SQLite

  pMerger->aTree[iOut] = iRes;
}

/*
** Allowed values for the eMode parameter to vdbeMergeEngineInit()
** and vdbePmaReaderIncrMergeInit().
**
** Only INCRINIT_NORMAL is valid in single-threaded builds (when
** SQLITE_MAX_WORKER_THREADS==0).  The other values are only used
** when there exists one or more separate worker threads.
*/
#define INCRINIT_NORMAL 0
#define INCRINIT_TASK   1
#define INCRINIT_ROOT   2

/* Forward reference.
** The vdbeIncrMergeInit() and vdbePmaReaderIncrMergeInit() routines call each

  MergeEngine *pMerger,           /* MergeEngine to initialize */
  int eMode                       /* One of the INCRINIT_XXX constants */
){
  int rc = SQLITE_OK;             /* Return code */
  int i;                          /* For looping over PmaReader objects */
  int nTree = pMerger->nTree;

  /* eMode is always INCRINIT_NORMAL in single-threaded mode */
  assert( SQLITE_MAX_WORKER_THREADS>0 || eMode==INCRINIT_NORMAL );

  /* Verify that the MergeEngine is assigned to a single thread */
  assert( pMerger->pTask==0 ); // || pMerger->pTask==pTask );
  pMerger->pTask = pTask;

  for(i=0; i<nTree; i++){
    if( SQLITE_MAX_WORKER_THREADS>0 && eMode==INCRINIT_ROOT ){
      /* PmaReaders should be normally initialized in order, as if they are
      ** reading from the same temp file this makes for more linear file IO.
      ** However, in the INCRINIT_ROOT case, if PmaReader aReadr[nTask-1] is
      ** in use it will block the vdbePmaReaderNext() call while it uses
      ** the main thread to fill its buffer. So calling PmaReaderNext()
      ** on this PmaReader before any of the multi-threaded PmaReaders takes
      ** better advantage of multi-processor hardware. */

** the current PmaReader set to point to the first key in its range.
**
** SQLITE_OK is returned if successful, or an SQLite error code otherwise.
*/
static int vdbePmaReaderIncrMergeInit(PmaReader *pReadr, int eMode){
  int rc = SQLITE_OK;
  IncrMerger *pIncr = pReadr->pIncr;

  /* eMode is always INCRINIT_NORMAL in single-threaded mode */
  assert( SQLITE_MAX_WORKER_THREADS>0 || eMode==INCRINIT_NORMAL );

  if( pIncr ){
    SortSubtask *pTask = pIncr->pTask;
    sqlite3 *db = pTask->pSorter->db;

    rc = vdbeMergeEngineInit(pTask, pIncr->pMerger, eMode);

    /* Set up the required files for pIncr. A multi-theaded IncrMerge object

      ** PmaReader is multi-threaded. The reason being that this function
      ** is already running in background thread pIncr->pTask->thread. */
      assert( eMode==INCRINIT_ROOT || eMode==INCRINIT_TASK );
      rc = vdbeIncrPopulate(pIncr);
    }
#endif

    if( rc==SQLITE_OK
     && (SQLITE_MAX_WORKER_THREADS==0 || eMode!=INCRINIT_TASK)
    ){
      rc = vdbePmaReaderNext(pReadr);
    }
  }
  return rc;
}

#if SQLITE_MAX_WORKER_THREADS>0

    pMain = vdbeMergeEngineNew(pSorter->nTask);
    if( pMain==0 ) rc = SQLITE_NOMEM;
  }
#endif

  for(iTask=0; rc==SQLITE_OK && iTask<pSorter->nTask; iTask++){
    SortSubtask *pTask = &pSorter->aTask[iTask];
    assert( pTask->nPMA>0 || SQLITE_MAX_WORKER_THREADS>0 );
    if( SQLITE_MAX_WORKER_THREADS==0 || pTask->nPMA ){
      MergeEngine *pRoot = 0;     /* Root node of tree for this task */
      int nDepth = vdbeSorterTreeDepth(pTask->nPMA);
      i64 iReadOff = 0;

      if( pTask->nPMA<=SORTER_MAX_MERGE_COUNT ){
        rc = vdbeMergeEngineLevel0(pTask, pTask->nPMA, &iReadOff, &pRoot);
      }else{