/ Check-in [9af50a87]
Login

Many hyperlinks are disabled.
Use anonymous login to enable hyperlinks.

Overview
Comment:Improvements to comments in the multi-threaded sorter. Also include a function name change for clarity. And add a test to help show that the MergeEngine object is only used by a single thread.
Downloads: Tarball | ZIP archive | SQL archive
Timelines: family | ancestors | descendants | both | threads
Files: files | file ages | folders
SHA1:9af50a878f67c1c2a4f1520160cc989650d7196a
User & Date: drh 2014-07-28 14:54:50
Context
2014-07-28
15:01
Merge recent trunk changes into the threads branch. check-in: 163c247b user: drh tags: threads
14:54
Improvements to comments in the multi-threaded sorter. Also include a function name change for clarity. And add a test to help show that the MergeEngine object is only used by a single thread. check-in: 9af50a87 user: drh tags: threads
2014-07-24
16:54
Merge all recent trunk changes into the threads branch. check-in: 77068589 user: drh tags: threads
Changes
Hide Diffs Unified Diffs Show Whitespace Changes Patch

Changes to src/vdbesort.c.

81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
...
186
187
188
189
190
191
192

193
194
195
196
197
198
199
...
243
244
245
246
247
248
249

250
251
252
253
254

255
256
257
258


259
260
261
262
263
264
265
...
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
...
590
591
592
593
594
595
596

597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
...
633
634
635
636
637
638
639

640
641
642
643
644
645
646
...
664
665
666
667
668
669
670

671
672
673
674
675
676
677
678
679
680

681
682
683
684
685
686
687
....
1022
1023
1024
1025
1026
1027
1028
1029




1030
1031
1032
1033
1034
1035
1036
....
1037
1038
1039
1040
1041
1042
1043

1044
1045
1046
1047
1048
1049
1050
....
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453




1454
1455
1456
1457
1458
1459
1460
....
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
....
2124
2125
2126
2127
2128
2129
2130
2131



2132
2133
2134
2135
2136
2137
2138
....
2332
2333
2334
2335
2336
2337
2338
2339
2340
2341
2342
2343
2344
2345
2346
** A PMA created at this point is known as a "level-0 PMA". Higher levels
** of PMAs may be created by merging existing PMAs together - for example
** merging two or more level-0 PMAs together creates a level-1 PMA.
**
** The threshold for the amount of main memory to use before flushing 
** records to a PMA is roughly the same as the limit configured for the
** page-cache of the main database. Specifically, the threshold is set to 
** the value returned multiplied by "PRAGMA main.page_size" multipled by 
** that returned by "PRAGMA main.cache_size", in bytes.
**
** If the sorter is running in single-threaded mode, then all PMAs generated
** are appended to a single temporary file. Or, if the sorter is running in
** multi-threaded mode then up to (N+1) temporary files may be opened, where
** N is the configured number of worker threads. In this case, instead of
** sorting the records and writing the PMA to a temporary file itself, the
................................................................................
** The MergeEngine object is used to combine two or more smaller PMAs into
** one big PMA using a merge operation.  Separate PMAs all need to be
** combined into one big PMA in order to be able to step through the sorted
** records in order.
**
** The aReadr[] array contains a PmaReader object for each of the PMAs being
** merged.  An aReadr[] object either points to a valid key or else is at EOF.

** For the purposes of the paragraphs below, we assume that the array is
** actually N elements in size, where N is the smallest power of 2 greater
** to or equal to the number of PMAs being merged. The extra aReadr[] elements
** are treated as if they are empty (always at EOF).
**
** The aTree[] array is also N elements in size. The value of N is stored in
** the MergeEngine.nTree variable.
................................................................................
**
** In other words, each time we advance to the next sorter element, log2(N)
** key comparison operations are required, where N is the number of segments
** being merged (rounded up to the next power of 2).
*/
struct MergeEngine {
  int nTree;                 /* Used size of aTree/aReadr (power of 2) */

  int *aTree;                /* Current state of incremental merge */
  PmaReader *aReadr;         /* Array of PmaReaders to merge data from */
};

/*

** Exactly VdbeSorter.nTask instances of this object are allocated
** as part of each VdbeSorter object. Instances are never allocated any
** other way. VdbeSorter.nTask is set to the number of worker threads allowed
** (see SQLITE_CONFIG_WORKER_THREADS) plus one (the main thread).


