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

Artifact 8a61a6d731cbe612217edf9eece6197f37c9489e:


/*
** 2011 July 9
**
** 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 contains code for the VdbeSorter object, used in concert with
** a VdbeCursor to sort large numbers of keys (as may be required, for
** example, by CREATE INDEX statements on tables too large to fit in main
** memory).
*/

#include "sqliteInt.h"
#include "vdbeInt.h"

#ifndef SQLITE_OMIT_MERGE_SORT

typedef struct VdbeSorterIter VdbeSorterIter;

/*
** NOTES ON DATA STRUCTURE USED FOR N-WAY MERGES:
**
** As keys are added to the sorter, they are written to disk in a series
** of sorted packed-memory-arrays (PMAs). The size of each PMA is roughly
** the same as the cache-size allowed for temporary databases. In order
** to allow the caller to extract keys from the sorter in sorted order,
** all PMAs currently stored on disk must be merged together. This comment
** describes the data structure used to do so. The structure supports 
** merging any number of arrays in a single pass with no redundant comparison 
** operations.
**
** The aIter[] array contains an iterator for each of the PMAs being merged.
** An aIter[] iterator 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 iterators being merged. The extra aIter[] 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 VdbeSorter.nTree variable.
**
** The final (N/2) elements of aTree[] contain the results of comparing
** pairs of iterator keys together. Element i contains the result of 
** comparing aIter[2*i-N] and aIter[2*i-N+1]. Whichever key is smaller, the
** aTree element is set to the index of it. 
**
** For the purposes of this comparison, EOF is considered greater than any
** other key value. If the keys are equal (only possible with two EOF
** values), it doesn't matter which index is stored.
**
** The (N/4) elements of aTree[] that preceed the final (N/2) described 
** above contains the index of the smallest of each block of 4 iterators.
** And so on. So that aTree[1] contains the index of the iterator that 
** currently points to the smallest key value. aTree[0] is unused.
**
** Example:
**
**     aIter[0] -> Banana
**     aIter[1] -> Feijoa
**     aIter[2] -> Elderberry
**     aIter[3] -> Currant
**     aIter[4] -> Grapefruit
**     aIter[5] -> Apple
**     aIter[6] -> Durian
**     aIter[7] -> EOF
**
**     aTree[] = { X, 5   0, 5    0, 3, 5, 6 }
**
** The current element is "Apple" (the value of the key indicated by 
** iterator 5). When the Next() operation is invoked, iterator 5 will
** be advanced to the next key in its segment. Say the next key is
** "Eggplant":
**
**     aIter[5] -> Eggplant
**
** The contents of aTree[] are updated first by comparing the new iterator
** 5 key to the current key of iterator 4 (still "Grapefruit"). The iterator
** 5 value is still smaller, so aTree[6] is set to 5. And so on up the tree.
** The value of iterator 6 - "Durian" - is now smaller than that of iterator
** 5, so aTree[3] is set to 6. Key 0 is smaller than key 6 (Banana<Durian),
** so the value written into element 1 of the array is 0. As follows:
**
**     aTree[] = { X, 0   0, 6    0, 3, 5, 6 }
**
** 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 VdbeSorter {
  int nWorking;                   /* Start a new b-tree after this many pages */
  int nBtree;                     /* Current size of b-tree contents as PMA */
  int nTree;                      /* Used size of aTree/aIter (power of 2) */
  VdbeSorterIter *aIter;          /* Array of iterators to merge */
  int *aTree;                     /* Current state of incremental merge */
  i64 iWriteOff;                  /* Current write offset within file pTemp1 */
  i64 iReadOff;                   /* Current read offset within file pTemp1 */
  sqlite3_file *pTemp1;           /* PMA file 1 */
  int nPMA;                       /* Number of PMAs stored in pTemp1 */
};

/*
** The following type is an iterator for a PMA. It caches the current key in 
** variables nKey/aKey. If the iterator is at EOF, pFile==0.
*/
struct VdbeSorterIter {
  i64 iReadOff;                   /* Current read offset */
  i64 iEof;                       /* 1 byte past EOF for this iterator */
  sqlite3_file *pFile;            /* File iterator is reading from */
  int nAlloc;                     /* Bytes of space at aAlloc */
  u8 *aAlloc;                     /* Allocated space */
  int nKey;                       /* Number of bytes in key */
  u8 *aKey;                       /* Pointer to current key */
};

