SQLite

        }

        /* Get the column number in the table (iColumn) and sort order
        ** (revIdx) for the j-th column of the index.
        */
        if( pIndex ){
          iColumn = pIndex->aiColumn[j];
          revIdx = pIndex->aSortOrder[j] & KEYINFO_ORDER_DESC;
          if( iColumn==pIndex->pTable->iPKey ) iColumn = XN_ROWID;
        }else{
          iColumn = XN_ROWID;
          revIdx = 0;
        }

        /* An unconstrained column that might be NULL means that this

            if( pOBExpr->iColumn!=iColumn ) continue;
          }else{
            Expr *pIdxExpr = pIndex->aColExpr->a[j].pExpr;
            if( sqlite3ExprCompareSkip(pOBExpr, pIdxExpr, iCur) ){
              continue;
            }
          }

          if( iColumn!=XN_ROWID ){
            pColl = sqlite3ExprNNCollSeq(pWInfo->pParse, pOrderBy->a[i].pExpr);
            if( sqlite3StrICmp(pColl->zName, pIndex->azColl[j])!=0 ) continue;
          }
          if( wctrlFlags & WHERE_DISTINCTBY ){
            pLoop->u.btree.nDistinctCol = j+1;
          }
          isMatch = 1;
          break;
        }
        if( isMatch && (wctrlFlags & WHERE_GROUPBY)==0 ){
          /* Make sure the sort order is compatible in an ORDER BY clause.
          ** Sort order is irrelevant for a GROUP BY clause. */

          if( revSet ){
            if( (rev ^ revIdx)!=(pOrderBy->a[i].sortFlags&KEYINFO_ORDER_DESC) ){
              isMatch = 0;
            }
          }else{
            rev = revIdx ^ (pOrderBy->a[i].sortFlags & KEYINFO_ORDER_DESC);
            if( rev ) *pRevMask |= MASKBIT(iLoop);
            revSet = 1;
          }
        }
        if( isMatch && (pOrderBy->a[i].sortFlags & KEYINFO_ORDER_BIGNULL) ){
          if( j==pLoop->u.btree.nEq ){
            pLoop->wsFlags |= WHERE_BIGNULL_SORT;
          }else{
            isMatch = 0;
          }
        }
        if( isMatch ){
          if( iColumn==XN_ROWID ){
            testcase( distinctColumns==0 );
            distinctColumns = 1;
          }
          obSat |= MASKBIT(i);



        }else{
          /* No match found */
          if( j==0 || j<nKeyCol ){
            testcase( isOrderDistinct!=0 );
            isOrderDistinct = 0;
          }
          break;

      assert( pIx->pSchema==pTab->pSchema );
      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
         && (pWInfo->wctrlFlags&WHERE_ORDERBY_MIN)==0
         && pWInfo->eDistinct!=WHERE_DISTINCT_ORDERED
        ){
          sqlite3VdbeChangeP5(v, OPFLAG_SEEKEQ); /* Hint to COMDB2 */
        }
        VdbeComment((v, "%s", pIx->zName));
#ifdef SQLITE_ENABLE_COLUMN_USED_MASK

      sqlite3VdbeResolveLabel(v, pLevel->addrCont);
      sqlite3VdbeAddOp3(v, pLevel->op, pLevel->p1, pLevel->p2, pLevel->p3);
      sqlite3VdbeChangeP5(v, pLevel->p5);
      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);
        sqlite3VdbeAddOp2(v, OP_Integer, 1, pLevel->regBignull);
        sqlite3VdbeAddOp2(v, OP_Goto, 0, pLevel->p2-1);
        sqlite3VdbeChangeP5(v, pLevel->p5);
        VdbeCoverage(v);
        VdbeCoverageIf(v, pLevel->op==OP_Next);
        VdbeCoverageIf(v, pLevel->op==OP_Prev);
        sqlite3VdbeJumpHere(v, addr);
      }
#ifndef SQLITE_DISABLE_SKIPAHEAD_DISTINCT
      if( addrSeek ) sqlite3VdbeJumpHere(v, addrSeek);
#endif
    }else{
      sqlite3VdbeResolveLabel(v, pLevel->addrCont);
    }
    if( pLoop->wsFlags & WHERE_IN_ABLE && pLevel->u.in.nIn>0 ){