**
** Essentially, this structure contains all those fields of the VdbeSorter
** structure for which each thread requires a separate instance. For example,
** each thread requries its own UnpackedRecord object to unpack records in
** as part of comparison operations.
**
** Before a background thread is launched, variable bDone is set to 0. Then, 
................................................................................
/* Maximum number of PMAs that a single MergeEngine can merge */
#define SORTER_MAX_MERGE_COUNT 16

static int vdbeIncrSwap(IncrMerger*);
static void vdbeIncrFree(IncrMerger *);

/*
** Free all memory belonging to the PmaReader object passed as the second
** argument. All structure fields are set to zero before returning.
*/
static void vdbePmaReaderClear(PmaReader *pReadr){
  sqlite3_free(pReadr->aAlloc);
  sqlite3_free(pReadr->aBuffer);
  if( pReadr->aMap ) sqlite3OsUnfetch(pReadr->pFile, 0, pReadr->aMap);
  vdbeIncrFree(pReadr->pIncr);
  memset(pReadr, 0, sizeof(PmaReader));
}

/*
** Read nByte bytes of data from the stream of data iterated by object p.
** If successful, set *ppOut to point to a buffer containing the data
** and return SQLITE_OK. Otherwise, if an error occurs, return an SQLite
** error code.
**
** The buffer indicated by *ppOut may only be considered valid until the
** next call to this function.
*/
static int vdbePmaReadBlob(
  PmaReader *p,                   /* PmaReader from which to take the blob */
  int nByte,                      /* Bytes of data to read */
  u8 **ppOut                      /* OUT: Pointer to buffer containing data */
){
................................................................................
** Or, if an error occurs, return an SQLite error code. The final value of
** *pp is undefined in this case.
*/
static int vdbeSorterMapFile(SortSubtask *pTask, SorterFile *pFile, u8 **pp){
  int rc = SQLITE_OK;
  if( pFile->iEof<=(i64)(pTask->pSorter->db->nMaxSorterMmap) ){
    rc = sqlite3OsFetch(pFile->pFd, 0, (int)pFile->iEof, (void**)pp);

  }
  return rc;
}

/*
** Seek PmaReader pReadr to offset iOff within file pFile. Return SQLITE_OK 
** if successful, or an SQLite error code if an error occurs.
*/
static int vdbePmaReaderSeek(
  SortSubtask *pTask,             /* Task context */
  PmaReader *pReadr,              /* Iterate to populate */
  SorterFile *pFile,              /* Sorter file to read from */
  i64 iOff                        /* Offset in pFile */
){
  int rc = SQLITE_OK;

  assert( pReadr->pIncr==0 || pReadr->pIncr->bEof==0 );

................................................................................
      int nRead = pgsz - iBuf;
      if( (pReadr->iReadOff + nRead) > pReadr->iEof ){
        nRead = (int)(pReadr->iEof - pReadr->iReadOff);
      }
      rc = sqlite3OsRead(
          pReadr->pFile, &pReadr->aBuffer[iBuf], nRead, pReadr->iReadOff
      );

    }
  }

  return rc;
}

/*
................................................................................
        bEof = 0;
      }
    }

    if( bEof ){
      /* This is an EOF condition */
      vdbePmaReaderClear(pReadr);

      return rc;
    }
  }

  if( rc==SQLITE_OK ){
    rc = vdbePmaReadVarint(pReadr, &nRec);
  }
  if( rc==SQLITE_OK ){
    pReadr->nKey = (int)nRec;
    rc = vdbePmaReadBlob(pReadr, (int)nRec, &pReadr->aKey);

  }

  return rc;
}

/*
** Initialize PmaReader pReadr to scan through the PMA stored in file pFile
................................................................................
}
#else
# define vdbeSorterJoinAll(x,rcin) (rcin)
# define vdbeSorterJoinThread(pTask) SQLITE_OK
#endif

/*
** Allocate a new MergeEngine object with space for nReader PmaReaders.