/* Minimum allowable value for the VdbeSorter.nWorking variable */
#define SORTER_MIN_WORKING 10

/* Maximum number of segments to merge in a single pass. */
#define SORTER_MAX_MERGE_COUNT 16

/*
** Free all memory belonging to the VdbeSorterIter object passed as the second
** argument. All structure fields are set to zero before returning.
*/
static void vdbeSorterIterZero(sqlite3 *db, VdbeSorterIter *pIter){
  sqlite3DbFree(db, pIter->aAlloc);
  memset(pIter, 0, sizeof(VdbeSorterIter));
}

/*
** Advance iterator pIter to the next key in its PMA. Return SQLITE_OK if
** no error occurs, or an SQLite error code if one does.
*/
static int vdbeSorterIterNext(
  sqlite3 *db,                    /* Database handle (for sqlite3DbMalloc() ) */
  VdbeSorterIter *pIter           /* Iterator to advance */
){
  int rc;                         /* Return Code */
  int nRead;                      /* Number of bytes read */
  int nRec;                       /* Size of record in bytes */
  int iOff;                       /* Size of serialized size varint in bytes */

  nRead = pIter->iEof - pIter->iReadOff;
  if( nRead>5 ) nRead = 5;
  if( nRead<=0 ){
    /* This is an EOF condition */
    vdbeSorterIterZero(db, pIter);
    return SQLITE_OK;
  }

  rc = sqlite3OsRead(pIter->pFile, pIter->aAlloc, nRead, pIter->iReadOff);
  iOff = getVarint32(pIter->aAlloc, nRec);

  if( rc==SQLITE_OK && (iOff+nRec)>nRead ){
    int nRead2;                   /* Number of extra bytes to read */
    if( (iOff+nRec)>pIter->nAlloc ){
      int nNew = pIter->nAlloc*2;
      while( (iOff+nRec)>nNew ) nNew = nNew*2;
      pIter->aAlloc = sqlite3DbReallocOrFree(db, pIter->aAlloc, nNew);
      if( !pIter->aAlloc ) return SQLITE_NOMEM;
      pIter->nAlloc = nNew;
    }

    nRead2 = iOff + nRec - nRead;
    rc = sqlite3OsRead(
        pIter->pFile, &pIter->aAlloc[nRead], nRead2, pIter->iReadOff+nRead
    );
  }

  assert( nRec>0 || rc!=SQLITE_OK );
  pIter->iReadOff += iOff+nRec;
  pIter->nKey = nRec;
  pIter->aKey = &pIter->aAlloc[iOff];
  return rc;
}

/*
** Write a single varint, value iVal, to file-descriptor pFile. Return
** SQLITE_OK if successful, or an SQLite error code if some error occurs.
**
** The value of *piOffset when this function is called is used as the byte
** offset in file pFile to write to. Before returning, *piOffset is 
** incremented by the number of bytes written.
*/
static int vdbeSorterWriteVarint(
  sqlite3_file *pFile,            /* File to write to */
  i64 iVal,                       /* Value to write as a varint */
  i64 *piOffset                   /* IN/OUT: Write offset in file pFile */
){
  u8 aVarint[9];                  /* Buffer large enough for a varint */
  int nVarint;                    /* Number of used bytes in varint */
  int rc;                         /* Result of write() call */

  nVarint = sqlite3PutVarint(aVarint, iVal);
  rc = sqlite3OsWrite(pFile, aVarint, nVarint, *piOffset);
  *piOffset += nVarint;

