Basic index selection for SELECT.

Added support to scanNode for restricting the start and end keys that
are scanned.

Added basic support for selecting an index to use for SELECT statement
processing and to restrict the start and end keys for the scan.

Fixes #1625.

sql/scan.go

@@ -52,7 +52,7 @@ func (q *qvalue) Datum() parser.Datum {
 }
 
 func (q *qvalue) String() string {
-	return q.datum.String()
+	return q.col.Name
 }
 
 type qvalMap map[ColumnID]*qvalue
@@ -64,9 +64,12 @@ type scanNode struct {
 	txn              *client.Txn
 	desc             *TableDescriptor
 	index            *IndexDescriptor
+	startKey         proto.Key
+	endKey           proto.Key
 	visibleCols      []ColumnDescriptor
 	isSecondaryIndex bool
 	columns          []string
+	columnIDs        []ColumnID
 	err              error
 	indexKey         []byte            // the index key of the current row
 	kvs              []client.KeyValue // the raw key/value pairs
@@ -204,15 +207,24 @@ func (n *scanNode) initScan() bool {
 	}
 
 	// Retrieve all of the keys that start with our index key prefix.
-	startKey := proto.Key(MakeIndexKeyPrefix(n.desc.ID, n.index.ID))
-	endKey := startKey.PrefixEnd()
-	n.kvs, n.err = n.txn.Scan(startKey, endKey, 0)
+	if len(n.startKey) == 0 {
+		n.startKey = proto.Key(MakeIndexKeyPrefix(n.desc.ID, n.index.ID))
+	}
+	if len(n.endKey) == 0 {
+		n.endKey = n.startKey.PrefixEnd()
+	}
+	n.kvs, n.err = n.txn.Scan(n.startKey, n.endKey, 0)
 	if n.err != nil {
 		return false
 	}
 
 	// Prepare our index key vals slice.
-	n.vals, n.err = makeIndexKeyVals(n.desc, *n.index)
+	n.columnIDs = n.index.ColumnIDs
+	if !n.index.Unique {
+		// Non-unique indexes have the primary key columns appended to their key.
+		n.columnIDs = append(n.columnIDs, n.desc.PrimaryIndex.ColumnIDs...)
+	}
+	n.vals, n.err = makeKeyVals(n.desc, n.columnIDs)
 	if n.err != nil {
 		return false
 	}
@@ -336,7 +348,7 @@ func (n *scanNode) processKV(kv client.KeyValue) bool {
 
 		// This is the first key for the row, initialize the column values that are
 		// part of the index key.
-		for i, id := range n.index.ColumnIDs {
+		for i, id := range n.columnIDs {
 			if qval := n.qvals[id]; qval != nil {
 				qval.datum = n.vals[i]
 			}