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SQLite training in Houston TX on 2019-11-05 (details)
Part of the 2019 Tcl Conference

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Overview
Comment:Revise the VDBE comments for NULL-scan so that they also work make sense when reading a NULLS FIRST plan.
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Timelines: family | ancestors | descendants | both | nulls-last
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SHA3-256: bfe793780fa6fc7a1e7d0ee60606dda272985de9573887dca118e9510fc609f2
User & Date: drh 2019-08-23 17:09:02
Context
2019-08-23
20:33
Move some things in parse.y so that TK_COLUMN and TK_AGG_COLUMN are assigned the same values as they are on trunk for a very small speedup. check-in: d26fdfa3 user: dan tags: nulls-last
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
Changes
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Changes to src/wherecode.c.

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      ** 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 );
................................................................................
    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 );
................................................................................
    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 );







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      ** 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 pass ctr"));
      }

      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 );
................................................................................
    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 2nd pass"));
        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 );
................................................................................
    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 1st pass"));
      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 );