Index: src/where.c
==================================================================
--- src/where.c
+++ src/where.c
@@ -4579,11 +4579,13 @@
     int addr;
     pLevel = &pWInfo->a[i];
     pLoop = pLevel->pWLoop;
     sqlite3VdbeResolveLabel(v, pLevel->addrCont);
     if( pLevel->op!=OP_Noop ){
-      if( pWInfo->eDistinct==WHERE_DISTINCT_ORDERED ){
+      if( pWInfo->eDistinct==WHERE_DISTINCT_ORDERED
+       && (pLoop->wsFlags & WHERE_INDEXED)!=0
+      ){
         int j, k, op;
         int r1 = pParse->nMem+1;
         int n = 0;
         ExprList *pX = pWInfo->pDistinctSet;
         for(j=0; j<pX->nExpr; j++){
@@ -4592,16 +4594,18 @@
         }
         for(j=0; j<n; j++){
           sqlite3VdbeAddOp3(v, OP_Column, pLevel->iIdxCur, j, r1+j);
         }
         pParse->nMem += n;
-        op = pLevel->op==OP_Prev ? OP_SeekLT : OP_SeekGT;
-        k = sqlite3VdbeAddOp4Int(v, op, pLevel->iIdxCur, 0, r1, n);
-        VdbeCoverageIf(v, op==OP_SeekLT);
-        VdbeCoverageIf(v, op==OP_SeekGT);
-        sqlite3VdbeAddOp2(v, OP_Goto, 1, pLevel->p2);
-        sqlite3VdbeJumpHere(v, k);
+        if( n>0 ){
+          op = pLevel->op==OP_Prev ? OP_SeekLT : OP_SeekGT;
+          k = sqlite3VdbeAddOp4Int(v, op, pLevel->iIdxCur, 0, r1, n);
+          VdbeCoverageIf(v, op==OP_SeekLT);
+          VdbeCoverageIf(v, op==OP_SeekGT);
+          sqlite3VdbeAddOp2(v, OP_Goto, 1, pLevel->p2);
+          sqlite3VdbeJumpHere(v, k);
+        }
       }else{
         sqlite3VdbeAddOp3(v, pLevel->op, pLevel->p1, pLevel->p2, pLevel->p3);
         sqlite3VdbeChangeP5(v, pLevel->p5);
         VdbeCoverage(v);
         VdbeCoverageIf(v, pLevel->op==OP_Next);

ADDED   test/distinct2.test
Index: test/distinct2.test
==================================================================
--- test/distinct2.test
+++ test/distinct2.test
@@ -0,0 +1,52 @@
+# 2016-04-15
+#
+# The author disclaims copyright to this source code.  In place of
+# a legal notice, here is a blessing:
+#
+#    May you do good and not evil.
+#    May you find forgiveness for yourself and forgive others.
+#    May you share freely, never taking more than you give.
+#
+#***********************************************************************
+# This file implements regression tests for SQLite library.  The
+# focus of this script is DISTINCT queries using the skip-ahead 
+# optimization.
+#
+
+set testdir [file dirname $argv0]
+source $testdir/tester.tcl
+
+set testprefix distinct2
+
+do_execsql_test 100 {
+  CREATE TABLE t1(x INTEGER PRIMARY KEY);
+  INSERT INTO t1 VALUES(0),(1),(2);
+  CREATE TABLE t2 AS
+     SELECT DISTINCT a.x AS aa, b.x AS bb
+      FROM t1 a, t1 b;
+  SELECT *, '|' FROM t2 ORDER BY aa, bb;
+} {0 0 | 0 1 | 0 2 | 1 0 | 1 1 | 1 2 | 2 0 | 2 1 | 2 2 |}
+do_execsql_test 110 {
+  DROP TABLE t2;
+  CREATE TABLE t2 AS
+     SELECT DISTINCT a.x AS aa, b.x AS bb
+       FROM t1 a, t1 b
+      WHERE a.x IN t1 AND b.x IN t1;
+  SELECT *, '|' FROM t2 ORDER BY aa, bb;
+} {0 0 | 0 1 | 0 2 | 1 0 | 1 1 | 1 2 | 2 0 | 2 1 | 2 2 |}
+do_execsql_test 120 {
+  CREATE TABLE t102 (i0 TEXT UNIQUE NOT NULL);
+  INSERT INTO t102 VALUES ('0'),('1'),('2');
+  DROP TABLE t2;
+  CREATE TABLE t2 AS
+    SELECT DISTINCT * 
+    FROM t102 AS t0 
+    JOIN t102 AS t4 ON (t2.i0 IN t102)
+    NATURAL JOIN t102 AS t3
+    JOIN t102 AS t1 ON (t0.i0 IN t102)
+    JOIN t102 AS t2 ON (t2.i0=+t0.i0 OR (t0.i0<>500 AND t2.i0=t1.i0));
+  SELECT *, '|' FROM t2 ORDER BY 1, 2, 3, 4, 5;
+} {0 0 0 0 | 0 0 1 0 | 0 0 1 1 | 0 0 2 0 | 0 0 2 2 | 0 1 0 0 | 0 1 1 0 | 0 1 1 1 | 0 1 2 0 | 0 1 2 2 | 0 2 0 0 | 0 2 1 0 | 0 2 1 1 | 0 2 2 0 | 0 2 2 2 | 1 0 0 0 | 1 0 0 1 | 1 0 1 1 | 1 0 2 1 | 1 0 2 2 | 1 1 0 0 | 1 1 0 1 | 1 1 1 1 | 1 1 2 1 | 1 1 2 2 | 1 2 0 0 | 1 2 0 1 | 1 2 1 1 | 1 2 2 1 | 1 2 2 2 | 2 0 0 0 | 2 0 0 2 | 2 0 1 1 | 2 0 1 2 | 2 0 2 2 | 2 1 0 0 | 2 1 0 2 | 2 1 1 1 | 2 1 1 2 | 2 1 2 2 | 2 2 0 0 | 2 2 0 2 | 2 2 1 1 | 2 2 1 2 | 2 2 2 2 |}
+
+
+finish_test