SQLite

      VdbeCoverage(v);
      VdbeCoverageIf(v, pLevel->op==OP_Next);
      VdbeCoverageIf(v, pLevel->op==OP_Prev);
      VdbeCoverageIf(v, pLevel->op==OP_VNext);
      if( pLevel->regBignull ){
        sqlite3VdbeResolveLabel(v, pLevel->addrBignull);
        addr = sqlite3VdbeAddOp1(v, OP_If, pLevel->regBignull);
        VdbeComment((v, "If NULL-scan done"));
        VdbeCoverage(v);
        sqlite3VdbeAddOp2(v, OP_Integer, 1, pLevel->regBignull);
        sqlite3VdbeAddOp2(v, OP_Goto, 0, pLevel->p2-1);
        VdbeComment((v, "Do the NULL-scan"));
        sqlite3VdbeJumpHere(v, addr);
      }
#ifndef SQLITE_DISABLE_SKIPAHEAD_DISTINCT
      if( addrSeek ) sqlite3VdbeJumpHere(v, addrSeek);
#endif
    }else{
      sqlite3VdbeResolveLabel(v, pLevel->addrCont);

  int iIdxCur;          /* The VDBE cursor used to access pIdx */
  int addrBrk;          /* Jump here to break out of the loop */
  int addrNxt;          /* Jump here to start the next IN combination */
  int addrSkip;         /* Jump here for next iteration of skip-scan */
  int addrCont;         /* Jump here to continue with the next loop cycle */
  int addrFirst;        /* First instruction of interior of the loop */
  int addrBody;         /* Beginning of the body of this loop */
  int regBignull;       /* big-null flag reg. True if NULLs not yet scanned */
  int addrBignull;      /* Jump here for next part of big-null scan */
#ifndef SQLITE_LIKE_DOESNT_MATCH_BLOBS
  u32 iLikeRepCntr;     /* LIKE range processing counter register (times 2) */
  int addrLikeRep;      /* LIKE range processing address */
#endif
  u8 iFrom;             /* Which entry in the FROM clause */
  u8 op, p3, p5;        /* Opcode, P3 & P5 of the opcode that ends the loop */

    int nExtraReg = 0;           /* Number of extra registers needed */
    int op;                      /* Instruction opcode */
    char *zStartAff;             /* Affinity for start of range constraint */
    char *zEndAff = 0;           /* Affinity for end of range constraint */
    u8 bSeekPastNull = 0;        /* True to seek past initial nulls */
    u8 bStopAtNull = 0;          /* Add condition to terminate at NULLs */
    int omitTable;               /* True if we use the index only */
    int regBignull = 0;          /* big-null flag register */

    pIdx = pLoop->u.btree.pIndex;
    iIdxCur = pLevel->iIdxCur;
    assert( nEq>=pLoop->nSkip );

    /* Find any inequality constraint terms for the start and end 
    ** of the range. 

      ** so no further seeking is needed */
    }else{
      if( pLoop->wsFlags & WHERE_IN_EARLYOUT ){
        sqlite3VdbeAddOp1(v, OP_SeekHit, iIdxCur);
      }
      if( regBignull ){
        sqlite3VdbeAddOp2(v, OP_Integer, 0, regBignull);
        VdbeComment((v, "NULL-scan flag"));
      }

      op = aStartOp[(start_constraints<<2) + (startEq<<1) + bRev];
      assert( op!=0 );
      sqlite3VdbeAddOp4Int(v, op, iIdxCur, addrNxt, regBase, nConstraint);
      VdbeCoverage(v);
      VdbeCoverageIf(v, op==OP_Rewind);  testcase( op==OP_Rewind );

    /* Top of the loop body */
    pLevel->p2 = sqlite3VdbeCurrentAddr(v);

    /* Check if the index cursor is past the end of the range. */
    if( nConstraint ){
      if( regBignull ){
        sqlite3VdbeAddOp2(v, OP_If, regBignull, sqlite3VdbeCurrentAddr(v)+3);
        VdbeComment((v, "If in NULL-scan"));
        VdbeCoverage(v);
      }
      op = aEndOp[bRev*2 + endEq];
      sqlite3VdbeAddOp4Int(v, op, iIdxCur, addrNxt, regBase, nConstraint);
      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( regBignull ){
      sqlite3VdbeAddOp2(v, OP_IfNot, regBignull, sqlite3VdbeCurrentAddr(v)+2);
      VdbeComment((v, "If not in NULL-scan"));
      VdbeCoverage(v);
      if( bStopAtNull ){
        op = aEndOp[bRev*2 + 0];
        assert( op==OP_IdxGE || op==OP_IdxLE );
        sqlite3VdbeAddOp4Int(v, op, iIdxCur, addrNxt, regBase, nConstraint);
        testcase( op==OP_IdxGE );  VdbeCoverageIf(v, op==OP_IdxGE );
        testcase( op==OP_IdxLE );  VdbeCoverageIf(v, op==OP_IdxLE );