  return rc;
}

/*
** Read a single varint from file-descriptor pFile. Return SQLITE_OK if
** successful, or an SQLite error code if some error occurs.
**
** The value of *piOffset when this function is called is used as the
** byte offset in file pFile from whence to read the varint. If successful
** (i.e. if no IO error occurs), then *piOffset is set to the offset of
** the first byte past the end of the varint before returning. *piVal is
** set to the integer value read. If an error occurs, the final values of
** both *piOffset and *piVal are undefined.
*/
static int vdbeSorterReadVarint(
  sqlite3_file *pFile,            /* File to read from */
  i64 iEof,                       /* Total number of bytes in file */
  i64 *piOffset,                  /* IN/OUT: Read offset in pFile */
  i64 *piVal                      /* OUT: Value read from file */
){
  u8 aVarint[9];                  /* Buffer large enough for a varint */
  i64 iOff = *piOffset;           /* Offset in file to read from */
  int nRead = 9;                  /* Number of bytes to read from file */
  int rc;                         /* Return code */

  assert( iEof>iOff );
  if( (iEof-iOff)<nRead ){
    nRead = iEof-iOff;
  }

  rc = sqlite3OsRead(pFile, aVarint, nRead, iOff);
  if( rc==SQLITE_OK ){
    *piOffset += getVarint(aVarint, (u64 *)piVal);
  }

  return rc;
}

/*
** Initialize iterator pIter to scan through the PMA stored in file pFile
** starting at offset iStart and ending at offset iEof-1. This function 
** leaves the iterator pointing to the first key in the PMA (or EOF if the 
** PMA is empty).
*/
static int vdbeSorterIterInit(
  sqlite3 *db,                    /* Database handle */
  VdbeSorter *pSorter,            /* Sorter object */
  i64 iStart,                     /* Start offset in pFile */
  VdbeSorterIter *pIter,          /* Iterator to populate */
  i64 *pnByte                     /* IN/OUT: Increment this value by PMA size */
){
  int rc;

  assert( pSorter->iWriteOff>iStart );
  assert( pIter->aAlloc==0 );
  pIter->pFile = pSorter->pTemp1;
  pIter->iReadOff = iStart;
  pIter->nAlloc = 128;
  pIter->aAlloc = (u8 *)sqlite3DbMallocRaw(db, pIter->nAlloc);
  if( !pIter->aAlloc ){
    rc = SQLITE_NOMEM;
  }else{
    i64 iEof = pSorter->iWriteOff;     /* EOF of file pSorter->pTemp1 */
    i64 nByte;                         /* Total size of PMA in bytes */
    rc = vdbeSorterReadVarint(pSorter->pTemp1, iEof, &pIter->iReadOff, &nByte);
    *pnByte += nByte;
    pIter->iEof = pIter->iReadOff + nByte;
  }
  if( rc==SQLITE_OK ){
    rc = vdbeSorterIterNext(db, pIter);
  }
  return rc;
}

/*
** This function is called to compare two iterator keys when merging 
** multiple b-tree segments. Parameter iOut is the index of the aTree[] 
** value to recalculate.
*/
static int vdbeSorterDoCompare(VdbeCursor *pCsr, int iOut){
  VdbeSorter *pSorter = pCsr->pSorter;
  int i1;
  int i2;
  int iRes;
  VdbeSorterIter *p1;
  VdbeSorterIter *p2;

  assert( iOut<pSorter->nTree && iOut>0 );

  if( iOut>=(pSorter->nTree/2) ){
    i1 = (iOut - pSorter->nTree/2) * 2;
    i2 = i1 + 1;
  }else{
    i1 = pSorter->aTree[iOut*2];
    i2 = pSorter->aTree[iOut*2+1];
  }

  p1 = &pSorter->aIter[i1];
  p2 = &pSorter->aIter[i2];

  if( p1->pFile==0 ){
    iRes = i2;
  }else if( p2->pFile==0 ){
    iRes = i1;
  }else{
    char aSpace[150];
    UnpackedRecord *r1;

    r1 = sqlite3VdbeRecordUnpack(
        pCsr->pKeyInfo, p1->nKey, p1->aKey, aSpace, sizeof(aSpace)
    );
    if( r1==0 ) return SQLITE_NOMEM;

    if( sqlite3VdbeRecordCompare(p2->nKey, p2->aKey, r1)>=0 ){
      iRes = i1;
    }else{
      iRes = i2;
    }
    sqlite3VdbeDeleteUnpackedRecord(r1);
  }

  pSorter->aTree[iOut] = iRes;
  return SQLITE_OK;
}

/*
** Initialize the temporary index cursor just opened as a sorter cursor.
*/
int sqlite3VdbeSorterInit(sqlite3 *db, VdbeCursor *pCsr){
  assert( pCsr->pKeyInfo && pCsr->pBt );
  pCsr->pSorter = sqlite3DbMallocZero(db, sizeof(VdbeSorter));
  return (pCsr->pSorter ? SQLITE_OK : SQLITE_NOMEM);
}

