SQLite

      pCur->eState = CURSOR_VALID;
      if( pCur->skipNext>0 ) return SQLITE_OK;
    }
  }

  pPage = pCur->pPage;
  idx = ++pCur->ix;
  if( !pPage->isInit ){
  if( !pPage->isInit || sqlite3FaultSim(412) ){
    /* The only known way for this to happen is for there to be a
    ** recursive SQL function that does a DELETE operation as part of a
    ** SELECT which deletes content out from under an active cursor
    ** in a corrupt database file where the table being DELETE-ed from
    ** has pages in common with the table being queried.  See TH3
    ** module cov1/btree78.test testcase 220 (2018-06-08) for an
    ** example. */

    goto jump_to_p2;
  }else if( eqOnly ){
    assert( pOp[1].opcode==OP_IdxLT || pOp[1].opcode==OP_IdxGT );
    pOp++; /* Skip the OP_IdxLt or OP_IdxGT that follows */
  }
  break;
}


/* Opcode: SeekScan  P1 * * * *
** Synopsis: Scan-ahead up to P1 rows
**
** This opcode is a prefix opcode to OP_SeekGE.  In other words, this
** opcode must be immediately followed by OP_SeekGE.  Furthermore, the
** OP_SeekGE must be followed by OP_IdxGT.  These constraints are
** checked by assert() statements.
**
** This opcode uses the P1 through P4 operands of the subsequent
** OP_SeekGE.  In the text that follows, the operands of the subsequent
** OP_SeekGE opcode are denoted as SeekOP.P1 through SeekOP.P4.   Only
** the P1 operand of this opcode is used, and it is denoted as This.P1.
**
** This opcode helps to optimize IN operators on a multi-column index
** where the IN operator is on the later terms of the index by avoiding
** unnecessary seeks on the btree, substituting steps to the next row
** of the b-tree instead.  A correct answer is obtained if this opcode
** is omitted or is a no-op.
**
** The SeekGE.P3 and SeekGE.P4 operands identify an unpacked key which
** is the desired entry that we want the cursor SeekGE.P1 to be pointing
** to.  Call this SeekGE.P4/P5 row the "target".
**
** If the SeekGE.P1 cursor is not currently pointing to a valid row,
** then this opcode is a no-op and control passes through into the OP_SeekGE.
**
** If the SeekGE.P1 cursor is pointing to a valid row, then that row
** might be the target row, or it might be near and slightly before the
** target row.  This opcode attempts to position the cursor on the target
** row by, perhaps stepping by invoking sqlite3BtreeStep() on the cursor
** between 0 and This.P1 times.
**
** There are three possible outcomes from this opcode:<ol>
**
** <li> If after This.P1 steps, the cursor is still point to a place that
**      is earlier in the btree than the target row,
**      then fall through into the subsquence OP_SeekGE opcode.
**
** <li> If the cursor is successfully moved to the target row by 0 or more
**      sqlite3BtreeNext() calls, then jump to the first instruction after the
**      OP_IdxGT opcode - or in other words, skip the next two opcodes.
**
** <li> If the cursor ends up past the target row (indicating the the target
**      row does not exist in the btree) then jump to SeekOP.P2. 
** </ol>
*/
case OP_SeekScan: {
  VdbeCursor *pC;
  int res;
  int n;
  UnpackedRecord r;