*/
static MergeEngine *vdbeMergeEngineNew(int nReader){
  int N = 2;                      /* Smallest power of two >= nReader */
  int nByte;                      /* Total bytes of space to allocate */
  MergeEngine *pNew;              /* Pointer to allocated object to return */

  assert( nReader<=SORTER_MAX_MERGE_COUNT );
................................................................................

  while( N<nReader ) N += N;
  nByte = sizeof(MergeEngine) + N * (sizeof(int) + sizeof(PmaReader));

  pNew = sqlite3FaultSim(100) ? 0 : (MergeEngine*)sqlite3MallocZero(nByte);
  if( pNew ){
    pNew->nTree = N;

    pNew->aReadr = (PmaReader*)&pNew[1];
    pNew->aTree = (int*)&pNew->aReadr[N];
  }
  return pNew;
}

/*
................................................................................
  vdbeSorterWorkDebug(pTask, "exit");
  assert( rc!=SQLITE_OK || pList->pList==0 );
  assert( rc!=SQLITE_OK || pTask->file.iEof==iSz );
  return rc;
}

/*
** Advance the MergeEngine PmaReader passed as the second argument to
** the next entry. Set *pbEof to true if this means the PmaReader has 
** reached EOF.
**
** Return SQLITE_OK if successful or an error code if an error occurs.
*/
static int vdbeSorterNext(
  SortSubtask *pTask, 
  MergeEngine *pMerger, 
  int *pbEof
){
  int rc;
  int iPrev = pMerger->aTree[1];/* Index of PmaReader to advance */





  /* Advance the current PmaReader */
  rc = vdbePmaReaderNext(&pMerger->aReadr[iPrev]);

  /* Update contents of aTree[] */
  if( rc==SQLITE_OK ){
    int i;                      /* Index of aTree[] to recalculate */
................................................................................
    ** In either case exit the loop. */
    if( pReader->pFile==0 ) break;
    if( (iEof + nKey + sqlite3VarintLen(nKey))>(iStart + pIncr->mxSz) ) break;

    /* Write the next key to the output. */
    vdbePmaWriteVarint(&writer, nKey);
    vdbePmaWriteBlob(&writer, pReader->aKey, nKey);
    rc = vdbeSorterNext(pTask, pIncr->pMerger, &dummy);
  }

  rc2 = vdbePmaWriterFinish(&writer, &pOut->iEof);
  if( rc==SQLITE_OK ) rc = rc2;
  vdbeSorterPopulateDebug(pTask, "exit");
  return rc;
}
................................................................................
** can be used to incrementally merge all PMAs on disk.
**
** If successful, SQLITE_OK is returned and *ppOut set to point to the
** MergeEngine object at the root of the tree before returning. Or, if an
** error occurs, an SQLite error code is returned and the final value 
** of *ppOut is undefined.
*/
static int vdbeSorterMergeTreeBuild(VdbeSorter *pSorter, MergeEngine **ppOut){



  MergeEngine *pMain = 0;
  int rc = SQLITE_OK;
  int iTask;

#if SQLITE_MAX_WORKER_THREADS>0
  /* If the sorter uses more than one task, then create the top-level 
  ** MergeEngine here. This MergeEngine will read data from exactly 
................................................................................
#if SQLITE_MAX_WORKER_THREADS>0
    if( pSorter->bUseThreads ){
      rc = vdbePmaReaderNext(pSorter->pReader);
      *pbEof = (pSorter->pReader->pFile==0);
    }else
#endif
    /*if( !pSorter->bUseThreads )*/ {
      rc = vdbeSorterNext(&pSorter->aTask[0], pSorter->pMerger, pbEof);
    }
  }else{
    SorterRecord *pFree = pSorter->list.pList;
    pSorter->list.pList = pFree->u.pNext;
    pFree->u.pNext = 0;
    if( pSorter->list.aMemory==0 ) vdbeSorterRecordFree(db, pFree);
    *pbEof = !pSorter->list.pList;







|







 