/*
** Free any cursor components allocated by sqlite3VdbeSorterXXX routines.
*/
void sqlite3VdbeSorterClose(sqlite3 *db, VdbeCursor *pCsr){
  VdbeSorter *pSorter = pCsr->pSorter;
  if( pSorter ){
    if( pSorter->aIter ){
      int i;
      for(i=0; i<pSorter->nTree; i++){
        vdbeSorterIterZero(db, &pSorter->aIter[i]);
      }
      sqlite3DbFree(db, pSorter->aIter);
    }
    if( pSorter->pTemp1 ){
      sqlite3OsCloseFree(pSorter->pTemp1);
    }
    sqlite3DbFree(db, pSorter);
    pCsr->pSorter = 0;
  }
}

/*
** Allocate space for a file-handle and open a temporary file. If successful,
** set *ppFile to point to the malloc'd file-handle and return SQLITE_OK.
** Otherwise, set *ppFile to 0 and return an SQLite error code.
*/
static int vdbeSorterOpenTempFile(sqlite3 *db, sqlite3_file **ppFile){
  int dummy;
  return sqlite3OsOpenMalloc(db->pVfs, 0, ppFile,
      SQLITE_OPEN_TEMP_JOURNAL |
      SQLITE_OPEN_READWRITE    | SQLITE_OPEN_CREATE |
      SQLITE_OPEN_EXCLUSIVE    | SQLITE_OPEN_DELETEONCLOSE, &dummy
  );
}


/*
** Write the current contents of the b-tree to a PMA. Return SQLITE_OK
** if successful, or an SQLite error code otherwise.
**
** The format of a PMA is:
**
**     * A varint. This varint contains the total number of bytes of content
**       in the PMA (not including the varint itself).
**
**     * One or more records packed end-to-end in order of ascending keys. 
**       Each record consists of a varint followed by a blob of data (the 
**       key). The varint is the number of bytes in the blob of data.
*/
static int vdbeSorterBtreeToPMA(sqlite3 *db, VdbeCursor *pCsr){
  int rc = SQLITE_OK;             /* Return code */
  VdbeSorter *pSorter = pCsr->pSorter;
  int res = 0;

  /* sqlite3BtreeFirst() cannot fail because sorter btrees are always held
  ** in memory and so an I/O error is not possible. */
  rc = sqlite3BtreeFirst(pCsr->pCursor, &res);
  if( NEVER(rc!=SQLITE_OK) || res ) return rc;
  assert( pSorter->nBtree>0 );

  /* If the first temporary PMA file has not been opened, open it now. */
  if( pSorter->pTemp1==0 ){
    rc = vdbeSorterOpenTempFile(db, &pSorter->pTemp1);
    assert( rc!=SQLITE_OK || pSorter->pTemp1 );
    assert( pSorter->iWriteOff==0 );
    assert( pSorter->nPMA==0 );
  }

  if( rc==SQLITE_OK ){
    i64 iWriteOff = pSorter->iWriteOff;
    void *aMalloc = 0;            /* Array used to hold a single record */
    int nMalloc = 0;              /* Allocated size of aMalloc[] in bytes */

    pSorter->nPMA++;
    for(
      rc = vdbeSorterWriteVarint(pSorter->pTemp1, pSorter->nBtree, &iWriteOff);
      rc==SQLITE_OK && res==0;
      rc = sqlite3BtreeNext(pCsr->pCursor, &res)
    ){
      i64 nKey;                   /* Size of this key in bytes */

      /* Write the size of the record in bytes to the output file */
      (void)sqlite3BtreeKeySize(pCsr->pCursor, &nKey);
      rc = vdbeSorterWriteVarint(pSorter->pTemp1, nKey, &iWriteOff);