  assert( pOp[1].opcode==OP_SeekGE );
  assert( pOp[2].opcode==OP_IdxGT );
  assert( pOp[1].p1==pOp[2].p1 );
  assert( pOp[1].p2==pOp[2].p2 );
  assert( pOp[1].p3==pOp[2].p3 );
  assert( pOp[1].p4.i==pOp[2].p4.i );
  assert( pOp->p1>0 );
  pC = p->apCsr[pOp[1].p1];
  assert( pC!=0 );
  assert( pC->eCurType==CURTYPE_BTREE );
  assert( !pC->isTable );
  if( !sqlite3BtreeCursorIsValidNN(pC->uc.pCursor) ){
#ifdef SQLITE_DEBUG
     if( db->flags&SQLITE_VdbeTrace ){
       printf("... cursor not valid - fall through\n");
     }        
#endif
    break;
  }
  n = pOp->p1;
  assert( n>=1 );
  r.pKeyInfo = pC->pKeyInfo;
  r.nField = (u16)pOp[1].p4.i;
  r.default_rc = 0;
  r.aMem = &aMem[pOp[1].p3];
#ifdef SQLITE_DEBUG
  {
    int i;
    for(i=0; i<r.nField; i++){
      assert( memIsValid(&r.aMem[i]) );
      REGISTER_TRACE(pOp[1].p3+i, &aMem[pOp[1].p3+i]);
    }
  }
#endif
  res = 0;  /* Not needed.  Only used to silence a warning. */
  while(1){
    rc = sqlite3VdbeIdxKeyCompare(db, pC, &r, &res);
    if( rc ) goto abort_due_to_error;
    if( res>0 ){
      seekscan_search_fail:
#ifdef SQLITE_DEBUG
      if( db->flags&SQLITE_VdbeTrace ){
        printf("... %d steps and then skip\n", pOp->p1 - n);
      }        
#endif
      VdbeBranchTaken(1,3);
      pOp++;
      goto jump_to_p2;
    }
    if( res==0 ){
#ifdef SQLITE_DEBUG
      if( db->flags&SQLITE_VdbeTrace ){
        printf("... %d steps and then success\n", pOp->p1 - n);
      }        
#endif
      VdbeBranchTaken(2,3);
      pOp += 2;
      break;
    }
    if( n<=0 ){
#ifdef SQLITE_DEBUG
      if( db->flags&SQLITE_VdbeTrace ){
        printf("... fall through after %d steps\n", pOp->p1);
      }        
#endif
      VdbeBranchTaken(0,3);
      break;
    }
    n--;
    rc = sqlite3BtreeNext(pC->uc.pCursor, 0);
    if( rc ){
      if( rc==SQLITE_DONE ){
        rc = SQLITE_OK;
        goto seekscan_search_fail;
      }else{
        goto abort_due_to_error;
      }
    }
  }
  
  break;
}


/* Opcode: SeekHit P1 P2 P3 * *
** Synopsis: set P2<=seekHit<=P3
**
** Increase or decrease the seekHit value for cursor P1, if necessary,
** so that it is no less than P2 and no greater than P3.
**

    int i;
    for(i=0; i<r.nField; i++){
      assert( memIsValid(&r.aMem[i]) );
      REGISTER_TRACE(pOp->p3+i, &aMem[pOp->p3+i]);
    }
  }
#endif
  res = 0;  /* Not needed.  Only used to silence a warning. */
  rc = sqlite3VdbeIdxKeyCompare(db, pC, &r, &res);

  /* Inlined version of sqlite3VdbeIdxKeyCompare() */
  {
    i64 nCellKey = 0;
    BtCursor *pCur;
    Mem m;

    assert( pC->eCurType==CURTYPE_BTREE );
    pCur = pC->uc.pCursor;
    assert( sqlite3BtreeCursorIsValid(pCur) );
    nCellKey = sqlite3BtreePayloadSize(pCur);
    /* nCellKey will always be between 0 and 0xffffffff because of the way
    ** that btreeParseCellPtr() and sqlite3GetVarint32() are implemented */
    if( nCellKey<=0 || nCellKey>0x7fffffff ){
      rc = SQLITE_CORRUPT_BKPT;
      goto abort_due_to_error;
    }
    sqlite3VdbeMemInit(&m, db, 0);
    rc = sqlite3VdbeMemFromBtreeZeroOffset(pCur, (u32)nCellKey, &m);
    if( rc ) goto abort_due_to_error;
    res = sqlite3VdbeRecordCompareWithSkip(m.n, m.z, &r, 0);
    sqlite3VdbeMemRelease(&m);
  }
  /* End of inlined sqlite3VdbeIdxKeyCompare() */

  assert( (OP_IdxLE&1)==(OP_IdxLT&1) && (OP_IdxGE&1)==(OP_IdxGT&1) );
  if( (pOp->opcode&1)==(OP_IdxLT&1) ){
    assert( pOp->opcode==OP_IdxLE || pOp->opcode==OP_IdxLT );
    res = -res;
  }else{
    assert( pOp->opcode==OP_IdxGE || pOp->opcode==OP_IdxGT );
    res++;
  }
  VdbeBranchTaken(res>0,2);
  if( rc ) goto abort_due_to_error;
  assert( rc==SQLITE_OK );
  if( res>0 ) goto jump_to_p2;
  break;
}

/* Opcode: Destroy P1 P2 P3 * *
**
** Delete an entire database table or index whose root page in the database