  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 register */
  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 p1, p2;           /* Operands of the opcode used to end the loop */
  union {               /* Information that depends on pWLoop->wsFlags */
    struct {
      int nIn;              /* Number of entries in aInLoop[] */
      struct InLoop {
        int iCur;              /* The VDBE cursor used by this IN operator */
        int addrInTop;         /* Top of the IN loop */
        int iBase;             /* Base register of multi-key index record */

#define WHERE_ONEROW       0x00001000  /* Selects no more than one row */
#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 */

#endif /* !defined(SQLITE_WHEREINT_H) */

    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;

    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. 
    */
    j = nEq;
    if( pLoop->wsFlags & WHERE_BTM_LIMIT ){
      pRangeStart = pLoop->aLTerm[j++];
      nExtraReg = MAX(nExtraReg, pLoop->u.btree.nBtm);

        j = pIdx->aiColumn[nEq];
        if( (j>=0 && pIdx->pTable->aCol[j].notNull==0) || j==XN_EXPR ){
          bSeekPastNull = 1;
        }
      }
    }
    assert( pRangeEnd==0 || (pRangeEnd->wtFlags & TERM_VNULL)==0 );

    /* If the WHERE_BIGNULL_SORT flag is set, then index column nEq uses
    ** a non-default "big-null" sort (either ASC NULLS LAST or DESC NULLS 
    ** FIRST). In both cases separate ordered scans are made of those
    ** index entries for which the column is null and for those for which
    ** it is not. For an ASC sort, the non-NULL entries are scanned first.
    ** For DESC, NULL entries are scanned first.
    */
    addrNxt = pLevel->addrNxt;
    if( (pLoop->wsFlags & (WHERE_TOP_LIMIT|WHERE_BTM_LIMIT))==0
     && (pLoop->wsFlags & WHERE_BIGNULL_SORT)!=0
    ){
      assert( bSeekPastNull==0 && nExtraReg==0 && nBtm==0 && nTop==0 );
      assert( pRangeEnd==0 && pRangeStart==0 );
      assert( pLoop->nSkip==0 );
      nExtraReg = 1;
      bSeekPastNull = 1;
      pLevel->regBignull = regBignull = ++pParse->nMem;
      addrNxt = pLevel->addrBignull = sqlite3VdbeMakeLabel(pParse);
    }

    /* If we are doing a reverse order scan on an ascending index, or
    ** a forward order scan on a descending index, interchange the 
    ** start and end terms (pRangeStart and pRangeEnd).
    */
    if( (nEq<pIdx->nKeyCol && bRev==(pIdx->aSortOrder[nEq]==SQLITE_SO_ASC))
     || (bRev && pIdx->nKeyCol==nEq)

    */
    codeCursorHint(pTabItem, pWInfo, pLevel, pRangeEnd);
    regBase = codeAllEqualityTerms(pParse,pLevel,bRev,nExtraReg,&zStartAff);
    assert( zStartAff==0 || sqlite3Strlen30(zStartAff)>=nEq );
    if( zStartAff && nTop ){
      zEndAff = sqlite3DbStrDup(db, &zStartAff[nEq]);
    }


    testcase( pRangeStart && (pRangeStart->eOperator & WO_LE)!=0 );
    testcase( pRangeStart && (pRangeStart->eOperator & WO_GE)!=0 );
    testcase( pRangeEnd && (pRangeEnd->eOperator & WO_LE)!=0 );
    testcase( pRangeEnd && (pRangeEnd->eOperator & WO_GE)!=0 );
    startEq = !pRangeStart || pRangeStart->eOperator & (WO_LE|WO_GE);
    endEq =   !pRangeEnd || pRangeEnd->eOperator & (WO_LE|WO_GE);

      }
      bSeekPastNull = 0;
    }else if( bSeekPastNull ){
      sqlite3VdbeAddOp2(v, OP_Null, 0, regBase+nEq);
      nConstraint++;
      startEq = 0;
      start_constraints = 1;
    }else if( regBignull ){
      sqlite3VdbeAddOp2(v, OP_Null, 0, regBase+nEq);
      start_constraints = 1;
      nConstraint++;
    }
    codeApplyAffinity(pParse, regBase, nConstraint - bSeekPastNull, zStartAff);
    if( pLoop->nSkip>0 && nConstraint==pLoop->nSkip ){
      /* The skip-scan logic inside the call to codeAllEqualityConstraints()
      ** above has already left the cursor sitting on the correct row,
      ** 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);
      }