      /* Make sure the aMalloc[] buffer is large enough for the record */
      if( rc==SQLITE_OK && nKey>nMalloc ){
        aMalloc = sqlite3DbReallocOrFree(db, aMalloc, nKey);
        if( !aMalloc ){ 
          rc = SQLITE_NOMEM; 
        }else{
          nMalloc = nKey;
        }
      }

      /* Write the record itself to the output file */
      if( rc==SQLITE_OK ){
        /* sqlite3BtreeKey() cannot fail because sorter btrees held in memory */
        rc = sqlite3BtreeKey(pCsr->pCursor, 0, nKey, aMalloc);
        if( ALWAYS(rc==SQLITE_OK) ){
          rc = sqlite3OsWrite(pSorter->pTemp1, aMalloc, nKey, iWriteOff);
          iWriteOff += nKey;
        }
      }

      if( rc!=SQLITE_OK ) break;
    }

    /* This assert verifies that unless an error has occurred, the size of 
    ** the PMA on disk is the same as the expected size stored in
    ** pSorter->nBtree. */ 
    assert( rc!=SQLITE_OK || pSorter->nBtree==(
          iWriteOff-pSorter->iWriteOff-sqlite3VarintLen(pSorter->nBtree)
    ));

    pSorter->iWriteOff = iWriteOff;
    sqlite3DbFree(db, aMalloc);
  }

  pSorter->nBtree = 0;
  return rc;
}

/*
** This function is called on a sorter cursor by the VDBE before each row 
** is inserted into VdbeCursor.pCsr. Argument nKey is the size of the key, in
** bytes, about to be inserted.
**
** If it is determined that the temporary b-tree accessed via VdbeCursor.pCsr
** is large enough, its contents are written to a sorted PMA on disk and the
** tree emptied. This prevents the b-tree (which must be small enough to
** fit entirely in the cache in order to support efficient inserts) from
** growing too large.
**
** An SQLite error code is returned if an error occurs. Otherwise, SQLITE_OK.
*/
int sqlite3VdbeSorterWrite(sqlite3 *db, VdbeCursor *pCsr, int nKey){
  int rc = SQLITE_OK;             /* Return code */
  VdbeSorter *pSorter = pCsr->pSorter;
  if( pSorter ){
    Pager *pPager = sqlite3BtreePager(pCsr->pBt);
    int nPage;                    /* Current size of temporary file in pages */

    /* Sorters never spill to disk */
    assert( sqlite3PagerFile(pPager)->pMethods==0 );

    /* Determine how many pages the temporary b-tree has grown to */
    sqlite3PagerPagecount(pPager, &nPage);

    /* If pSorter->nWorking is still zero, but the temporary file has been
    ** created in the file-system, then the most recent insert into the
    ** current b-tree segment probably caused the cache to overflow (it is
    ** also possible that sqlite3_release_memory() was called). So set the
    ** size of the working set to a little less than the current size of the 
    ** file in pages.  */
    if( pSorter->nWorking==0 && sqlite3PagerUnderStress(pPager) ){
      pSorter->nWorking = nPage-5;
      if( pSorter->nWorking<SORTER_MIN_WORKING ){
        pSorter->nWorking = SORTER_MIN_WORKING;
      }
    }

    /* If the number of pages used by the current b-tree segment is greater
    ** than the size of the working set (VdbeSorter.nWorking), start a new
    ** segment b-tree.  */
    if( pSorter->nWorking && nPage>=pSorter->nWorking ){
      BtCursor *p = pCsr->pCursor;/* Cursor structure to close and reopen */
      int iRoot;                  /* Root page of new tree */

      /* Copy the current contents of the b-tree into a PMA in sorted order.
      ** Close the currently open b-tree cursor. */
      rc = vdbeSorterBtreeToPMA(db, pCsr);
      sqlite3BtreeCloseCursor(p);

      if( rc==SQLITE_OK ){
        rc = sqlite3BtreeDropTable(pCsr->pBt, 2, 0);
#ifdef SQLITE_DEBUG
        sqlite3PagerPagecount(pPager, &nPage);
        assert( rc!=SQLITE_OK || nPage==1 );
#endif
      }
      if( rc==SQLITE_OK ){
        rc = sqlite3BtreeCreateTable(pCsr->pBt, &iRoot, BTREE_BLOBKEY);
      }
      if( rc==SQLITE_OK ){
        assert( iRoot==2 );
        rc = sqlite3BtreeCursor(pCsr->pBt, iRoot, 1, pCsr->pKeyInfo, p);
      }
    }

