx *Index, c *pql.Call) error {
 	return nil
 }
 
+func (e *executor) translateGroupByCall(index string, idx *Index, c *pql.Call) error {
+	if c.Name != "GroupBy" {
+		panic("translateGroupByCall called with '" + c.Name + "'")
+	}
+
+	for _, child := range c.Children {
+		if err := e.translateCall(index, idx, child); err != nil {
+			return errors.Wrapf(err, "translating %s", child)
+		}
+	}
+
+	prev, ok := c.Args["previous"]
+	if !ok {
+		return nil // nothing else to be translated
+	}
+	previous, ok := prev.([]interface{})
+	if !ok {
+		return errors.Errorf("'previous' argument must be list, but got %T", prev)
+	}
+	if len(c.Children) != len(previous) {
+		return errors.Errorf("mismatched lengths for previous: %d and children: %d in %s", len(previous), len(c.Children), c)
+	}
+
+	fields := make([]*Field, len(c.Children))
+	for i, child := range c.Children {
+		fieldname := callArgString(child, "field")
+		field := idx.Field(fieldname)
+		if field == nil {
+			return errors.Wrapf(ErrFieldNotFound, "getting field '%s' from '%s'", fieldname, child)
+		}
+		fields[i] = field
+	}
+
+	for i, field := range fields {
+		prev := previous[i]
+		if field.keys() {
+			prevStr, ok := prev.(string)
+			if !ok {
+				return errors.New("prev value must be a string when field 'keys' option enabled")
+			}
+			ids, err := e.TranslateStore.TranslateRowsToUint64(index, field.Name(), []string{prevStr})
+			if err != nil {
+				return errors.Wrapf(err, "translating row key '%s'", prevStr)
+			}
+			previous[i] = ids[0]
+		} else {
+			if prevStr, ok := prev.(string); ok {
+				return errors.Errorf("got string row val '%s' in 'previous' for field %s which doesn't use string keys", prevStr, field.Name())
+			}
+		}
+
+	}
+	return nil
+}
+
 func (e *executor) translateResult(index string, idx *Index, call *pql.Call, result interface{}) (interface{}, error) {
 	switch result := result.(type) {
 	case *Row:
diff --git a/executor_internal_test.go b/executor_internal_test.go
new file mode 100644
index 000000000..e90948561
--- /dev/null
+++ b/executor_internal_test.go
@@ -0,0 +1,122 @@
+package pilosa
+
+import (
+	"fmt"
+	"io/ioutil"
+	"strings"
+	"testing"
+
+	"github.com/pilosa/pilosa/pql"
+)
+
+func TestExecutor_TranslateGroupByCall(t *testing.T) {
+	e := &executor{
+		Holder: NewHolder(),
+	}
+	e.Holder.Path, _ = ioutil.TempDir("", "")
+	err := e.Holder.Open()
+	if err != nil {
+		t.Fatalf("opening holder: %v", err)
+	}
+
+	e.TranslateStore = e.Holder.translateFile
+	tf, _ := ioutil.TempFile("", "")
+	e.Holder.translateFile.Path = tf.Name()
+	err = e.Holder.translateFile.Open()
+	if err != nil {
+		t.Fatalf("opening translateFile: %v", err)
+	}
+
+	idx, err := e.Holder.CreateIndex("i", IndexOptions{})
+	if err != nil {
+		t.Fatalf("creating index: %v", err)
+	}
+
+	_, erra := idx.CreateField("ak", OptFieldKeys())
+	_, errb := idx.CreateField("b")
+	_, errc := idx.CreateField("ck", OptFieldKeys())
+	if erra != nil || errb != nil || errc != nil {
+		t.Fatalf("creating fields %v, %v, %v", erra, errb, errc)
+	}
+
+	_, erra = e.TranslateStore.TranslateRowsToUint64("i", "ak", []string{"la"})
+	_, errb = e.TranslateStore.TranslateRowsToUint64("i", "ck", []string{"ha"})
+	if erra != nil || errb != nil {
+		t.Fatalf("translating rows %v, %v", erra, errb)
+	}
+
+	query, err := pql.ParseString(`GroupBy(Rows(field=ak), Rows(field=b), Rows(field=ck), previous=["la", 0, "ha"])`)
+	if err != nil {
+		t.Fatalf("parsing query: %v", err)
+	}
+	c := query.Calls[0]
+	err = e.translateGroupByCall("i", idx, c)
+	if err != nil {
+		t.Fatalf("translating call: %v", err)
+	}
+	if len(c.Args["previous"].([]interface{})) != 3 {
+		t.Fatalf("unexpected length for 'previous' arg %v", c.Args["previous"])
+	}
+	for i, v := range c.Args["previous"].([]interface{}) {
+		if !isInt(v) {
+			t.Fatalf("expected all items in previous to be ints, but '%v' at index %d is %[1]T", v, i)
+		}
+	}
+
+	errTests := []struct {
+		pql string
+		err string
+	}{
+		{
+			pql: `GroupBy(Rows(field=notfound), previous=1)`,
+			err: "'previous' argument must be list",
+		},
+		{
+			pql: `GroupBy(Rows(field=ak), previous=["la", 0])`,
+			err: "mismatched lengths",
+		},
+		{
+			pql: `GroupBy(Rows(field=ak), previous=[1])`,
+			err: "prev value must be a string",
+		},
+		{
+			pql: `GroupBy(Rows(field=notfound), previous=[1])`,
+			err: ErrFieldNotFound.Error(),
+		},
+		// TODO: an unknown key will actually allocate an id. this is probably bad.
+		// {
+		// 	pql: `GroupBy(Rows(field=ak), previous=["zoop"])`,
+		// 	err: "translating row key '",
+		// },
+		{
+			pql: `GroupBy(Rows(field=b), previous=["la"])`,
+			err: "which doesn't use string keys",
+		},
+	}
+
+	for i, test := range errTests {
+		t.Run(fmt.Sprintf("#%d_%s", i, test.err), func(t *testing.T) {
+			query, err := pql.ParseString(test.pql)
+			if err != nil {
+				t.Fatalf("parsing query: %v", err)
+			}
+			c := query.Calls[0]
+			err = e.translateGroupByCall("i", idx, c)
+			if err == nil {
+				t.Fatalf("expected error, but translated call is '%s", c)
+			}
+			if !strings.Contains(err.Error(), test.err) {
+				t.Fatalf("expected '%s', got '%v'", test.err, err)
+			}
+		})
+	}
+}
+
+func isInt(a interface{}) bool {
+	switch a.(type) {
+	case int, int64, uint, uint64:
+		return true
+	default:
+		return false
+	}
+}