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Overview
Comment:Additional simplifications of the WHERE loop code generator logic for NULLS LAST.
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SHA3-256: 1383680d927d7d703933be7653c0019199845e7f7e82bcc76d9ca3808093e9d8
User & Date: drh 2019-08-23 17:00:22
Context
2019-08-23
17:09
Revise the VDBE comments for NULL-scan so that they also work make sense when reading a NULLS FIRST plan. check-in: bfe79378 user: drh tags: nulls-last
17:00
Additional simplifications of the WHERE loop code generator logic for NULLS LAST. check-in: 1383680d user: drh tags: nulls-last
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
Changes
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Changes to src/wherecode.c.

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      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 );

      assert( bSeekPastNull==0 || bStopAtNull==0 );
      if( regBignull ){
        assert( bSeekPastNull==1 || bStopAtNull==1 );

        assert( bStopAtNull==startEq );
        sqlite3VdbeAddOp2(v, OP_Goto, 0, sqlite3VdbeCurrentAddr(v)+2);
        op = aStartOp[(nConstraint>1)*4 + 2 + bRev];
        sqlite3VdbeAddOp4Int(v, op, iIdxCur, addrNxt, regBase, 
                             nConstraint-startEq);
        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_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 );
      }else{
        op = aEndOp[bRev*2 + endEq];
        sqlite3VdbeAddOp4Int(v, op, iIdxCur, addrNxt, regBase, nConstraint+1);
        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 ){
      sqlite3VdbeAddOp2(v, OP_SeekHit, iIdxCur, 1);
    }

    /* Seek the table cursor, if required */







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      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 );

      assert( bSeekPastNull==0 || bStopAtNull==0 );
      if( regBignull ){
        assert( bSeekPastNull==1 || bStopAtNull==1 );
        assert( bSeekPastNull==!bStopAtNull );
        assert( bStopAtNull==startEq );
        sqlite3VdbeAddOp2(v, OP_Goto, 0, sqlite3VdbeCurrentAddr(v)+2);
        op = aStartOp[(nConstraint>1)*4 + 2 + bRev];
        sqlite3VdbeAddOp4Int(v, op, iIdxCur, addrNxt, regBase, 
                             nConstraint-startEq);
        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 ){
        /* Except, skip the end-of-range check while doing the NULL-scan */
        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 ){
      /* During a NULL-scan, check to see if we have reached the end of
      ** the NULLs */
      assert( bSeekPastNull==!bStopAtNull );
      assert( bSeekPastNull+bStopAtNull==1 );
      assert( nConstraint+bSeekPastNull>0 );
      sqlite3VdbeAddOp2(v, OP_If, regBignull, sqlite3VdbeCurrentAddr(v)+2);
      VdbeComment((v, "If NULL-scan pending"));
      VdbeCoverage(v);

      op = aEndOp[bRev*2 + bSeekPastNull];

      sqlite3VdbeAddOp4Int(v, op, iIdxCur, addrNxt, regBase,



                           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 ){
      sqlite3VdbeAddOp2(v, OP_SeekHit, iIdxCur, 1);
    }

    /* Seek the table cursor, if required */