    pSorter->nBtree += sqlite3VarintLen(nKey) + nKey;
  }
  return rc;
}

/*
** Helper function for sqlite3VdbeSorterRewind(). 
*/
static int vdbeSorterInitMerge(
  sqlite3 *db,                    /* Database handle */
  VdbeCursor *pCsr,               /* Cursor handle for this sorter */
  i64 *pnByte                     /* Sum of bytes in all opened PMAs */
){
  VdbeSorter *pSorter = pCsr->pSorter;
  int rc = SQLITE_OK;             /* Return code */
  int i;                          /* Used to iterator through aIter[] */
  i64 nByte = 0;                  /* Total bytes in all opened PMAs */

  /* Initialize the iterators. */
  for(i=0; rc==SQLITE_OK && i<SORTER_MAX_MERGE_COUNT; i++){
    VdbeSorterIter *pIter = &pSorter->aIter[i];
    rc = vdbeSorterIterInit(db, pSorter, pSorter->iReadOff, pIter, &nByte);
    pSorter->iReadOff = pIter->iEof;
    assert( pSorter->iReadOff<=pSorter->iWriteOff || rc!=SQLITE_OK );
    if( pSorter->iReadOff>=pSorter->iWriteOff ) break;
  }

  /* Initialize the aTree[] array. */
  for(i=pSorter->nTree-1; rc==SQLITE_OK && i>0; i--){
    rc = vdbeSorterDoCompare(pCsr, i);
  }

  *pnByte = nByte;
  return rc;
}

/*
** Once the sorter has been populated, this function is called to prepare
** for iterating through its contents in sorted order.
*/
int sqlite3VdbeSorterRewind(sqlite3 *db, VdbeCursor *pCsr, int *pbEof){
  VdbeSorter *pSorter = pCsr->pSorter;
  int rc;                         /* Return code */
  sqlite3_file *pTemp2 = 0;       /* Second temp file to use */
  i64 iWrite2 = 0;                /* Write offset for pTemp2 */
  int nIter;                      /* Number of iterators used */
  int nByte;                      /* Bytes of space required for aIter/aTree */
  int N = 2;                      /* Power of 2 >= nIter */

  assert( pSorter );

  /* Write the current b-tree to a PMA. Close the b-tree cursor. */
  rc = vdbeSorterBtreeToPMA(db, pCsr);
  sqlite3BtreeCloseCursor(pCsr->pCursor);
  if( rc!=SQLITE_OK ) return rc;
  if( pSorter->nPMA==0 ){
    *pbEof = 1;
    return SQLITE_OK;
  }

  /* Allocate space for aIter[] and aTree[]. */
  nIter = pSorter->nPMA;
  if( nIter>SORTER_MAX_MERGE_COUNT ) nIter = SORTER_MAX_MERGE_COUNT;
  assert( nIter>0 );
  while( N<nIter ) N += N;
  nByte = N * (sizeof(int) + sizeof(VdbeSorterIter));
  pSorter->aIter = (VdbeSorterIter *)sqlite3DbMallocZero(db, nByte);
  if( !pSorter->aIter ) return SQLITE_NOMEM;
  pSorter->aTree = (int *)&pSorter->aIter[N];
  pSorter->nTree = N;

  do {
    int iNew;                     /* Index of new, merged, PMA */

    for(iNew=0; 
        rc==SQLITE_OK && iNew*SORTER_MAX_MERGE_COUNT<pSorter->nPMA; 
        iNew++
    ){
      i64 nWrite;                 /* Number of bytes in new PMA */