        M = pProbe->aiRowLogEst[saved_nEq];
        logK = estLog(nIn);
        safetyMargin = 10;  /* TUNING: extra weight for indexed IN */
        if( M + logK + safetyMargin < nIn + rLogSize ){
          WHERETRACE(0x40,
            ("Scan preferred over IN operator on column %d of \"%s\" (%d<%d)\n",
             saved_nEq, pProbe->zName, M+logK+10, nIn+rLogSize));
          continue;
          pNew->wsFlags |= WHERE_IN_SEEKSCAN;
        }else{
          WHERETRACE(0x40,
            ("IN operator preferred on column %d of \"%s\" (%d>=%d)\n",
             saved_nEq, pProbe->zName, M+logK+10, nIn+rLogSize));
        }
      }
      pNew->wsFlags |= WHERE_COLUMN_IN;

      assert( iIndexCur>=0 );
      if( op ){
        sqlite3VdbeAddOp3(v, op, iIndexCur, pIx->tnum, iDb);
        sqlite3VdbeSetP4KeyInfo(pParse, pIx);
        if( (pLoop->wsFlags & WHERE_CONSTRAINT)!=0
         && (pLoop->wsFlags & (WHERE_COLUMN_RANGE|WHERE_SKIPSCAN))==0
         && (pLoop->wsFlags & WHERE_BIGNULL_SORT)==0
         && (pLoop->wsFlags & WHERE_IN_SEEKSCAN)==0
         && (pWInfo->wctrlFlags&WHERE_ORDERBY_MIN)==0
         && pWInfo->eDistinct!=WHERE_DISTINCT_ORDERED
        ){
          sqlite3VdbeChangeP5(v, OPFLAG_SEEKEQ);
        }
        VdbeComment((v, "%s", pIx->zName));
#ifdef SQLITE_ENABLE_COLUMN_USED_MASK

      struct InLoop *pIn;
      int j;
      sqlite3VdbeResolveLabel(v, pLevel->addrNxt);
      for(j=pLevel->u.in.nIn, pIn=&pLevel->u.in.aInLoop[j-1]; j>0; j--, pIn--){
        sqlite3VdbeJumpHere(v, pIn->addrInTop+1);
        if( pIn->eEndLoopOp!=OP_Noop ){
          if( pIn->nPrefix ){
            int bEarlyOut = 
                (pLoop->wsFlags & WHERE_VIRTUALTABLE)==0
            assert( pLoop->wsFlags & WHERE_IN_EARLYOUT );
                 && (pLoop->wsFlags & WHERE_IN_EARLYOUT)!=0;
            if( pLevel->iLeftJoin ){
              /* For LEFT JOIN queries, cursor pIn->iCur may not have been
              ** opened yet. This occurs for WHERE clauses such as
              ** "a = ? AND b IN (...)", where the index is on (a, b). If
              ** the RHS of the (a=?) is NULL, then the "b IN (...)" may
              ** never have been coded, but the body of the loop run to
              ** return the null-row. So, if the cursor is not open yet,
              ** jump over the OP_Next or OP_Prev instruction about to
              ** be coded.  */
              sqlite3VdbeAddOp2(v, OP_IfNotOpen, pIn->iCur, 
                  sqlite3VdbeCurrentAddr(v) + 2 + 
                  sqlite3VdbeCurrentAddr(v) + 2 + bEarlyOut);
                     ((pLoop->wsFlags & WHERE_VIRTUALTABLE)==0)
              );
              VdbeCoverage(v);
            }
            if( (pLoop->wsFlags & WHERE_VIRTUALTABLE)==0 ){
            if( bEarlyOut ){
              sqlite3VdbeAddOp4Int(v, OP_IfNoHope, pLevel->iIdxCur,
                  sqlite3VdbeCurrentAddr(v)+2,
                  pIn->iBase, pIn->nPrefix);
              VdbeCoverage(v);
            }
          }
          sqlite3VdbeAddOp2(v, pIn->eEndLoopOp, pIn->iCur, pIn->addrInTop);

#define WHERE_MULTI_OR     0x00002000  /* OR using multiple indices */
#define WHERE_AUTO_INDEX   0x00004000  /* Uses an ephemeral index */
#define WHERE_SKIPSCAN     0x00008000  /* Uses the skip-scan algorithm */
#define WHERE_UNQ_WANTED   0x00010000  /* WHERE_ONEROW would have been helpful*/
#define WHERE_PARTIALIDX   0x00020000  /* The automatic index is partial */
#define WHERE_IN_EARLYOUT  0x00040000  /* Perhaps quit IN loops early */
#define WHERE_BIGNULL_SORT 0x00080000  /* Column nEq of index is BIGNULL */
#define WHERE_IN_SEEKSCAN  0x00100000  /* Seek-scan optimization for IN */

