Index: src/sqliteInt.h
==================================================================
--- src/sqliteInt.h
+++ src/sqliteInt.h
@@ -364,10 +364,18 @@
 */
 #ifndef SQLITE_TEMP_STORE
 # define SQLITE_TEMP_STORE 1
 #endif
 
+/*
+** If all temporary storage is in-memory, then omit the external merge-sort
+** logic since it is superfluous.
+*/
+#if SQLITE_TEMP_STORE==3 && !defined(SQLITE_OMIT_MERGE_SORT)
+# define SQLITE_OMIT_MERGE_SORT
+#endif
+
 /*
 ** GCC does not define the offsetof() macro so we'll have to do it
 ** ourselves.
 */
 #ifndef offsetof

Index: src/vdbe.c
==================================================================
--- src/vdbe.c
+++ src/vdbe.c
@@ -3154,10 +3154,16 @@
 ** This opcode works the same as OP_OpenEphemeral.  It has a
 ** different name to distinguish its use.  Tables created using
 ** by this opcode will be used for automatically created transient
 ** indices in joins.
 */
+/* Opcode: OpenSorter P1 P2 * P4 *
+**
+** This opcode works like OP_OpenEphemeral except that it opens
+** a transient index that is specifically designed to sort large
+** tables using an external merge-sort algorithm.
+*/
 case OP_OpenSorter: 
 case OP_OpenAutoindex: 
 case OP_OpenEphemeral: {
   VdbeCursor *pCx;
   static const int vfsFlags =