      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 );
      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 );
      VdbeCoverageIf(v, op==OP_SeekLT);  testcase( op==OP_SeekLT );

      if( regBignull ){








        sqlite3VdbeAddOp2(v, OP_Goto, 0, sqlite3VdbeCurrentAddr(v)+2);
        if( bStopAtNull ){
          start_constraints = (nConstraint>1);
          op = aStartOp[(start_constraints<<2) + (startEq<<1) + bRev];

          sqlite3VdbeAddOp4Int(v, op, iIdxCur, addrNxt, regBase, nConstraint-1);
        }else{
          op = aStartOp[(start_constraints<<2) + ((!startEq)<<1) + bRev];
          sqlite3VdbeAddOp4Int(v, op, iIdxCur, addrNxt, regBase, nConstraint);


        }
      }
    }

    /* Load the value for the inequality constraint at the end of the
    ** range (if any).
    */
    nConstraint = nEq;

      if( sqlite3ExprIsVector(pRight)==0 ){
        disableTerm(pLevel, pRangeEnd);
      }else{
        endEq = 1;
      }
    }else if( bStopAtNull ){
      if( regBignull==0 ){
        sqlite3VdbeAddOp2(v, OP_Null, 0, regBase+nEq);
        endEq = 0;
      }
      nConstraint++;
    }
    sqlite3DbFree(db, zStartAff);
    sqlite3DbFree(db, zEndAff);

    /* 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);
      }
      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);
      if( bStopAtNull ){
        op = aEndOp[bRev*2 + 0];
        sqlite3VdbeAddOp4Int(v, op, iIdxCur, addrNxt, regBase, nConstraint);
      }else{
        op = aEndOp[bRev*2 + endEq];
        sqlite3VdbeAddOp4Int(v, op, iIdxCur, addrNxt, regBase, nConstraint+1);
      }
    }

    if( pLoop->wsFlags & WHERE_IN_EARLYOUT ){
      sqlite3VdbeAddOp2(v, OP_SeekHit, iIdxCur, 1);
    }

    /* Seek the table cursor, if required */

# 2019 August 10
#
# 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.
#





set testdir [file dirname $argv0]
source $testdir/tester.tcl
set testprefix nulls1

if 1 {

do_execsql_test 1.0 {
  DROP TABLE IF EXISTS t3;
  CREATE TABLE t3(a INTEGER);
  INSERT INTO t3 VALUES(NULL), (10), (30), (20), (NULL);
} {}

for {set a 0} {$a < 3} {incr a} {
  foreach {tn limit} {
    1 ""
    2 "LIMIT 10"
  } {
    do_execsql_test 1.$a.$tn.1 "
      SELECT a FROM t3 ORDER BY a nULLS FIRST $limit
    " {{}   {}   10   20   30}

    " {{}   {}   30   20   10}
    
    do_execsql_test 1.$a.$tn.4 "
      SELECT a FROM t3 ORDER BY a DESC nULLS LAST $limit
    " {30   20   10   {}   {}}
  }

  switch $a {
    0 {
      execsql { CREATE INDEX i1 ON t3(a) }
    }
    1 {
      execsql { DROP INDEX i1 ; CREATE INDEX i1 ON t3(a DESC) }
    }
  }
}

}

#-------------------------------------------------------------------------
reset_db
do_execsql_test 2.0 {
  CREATE TABLE t2(a, b, c);
  CREATE INDEX i2 ON t2(a, b);
  INSERT INTO t2 VALUES(1, 1, 1);
  INSERT INTO t2 VALUES(1, NULL, 2);
  INSERT INTO t2 VALUES(1, NULL, 3);
  INSERT INTO t2 VALUES(1, 4, 4);
}

do_execsql_test 2.1 {
  SELECT * FROM t2 WHERE a=1 ORDER BY b NULLS LAST
} {
  1 1 1    1 4 4   1 {} 2   1 {} 3
}

do_execsql_test 2.2 {
  SELECT * FROM t2 WHERE a=1 ORDER BY b DESC NULLS FIRST
} {
  1 {} 3
  1 {} 2     
  1 4 4     
  1 1 1
}


finish_test