      /* If there are SORTER_MAX_MERGE_COUNT or less PMAs in file pTemp1,
      ** initialize an iterator for each of them and break out of the loop.
      ** These iterators will be incrementally merged as the VDBE layer calls
      ** sqlite3VdbeSorterNext().
      **
      ** Otherwise, if pTemp1 contains more than SORTER_MAX_MERGE_COUNT PMAs,
      ** initialize interators for SORTER_MAX_MERGE_COUNT of them. These PMAs
      ** are merged into a single PMA that is written to file pTemp2.
      */
      rc = vdbeSorterInitMerge(db, pCsr, &nWrite);
      assert( rc!=SQLITE_OK || pSorter->aIter[ pSorter->aTree[1] ].pFile );
      if( rc!=SQLITE_OK || pSorter->nPMA<=SORTER_MAX_MERGE_COUNT ){
        break;
      }

      /* Open the second temp file, if it is not already open. */
      if( pTemp2==0 ){
        assert( iWrite2==0 );
        rc = vdbeSorterOpenTempFile(db, &pTemp2);
      }

      if( rc==SQLITE_OK ){
        rc = vdbeSorterWriteVarint(pTemp2, nWrite, &iWrite2);
      }

      if( rc==SQLITE_OK ){
        int bEof = 0;
        while( rc==SQLITE_OK && bEof==0 ){
          int nToWrite;
          VdbeSorterIter *pIter = &pSorter->aIter[ pSorter->aTree[1] ];
          assert( pIter->pFile );
          nToWrite = pIter->nKey + sqlite3VarintLen(pIter->nKey);
          rc = sqlite3OsWrite(pTemp2, pIter->aAlloc, nToWrite, iWrite2);
          iWrite2 += nToWrite;
          if( rc==SQLITE_OK ){
            rc = sqlite3VdbeSorterNext(db, pCsr, &bEof);
          }
        }
      }
    }

    if( pSorter->nPMA<=SORTER_MAX_MERGE_COUNT ){
      break;
    }else{
      sqlite3_file *pTmp = pSorter->pTemp1;
      pSorter->nPMA = iNew;
      pSorter->pTemp1 = pTemp2;
      pTemp2 = pTmp;
      pSorter->iWriteOff = iWrite2;
      pSorter->iReadOff = 0;
      iWrite2 = 0;
    }
  }while( rc==SQLITE_OK );

  if( pTemp2 ){
    sqlite3OsCloseFree(pTemp2);
  }
  *pbEof = (pSorter->aIter[pSorter->aTree[1]].pFile==0);
  return rc;
}

/*
** Advance to the next element in the sorter.
*/
int sqlite3VdbeSorterNext(sqlite3 *db, VdbeCursor *pCsr, int *pbEof){
  VdbeSorter *pSorter = pCsr->pSorter;
  int iPrev = pSorter->aTree[1];  /* Index of iterator to advance */
  int i;                          /* Index of aTree[] to recalculate */
  int rc;                         /* Return code */

  rc = vdbeSorterIterNext(db, &pSorter->aIter[iPrev]);
  for(i=(pSorter->nTree+iPrev)/2; rc==SQLITE_OK && i>0; i=i/2){
    rc = vdbeSorterDoCompare(pCsr, i);
  }

  *pbEof = (pSorter->aIter[pSorter->aTree[1]].pFile==0);
  return rc;
}

/*
** Copy the current sorter key into the memory cell pOut.
*/
int sqlite3VdbeSorterRowkey(VdbeCursor *pCsr, Mem *pOut){
  VdbeSorter *pSorter = pCsr->pSorter;
  VdbeSorterIter *pIter;

  pIter = &pSorter->aIter[ pSorter->aTree[1] ];

  /* Coverage testing note: As things are currently, this call will always
  ** succeed. This is because the memory cell passed by the VDBE layer 
  ** happens to be the same one as was used to assemble the keys before they
  ** were passed to the sorter - meaning it is always large enough for the
  ** largest key. But this could change very easily, so we leave the call
  ** to sqlite3VdbeMemGrow() in. */
  if( NEVER(sqlite3VdbeMemGrow(pOut, pIter->nKey, 0)) ){
    return SQLITE_NOMEM;
  }
  pOut->n = pIter->nKey;
  MemSetTypeFlag(pOut, MEM_Blob);
  memcpy(pOut->z, pIter->aKey, pIter->nKey);

  return SQLITE_OK;
}

#endif /* #ifndef SQLITE_OMIT_MERGE_SORT */