If the LHS for an EXCEPT or INTERSECT operator is empty, skip over the

computation of the RHS.

FossilOrigin-Name: 13f096ae8a850a05d4a8684561066f11693ee66289e6568c44ef32822cca06f6

manifest

@@ -1,5 +1,5 @@
-C Improve\sthe\sbytecode\sfor\sjoins\ssuch\sthat\sit\sexits\searlier\sif\sit\sdetermines\nthat\sno\soutput\sis\spossible.
-D 2025-07-02T11:47:54.123
+C If\sthe\sLHS\sfor\san\sEXCEPT\sor\sINTERSECT\soperator\sis\sempty,\sskip\sover\sthe\ncomputation\sof\sthe\sRHS.
+D 2025-07-02T13:19:24.955
 F .fossil-settings/binary-glob 61195414528fb3ea9693577e1980230d78a1f8b0a54c78cf1b9b24d0a409ed6a x
 F .fossil-settings/empty-dirs dbb81e8fc0401ac46a1491ab34a7f2c7c0452f2f06b54ebb845d024ca8283ef1
 F .fossil-settings/ignore-glob 35175cdfcf539b2318cb04a9901442804be81cd677d8b889fcc9149c21f239ea
@@ -726,8 +726,8 @@ F src/auth.c 54ab9c6c5803b47c0d45b76ce27eff22a03b4b1f767c5945a3a4eb13aa4c78dc
 F src/backup.c 5c97e8023aab1ce14a42387eb3ae00ba5a0644569e3476f38661fa6f824c3523
 F src/bitvec.c e242d4496774dfc88fa278177dd23b607dce369ccafb3f61b41638eea2c9b399
 F src/btmutex.c 30dada73a819a1ef5b7583786370dce1842e12e1ad941e4d05ac29695528daea
-F src/btree.c 81fb44041929a605e293185bd4091cb24c468a108afe8ba1d93a81a8823de47f
-F src/btree.h 18e5e7b2124c23426a283523e5f31a4bff029131b795bb82391f9d2f3136fc50
+F src/btree.c 20c243f1531c806b112dc5f2cd8e2e81618284d8d04897755338ca40e7eaf29b
+F src/btree.h 53a4f92a4c79470c18f88e0cf42755e2a74833044b0b4610aa328083379c7767
 F src/btreeInt.h 9c0f9ea5c9b5f4dcaea18111d43efe95f2ac276cd86d770dce10fd99ccc93886
 F src/build.c 67c1db4c5e89a8519fe9b6dafc287f6bc3627696b5b8536dc5e06db570d8c05f
 F src/callback.c acae8c8dddda41ee85cfdf19b926eefe830f371069f8aadca3aa39adf5b1c859
@@ -785,7 +785,7 @@ F src/printf.c 71b6d3a0093bf23f473e25480ca0024e8962681506c75f4ffd3d343a3f0ab113
 F src/random.c 606b00941a1d7dd09c381d3279a058d771f406c5213c9932bbd93d5587be4b9c
 F src/resolve.c d40fe18d7c2fd0339f5846ffcf7d6809866e380acdf14c76fb2af87e9fe13f64
 F src/rowset.c 8432130e6c344b3401a8874c3cb49fefe6873fec593294de077afea2dce5ec97
-F src/select.c 0258c6c36372e64e3ecc5f9ed4ebb598c0688e112e28f9c9c0f9b61dc6500609
+F src/select.c 68b43b6a2de2440fb05aa4745e164fc2347d739b990b63ba494a7b18fc55b200
 F src/shell.c.in 4f14a1f5196b6006abc8e73cc8fd6c1a62cf940396f8ba909d6711f35f074bb6
 F src/sqlite.h.in 5c54f2461a1ea529bab8499148a2b238e2d4bb571d59e8ea5322d0c190abb693
 F src/sqlite3.rc 015537e6ac1eec6c7050e17b616c2ffe6f70fca241835a84a4f0d5937383c479
@@ -852,7 +852,7 @@ F src/upsert.c 215328c3f91623c520ec8672c44323553f12caeb4f01b1090ebdca99fdf7b4f1
 F src/utf.c 7267c3fb9e2467020507601af3354c2446c61f444387e094c779dccd5ca62165
 F src/util.c 36fb1150062957280777655976f3f9a75db236cb8207a0770ceae8d5ec17fcd3
 F src/vacuum.c 1bacdd0a81d2b5dc1c508fbf0d938c89fa78dd8d5b46ec92686d44030d4f4789
-F src/vdbe.c d2c13c0001f5ec40ec1f010a7f9badcff3ce09bbd7a78528682ad49d8566df54
+F src/vdbe.c 5fb85ec6f2f6cdfb8efac6c4dd4f7eed388dfb5041bf71e1c55f213eb0ed5421
 F src/vdbe.h 93761ed7c6b8bc19524912fd9b9b587d41bf4f1d0ade650a00dadc10518d8958
 F src/vdbeInt.h 0bc581a9763be385e3af715e8c0a503ba8422c2b7074922faf4bb0d6ae31b15e
 F src/vdbeapi.c f9a4881a9674fec3fa13da35044a1484d3c4b95f9ec891cc8ffb02ef2b7a41df
@@ -2208,8 +2208,11 @@ F tool/version-info.c 3b36468a90faf1bbd59c65fd0eb66522d9f941eedd364fabccd7227350
 F tool/warnings-clang.sh bbf6a1e685e534c92ec2bfba5b1745f34fb6f0bc2a362850723a9ee87c1b31a7
 F tool/warnings.sh 1ad0169b022b280bcaaf94a7fa231591be96b514230ab5c98fbf15cd7df842dd
 F tool/win/sqlite.vsix deb315d026cc8400325c5863eef847784a219a2f
-P d27d34fb746280e7e81335db4e195914b15403ef0da7b2955550553dd78fbe9a 63306e447efb3ac17e789a331ed3bb65459eb8b79d66e9c185ba3bd852f34ce3
-R b92b209ccf559f2de96150026272807a
+P 2d2b61cba44a756a3a41ef5c95bbb0c0b7111f4b679c578fec9bd0b214cca367
+R c6049700927657d653d5f08e5f12e785
+T *branch * empty-table-optimizations
+T *sym-empty-table-optimizations *
+T -sym-trunk *
 U drh
-Z 48380f5a742f367bd4ced6b10cb684be
+Z de7126eae59d141656689f55b14fd34e
 # Remove this line to create a well-formed Fossil manifest.

manifest.uuid

@@ -1 +1 @@
-2d2b61cba44a756a3a41ef5c95bbb0c0b7111f4b679c578fec9bd0b214cca367
+13f096ae8a850a05d4a8684561066f11693ee66289e6568c44ef32822cca06f6

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.

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*);

src/select.c

@@ -3036,8 +3036,10 @@ 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 );
         priorOp = SRT_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;
       }

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