>







 







>





>



|
>
>







 







|











|




|







 







>










|







 







>







 







>










>







 







|
>
>
>
>







 







>







 







|
|
|



|
|
|
|



>
>
>
>







 







|







 







|
>
>
>







 







|







81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
...
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
...
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
...
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
...
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
...
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
...
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
....
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
....
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
....
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
....
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
....
2142
2143
2144
2145
2146
2147
2148
2149
2150
2151
2152
2153
2154
2155
2156
2157
2158
2159
....
2353
2354
2355
2356
2357
2358
2359
2360
2361
2362
2363
2364
2365
2366
2367
** A PMA created at this point is known as a "level-0 PMA". Higher levels
** of PMAs may be created by merging existing PMAs together - for example
** merging two or more level-0 PMAs together creates a level-1 PMA.
**
** The threshold for the amount of main memory to use before flushing 
** records to a PMA is roughly the same as the limit configured for the
** page-cache of the main database. Specifically, the threshold is set to 
** the value returned by "PRAGMA main.page_size" multipled by 
** that returned by "PRAGMA main.cache_size", in bytes.
**
** If the sorter is running in single-threaded mode, then all PMAs generated
** are appended to a single temporary file. Or, if the sorter is running in
** multi-threaded mode then up to (N+1) temporary files may be opened, where
** N is the configured number of worker threads. In this case, instead of
** sorting the records and writing the PMA to a temporary file itself, the
................................................................................
** The MergeEngine object is used to combine two or more smaller PMAs into
** one big PMA using a merge operation.  Separate PMAs all need to be
** combined into one big PMA in order to be able to step through the sorted
** records in order.
**
** The aReadr[] array contains a PmaReader object for each of the PMAs being
** merged.  An aReadr[] object either points to a valid key or else is at EOF.
** ("EOF" means "End Of File".  When aReadr[] is at EOF there is no more data.)
** For the purposes of the paragraphs below, we assume that the array is
** actually N elements in size, where N is the smallest power of 2 greater
** to or equal to the number of PMAs being merged. The extra aReadr[] elements
** are treated as if they are empty (always at EOF).
**
** The aTree[] array is also N elements in size. The value of N is stored in
** the MergeEngine.nTree variable.
................................................................................
**
** In other words, each time we advance to the next sorter element, log2(N)
** key comparison operations are required, where N is the number of segments
** being merged (rounded up to the next power of 2).
*/
struct MergeEngine {
  int nTree;                 /* Used size of aTree/aReadr (power of 2) */
  SortSubtask *pTask;        /* Used by this thread only */
  int *aTree;                /* Current state of incremental merge */
  PmaReader *aReadr;         /* Array of PmaReaders to merge data from */
};

/*
** This object represents a single thread of control in a sort operation.
** Exactly VdbeSorter.nTask instances of this object are allocated
** as part of each VdbeSorter object. Instances are never allocated any
** other way. VdbeSorter.nTask is set to the number of worker threads allowed
** (see SQLITE_CONFIG_WORKER_THREADS) plus one (the main thread).  Thus for
** single-threaded operation, there is exactly one instance of this object
** and for multi-threaded operation there are two or more instances.
**
** Essentially, this structure contains all those fields of the VdbeSorter
** structure for which each thread requires a separate instance. For example,
** each thread requries its own UnpackedRecord object to unpack records in
** as part of comparison operations.
**
** Before a background thread is launched, variable bDone is set to 0. Then, 
................................................................................
/* Maximum number of PMAs that a single MergeEngine can merge */
#define SORTER_MAX_MERGE_COUNT 16

static int vdbeIncrSwap(IncrMerger*);
static void vdbeIncrFree(IncrMerger *);

