From 3a1f0e68be16aabe0f77b8b18135026ab6df09ca Mon Sep 17 00:00:00 2001
From: Preetam Jinka <pj@preet.am>
Date: Wed, 4 Apr 2018 23:40:33 -0700
Subject: [PATCH 1/2] init executor

---
 internal/query/executor.go | 33 +++++++++++++++++++++++++++++++++
 1 file changed, 33 insertions(+)
 create mode 100644 internal/query/executor.go

diff --git a/internal/query/executor.go b/internal/query/executor.go
new file mode 100644
index 0000000..3f561bf
--- /dev/null
+++ b/internal/query/executor.go
@@ -0,0 +1,33 @@
+package query
+
+// Executor is a query executor.
+type Executor struct{}
+
+func NewExecutor(t Table) *Executor {
+	return &Executor{}
+}
+
+func (e *Executor) Execute(query Desc) error {
+	// TODO
+	return nil
+}
+
+type Table interface {
+	NewCursor() (Cursor, error)
+	NewCursorForFields(fields []string) (Cursor, error)
+}
+
+type Cursor interface {
+	GetRow() (Row, error)
+	Next() bool
+	Err() error
+}
+
+type Seeker interface {
+	Seek(Row) error
+}
+
+type Row interface {
+	Fields() []string
+	Get(field string) (interface{}, error)
+}

From aebc2754261b8428f41f97001c12da7d56464464 Mon Sep 17 00:00:00 2001
From: Preetam Jinka <pj@preet.am>
Date: Thu, 5 Apr 2018 07:15:33 -0700
Subject: [PATCH 2/2] more wip

---
 internal/query/executor.go | 39 +++++++++++++++++++++++++++++++++++---
 1 file changed, 36 insertions(+), 3 deletions(-)

diff --git a/internal/query/executor.go b/internal/query/executor.go
index 3f561bf..8224fb5 100644
--- a/internal/query/executor.go
+++ b/internal/query/executor.go
@@ -1,14 +1,47 @@
 package query
 
 // Executor is a query executor.
-type Executor struct{}
+type Executor struct {
+	table Table
+}
 
-func NewExecutor(t Table) *Executor {
-	return &Executor{}
+func NewExecutor(table Table) *Executor {
+	return &Executor{
+		table: table,
+	}
 }
 
 func (e *Executor) Execute(query Desc) error {
+	// Get a cursor
+	var cur Cursor
+	var err error
+	if len(query.Columns) == 0 && len(query.GroupBy) == 0 {
+		// No aggregations. Use a plain cursor.
+		cur, err = e.table.NewCursor()
+		if err != nil {
+			return err
+		}
+	} else {
+		fieldsMap := map[string]struct{}{}
+		for _, col := range query.Columns {
+			fieldsMap[col.Name] = struct{}{}
+		}
+		for _, col := range query.GroupBy {
+			fieldsMap[col.Name] = struct{}{}
+		}
+		fields := make([]string, 0, len(fieldsMap))
+		for field := range fieldsMap {
+			fields = append(fields, field)
+		}
+		cur, err = e.table.NewCursorForFields(fields)
+		if err != nil {
+			return err
+		}
+	}
+
 	// TODO
+	_ = cur
+
 	return nil
 }