4 files changed, 51 insertions, 0 deletions

diff --git a/src/btree.c b/src/btree.c
index 1806a914a..1dc7b851f 100644
--- a/src/btree.c
+++ b/src/btree.c
@@ -5667,6 +5667,23 @@ int sqlite3BtreeFirst(BtCursor *pCur, int *pRes){
   return rc;
 }
 
+/* Return true if the BTree pointed to by cursor pCur contains zero
+** rows of content.  Return false if the table contains content or if
+** if there is some kind of error.  This routine is used as an optimization.
+** Returning false (a false negative) will always result in a correct
+** answer, though perhaps more slowly.  But a false positive (an incorrect
+** return of true) can yield an incorrect answer.
+*/
+int sqlite3BtreeIsEmpty(BtCursor *pCur){
+  int rc;
+
+  assert( cursorOwnsBtShared(pCur) );
+  assert( sqlite3_mutex_held(pCur->pBtree->db->mutex) );
+  if( pCur->eState==CURSOR_VALID ) return 0;
+  rc = moveToRoot(pCur);
+  return rc==SQLITE_EMPTY;
+}
+
 #ifdef SQLITE_DEBUG
 /* The cursors is CURSOR_VALID and has BTCF_AtLast set.  Verify that
 ** this flags are true for a consistent database.
diff --git a/src/btree.h b/src/btree.h
index 241261dc6..23fb12791 100644
--- a/src/btree.h
+++ b/src/btree.h
@@ -317,6 +317,7 @@ struct BtreePayload {
 int sqlite3BtreeInsert(BtCursor*, const BtreePayload *pPayload,
                        int flags, int seekResult);
 int sqlite3BtreeFirst(BtCursor*, int *pRes);
+int sqlite3BtreeIsEmpty(BtCursor *pCur);
 int sqlite3BtreeLast(BtCursor*, int *pRes);
 int sqlite3BtreeNext(BtCursor*, int flags);
 int sqlite3BtreeEof(BtCursor*);
diff --git a/src/select.c b/src/select.c
index 95b2925e3..e2dd24b3e 100644
--- a/src/select.c
+++ b/src/select.c
@@ -3036,7 +3036,9 @@ static int multiSelect(
         int priorOp;     /* The SRT_ operation to apply to prior selects */
         Expr *pLimit;    /* Saved values of p->nLimit  */
         int addr;
+        int emptyBypass = 0;   /* IfEmpty opcode to bypass RHS */
         SelectDest uniondest;
+
  
         testcase( p->op==TK_EXCEPT );
         testcase( p->op==TK_UNION );
@@ -3075,6 +3077,8 @@ static int multiSelect(
         */
         if( p->op==TK_EXCEPT ){
           op = SRT_Except;
+          emptyBypass = sqlite3VdbeAddOp1(v, OP_IfEmpty, unionTab);
+          VdbeCoverage(v);
         }else{
           assert( p->op==TK_UNION );
           op = SRT_Union;
@@ -3117,6 +3121,7 @@ static int multiSelect(
           sqlite3VdbeResolveLabel(v, iCont);
           sqlite3VdbeAddOp2(v, OP_Next, unionTab, iStart); VdbeCoverage(v);
           sqlite3VdbeResolveLabel(v, iBreak);
+          if( emptyBypass ) sqlite3VdbeJumpHere(v, emptyBypass);
           sqlite3VdbeAddOp2(v, OP_Close, unionTab, 0);
         }
         break;
@@ -3128,6 +3133,7 @@ static int multiSelect(
         int addr;
         SelectDest intersectdest;
         int r1;
+        int emptyBypass;
  
         /* INTERSECT is different from the others since it requires
         ** two temporary tables.  Hence it has its own case.  Begin
@@ -3151,6 +3157,7 @@ static int multiSelect(
         if( rc ){
           goto multi_select_end;
         }
+        emptyBypass = sqlite3VdbeAddOp1(v, OP_IfEmpty, tab1); VdbeCoverage(v);
  
         /* Code the current SELECT into temporary table "tab2"
         */
@@ -3194,6 +3201,7 @@ static int multiSelect(
         sqlite3VdbeAddOp2(v, OP_Next, tab1, iStart); VdbeCoverage(v);
         sqlite3VdbeResolveLabel(v, iBreak);
         sqlite3VdbeAddOp2(v, OP_Close, tab2, 0);
+        sqlite3VdbeJumpHere(v, emptyBypass);
         sqlite3VdbeAddOp2(v, OP_Close, tab1, 0);
         break;
       }
diff --git a/src/vdbe.c b/src/vdbe.c
index 0020b52bf..895ef109c 100644
--- a/src/vdbe.c
+++ b/src/vdbe.c
@@ -6398,6 +6398,31 @@ case OP_Rewind: {        /* jump0, ncycle */
   break;
 }
 
+/* Opcode: IfEmpty P1 P2 * * *
+** Synopsis: if( empty(P1) ) goto P2
+**
+** Check to see if the b-tree table that cursor P1 references is empty
+** and jump to P2 if it is.
+*/
+case OP_IfEmpty: {        /* jump */
+  VdbeCursor *pC;
+  BtCursor *pCrsr;
+  int res;
+
+  assert( pOp->p1>=0 && pOp->p1<p->nCursor );
+  assert( pOp->p2>=0 && pOp->p2<p->nOp );
+
+  pC = p->apCsr[pOp->p1];
+  assert( pC!=0 );
+  assert( pC->eCurType==CURTYPE_BTREE );
+  pCrsr = pC->uc.pCursor;
+  assert( pCrsr );
+  res = sqlite3BtreeIsEmpty(pCrsr);
+  VdbeBranchTaken(res!=0,2);
+  if( res ) goto jump_to_p2;
+  break;
+}
+
 /* Opcode: Next P1 P2 P3 * P5
 **
 ** Advance cursor P1 so that it points to the next key/data pair in its