/*
** Free all memory belonging to the PmaReader object passed as the
** argument. All structure fields are set to zero before returning.
*/
static void vdbePmaReaderClear(PmaReader *pReadr){
  sqlite3_free(pReadr->aAlloc);
  sqlite3_free(pReadr->aBuffer);
  if( pReadr->aMap ) sqlite3OsUnfetch(pReadr->pFile, 0, pReadr->aMap);
  vdbeIncrFree(pReadr->pIncr);
  memset(pReadr, 0, sizeof(PmaReader));
}

/*
** Read the next nByte bytes of data from the PMA p.
** If successful, set *ppOut to point to a buffer containing the data
** and return SQLITE_OK. Otherwise, if an error occurs, return an SQLite
** error code.
**
** The buffer returned in *ppOut is only valid until the
** next call to this function.
*/
static int vdbePmaReadBlob(
  PmaReader *p,                   /* PmaReader from which to take the blob */
  int nByte,                      /* Bytes of data to read */
  u8 **ppOut                      /* OUT: Pointer to buffer containing data */
){
................................................................................
** Or, if an error occurs, return an SQLite error code. The final value of
** *pp is undefined in this case.
*/
static int vdbeSorterMapFile(SortSubtask *pTask, SorterFile *pFile, u8 **pp){
  int rc = SQLITE_OK;
  if( pFile->iEof<=(i64)(pTask->pSorter->db->nMaxSorterMmap) ){
    rc = sqlite3OsFetch(pFile->pFd, 0, (int)pFile->iEof, (void**)pp);
    testcase( rc!=SQLITE_OK );
  }
  return rc;
}

/*
** Seek PmaReader pReadr to offset iOff within file pFile. Return SQLITE_OK 
** if successful, or an SQLite error code if an error occurs.
*/
static int vdbePmaReaderSeek(
  SortSubtask *pTask,             /* Task context */
  PmaReader *pReadr,              /* Reader whose cursor is to be moved */
  SorterFile *pFile,              /* Sorter file to read from */
  i64 iOff                        /* Offset in pFile */
){
  int rc = SQLITE_OK;

  assert( pReadr->pIncr==0 || pReadr->pIncr->bEof==0 );

................................................................................
      int nRead = pgsz - iBuf;
      if( (pReadr->iReadOff + nRead) > pReadr->iEof ){
        nRead = (int)(pReadr->iEof - pReadr->iReadOff);
      }
      rc = sqlite3OsRead(
          pReadr->pFile, &pReadr->aBuffer[iBuf], nRead, pReadr->iReadOff
      );
      testcase( rc!=SQLITE_OK );
    }
  }

  return rc;
}

/*
................................................................................
        bEof = 0;
      }
    }

    if( bEof ){
      /* This is an EOF condition */
      vdbePmaReaderClear(pReadr);
      testcase( rc!=SQLITE_OK );
      return rc;
    }
  }

  if( rc==SQLITE_OK ){
    rc = vdbePmaReadVarint(pReadr, &nRec);
  }
  if( rc==SQLITE_OK ){
    pReadr->nKey = (int)nRec;
    rc = vdbePmaReadBlob(pReadr, (int)nRec, &pReadr->aKey);
    testcase( rc!=SQLITE_OK );
  }

  return rc;
}

/*
** Initialize PmaReader pReadr to scan through the PMA stored in file pFile
................................................................................
}
#else
# define vdbeSorterJoinAll(x,rcin) (rcin)
# define vdbeSorterJoinThread(pTask) SQLITE_OK
#endif

/*
** Allocate a new MergeEngine object capable of handling up to
** nReader PmaReader inputs.
**
** nReader is automatically rounded up to the next power of two.
** nReader may not exceed SORTER_MAX_MERGE_COUNT even after rounding up.
*/
static MergeEngine *vdbeMergeEngineNew(int nReader){
  int N = 2;                      /* Smallest power of two >= nReader */
  int nByte;                      /* Total bytes of space to allocate */
  MergeEngine *pNew;              /* Pointer to allocated object to return */

  assert( nReader<=SORTER_MAX_MERGE_COUNT );
................................................................................

