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Overview
Comment:Invert the meaning of the regBignull flag so that it is 1 when doing the normal scan and 1 when scanning nulls. This enables the re-do jump at the bottom of the loop to be coded with a single OP_IfNotZero opcode, rather than a sequence of OP_If, OP_Integer, OP_Goto.
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SHA3-256: bf875e1a259a4167694e06349458452dc36c1d38aa6843518d9ae46ce74e5559
User & Date: drh 2019-08-23 13:32:03
Context
2019-08-23
16:12
Simplification of the WHERE loop code generator for NULLS LAST saves a few CPU cycles and about a hundred bytes of code space. check-in: e3ed2f49 user: drh tags: nulls-last
13:32
Invert the meaning of the regBignull flag so that it is 1 when doing the normal scan and 1 when scanning nulls. This enables the re-do jump at the bottom of the loop to be coded with a single OP_IfNotZero opcode, rather than a sequence of OP_If, OP_Integer, OP_Goto. check-in: bf875e1a user: drh tags: nulls-last
13:08
Extra VDBE comments in the NULLS LAST logic provide landmarks to help understand the EXPLAIN output. check-in: 649b08ea user: drh tags: nulls-last
Changes
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Changes to src/where.c.

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      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);
        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);
    }







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      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);




        sqlite3VdbeAddOp2(v, OP_IfNotZero, pLevel->regBignull, pLevel->p2-1);


      }
#ifndef SQLITE_DISABLE_SKIPAHEAD_DISTINCT
      if( addrSeek ) sqlite3VdbeJumpHere(v, addrSeek);
#endif
    }else{
      sqlite3VdbeResolveLabel(v, pLevel->addrCont);
    }

Changes to src/whereInt.h.

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  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 */







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  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 a NULL-scan is needed */
  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 */

Changes to src/wherecode.c.

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      ** 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);
        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 );







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      ** 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, 1, regBignull);
        VdbeComment((v, "NULL-scan needed 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 );
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    /* 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_IfNot, regBignull, sqlite3VdbeCurrentAddr(v)+3);
        VdbeComment((v, "If NULL-scan active"));
        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_If, regBignull, sqlite3VdbeCurrentAddr(v)+2);
      VdbeComment((v, "If NULL-scan pending"));
      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 );