#endif /* !defined(SQLITE_WHEREINT_H) */

    VdbeCoverageIf(v, !bRev);
    assert( (pLoop->wsFlags & WHERE_MULTI_OR)==0 );

    pLoop->wsFlags |= WHERE_IN_ABLE;
    if( pLevel->u.in.nIn==0 ){
      pLevel->addrNxt = sqlite3VdbeMakeLabel(pParse);
    }
    if( iEq>0 ){
    if( iEq>0 && (pLoop->wsFlags & WHERE_IN_SEEKSCAN)==0 ){
      pLoop->wsFlags |= WHERE_IN_EARLYOUT;
    }

    i = pLevel->u.in.nIn;
    pLevel->u.in.nIn += nEq;
    pLevel->u.in.aInLoop =
       sqlite3DbReallocOrFree(pParse->db, pLevel->u.in.aInLoop,

            }
          }else{
            pIn->eEndLoopOp = OP_Noop;
          }
          pIn++;
        }
      }
      if( iEq>0 ){
      if( iEq>0 && (pLoop->wsFlags & WHERE_IN_SEEKSCAN)==0 ){
        sqlite3VdbeAddOp3(v, OP_SeekHit, pLevel->iIdxCur, 0, iEq);
      }
    }else{
      pLevel->u.in.nIn = 0;
    }
    sqlite3DbFree(pParse->db, aiMap);
#endif

      if( regBignull ){
        sqlite3VdbeAddOp2(v, OP_Integer, 1, regBignull);
        VdbeComment((v, "NULL-scan pass ctr"));
      }

      op = aStartOp[(start_constraints<<2) + (startEq<<1) + bRev];
      assert( op!=0 );
      if( (pLoop->wsFlags & WHERE_IN_SEEKSCAN)!=0 ){
        assert( op==OP_SeekGE );
        assert( regBignull==0 );
        /* TUNING:  The OP_SeekScan opcode seeks to reduce the number
        ** of expensive seek operations by replacing a single seek with
        ** 1 or more step operations.  The question is, how many steps
        ** should we try before giving up and going with a seek.  The cost
        ** of a seek is proportional to the logarithm of the of the number
        ** of entries in the tree, so basing the number of steps to try
        ** on the estimated number of rows in the btree seems like a good
        ** guess. */
        sqlite3VdbeAddOp1(v, OP_SeekScan, (pIdx->aiRowLogEst[0]+9)/10);
        VdbeCoverage(v);
      }
      sqlite3VdbeAddOp4Int(v, op, iIdxCur, addrNxt, regBase, nConstraint);
      VdbeCoverage(v);
      VdbeCoverageIf(v, op==OP_Rewind);  testcase( op==OP_Rewind );
      VdbeCoverageIf(v, op==OP_Last);    testcase( op==OP_Last );
      VdbeCoverageIf(v, op==OP_SeekGT);  testcase( op==OP_SeekGT );
      VdbeCoverageIf(v, op==OP_SeekGE);  testcase( op==OP_SeekGE );
      VdbeCoverageIf(v, op==OP_SeekLE);  testcase( op==OP_SeekLE );

                           nConstraint+bSeekPastNull);
      testcase( op==OP_IdxGT );  VdbeCoverageIf(v, op==OP_IdxGT );
      testcase( op==OP_IdxGE );  VdbeCoverageIf(v, op==OP_IdxGE );
      testcase( op==OP_IdxLT );  VdbeCoverageIf(v, op==OP_IdxLT );
      testcase( op==OP_IdxLE );  VdbeCoverageIf(v, op==OP_IdxLE );
    }

    if( pLoop->wsFlags & WHERE_IN_EARLYOUT ){
    if( (pLoop->wsFlags & WHERE_IN_EARLYOUT)!=0 ){
      sqlite3VdbeAddOp3(v, OP_SeekHit, iIdxCur, nEq, nEq);
    }

    /* Seek the table cursor, if required */
    omitTable = (pLoop->wsFlags & WHERE_IDX_ONLY)!=0 
           && (pWInfo->wctrlFlags & WHERE_OR_SUBCLAUSE)==0;
    if( omitTable ){