  while( N<nReader ) N += N;
  nByte = sizeof(MergeEngine) + N * (sizeof(int) + sizeof(PmaReader));

  pNew = sqlite3FaultSim(100) ? 0 : (MergeEngine*)sqlite3MallocZero(nByte);
  if( pNew ){
    pNew->nTree = N;
    pNew->pTask = 0;
    pNew->aReadr = (PmaReader*)&pNew[1];
    pNew->aTree = (int*)&pNew->aReadr[N];
  }
  return pNew;
}

/*
................................................................................
  vdbeSorterWorkDebug(pTask, "exit");
  assert( rc!=SQLITE_OK || pList->pList==0 );
  assert( rc!=SQLITE_OK || pTask->file.iEof==iSz );
  return rc;
}

/*
** Advance the MergeEngine pMerge (passed as the second argument) to
** its next entry.  Set *pbEof to true there is no next entry because
** the MergeEngine has reached the end of all its inputs.
**
** Return SQLITE_OK if successful or an error code if an error occurs.
*/
static int vdbeMergeEngineStep(
  SortSubtask *pTask,        /* The thread in which this MergeEngine runs */
  MergeEngine *pMerger,      /* The merge engine to advance to the next row */
  int *pbEof                 /* Set TRUE at EOF.  Set false for more content */
){
  int rc;
  int iPrev = pMerger->aTree[1];/* Index of PmaReader to advance */

  /* A MergeEngine object is only used by a single thread */
  assert( pMerger->pTask==0 || pMerger->pTask==pTask );
  pMerger->pTask = pTask;

  /* Advance the current PmaReader */
  rc = vdbePmaReaderNext(&pMerger->aReadr[iPrev]);

  /* Update contents of aTree[] */
  if( rc==SQLITE_OK ){
    int i;                      /* Index of aTree[] to recalculate */
................................................................................
    ** In either case exit the loop. */
    if( pReader->pFile==0 ) break;
    if( (iEof + nKey + sqlite3VarintLen(nKey))>(iStart + pIncr->mxSz) ) break;

    /* Write the next key to the output. */
    vdbePmaWriteVarint(&writer, nKey);
    vdbePmaWriteBlob(&writer, pReader->aKey, nKey);
    rc = vdbeMergeEngineStep(pTask, pIncr->pMerger, &dummy);
  }

  rc2 = vdbePmaWriterFinish(&writer, &pOut->iEof);
  if( rc==SQLITE_OK ) rc = rc2;
  vdbeSorterPopulateDebug(pTask, "exit");
  return rc;
}
................................................................................
** can be used to incrementally merge all PMAs on disk.
**
** If successful, SQLITE_OK is returned and *ppOut set to point to the
** MergeEngine object at the root of the tree before returning. Or, if an
** error occurs, an SQLite error code is returned and the final value 
** of *ppOut is undefined.
*/
static int vdbeSorterMergeTreeBuild(
  VdbeSorter *pSorter,       /* The VDBE cursor that implements the sort */
  MergeEngine **ppOut        /* Write the MergeEngine here */
){
  MergeEngine *pMain = 0;
  int rc = SQLITE_OK;
  int iTask;

#if SQLITE_MAX_WORKER_THREADS>0
  /* If the sorter uses more than one task, then create the top-level 
  ** MergeEngine here. This MergeEngine will read data from exactly 
................................................................................
#if SQLITE_MAX_WORKER_THREADS>0
    if( pSorter->bUseThreads ){
      rc = vdbePmaReaderNext(pSorter->pReader);
      *pbEof = (pSorter->pReader->pFile==0);
    }else
#endif
    /*if( !pSorter->bUseThreads )*/ {
      rc = vdbeMergeEngineStep(&pSorter->aTask[0], pSorter->pMerger, pbEof);
    }
  }else{
    SorterRecord *pFree = pSorter->list.pList;
    pSorter->list.pList = pFree->u.pNext;
    pFree->u.pNext = 0;
    if( pSorter->list.aMemory==0 ) vdbeSorterRecordFree(db, pFree);
    *pbEof = !pSorter->list.pList;