return NULL;

        if( nFID < 0 || nFID >= psSummary->count )
            return NULL;

        poSummaryFeature->SetField( 0, psSummary->distinct_list[nFID] );
        poSummaryFeature->SetFID( nFID );

        return poSummaryFeature->Clone();
    }

/* -------------------------------------------------------------------- */
/*      Are we running in sorted mode?  If so, run the fid through      */
/*      the index.                                                      */
/* -------------------------------------------------------------------- */
    if( panFIDIndex != NULL )
    {
        if( nFID < 0 || nFID >= nIndexSize )
            return NULL;
        else
            nFID = panFIDIndex[nFID];
    }

/* -------------------------------------------------------------------- */
/*      Handle request for random record.                               */
/* -------------------------------------------------------------------- */
    OGRFeature *poSrcFeature = poSrcLayer->GetFeature( nFID );
    OGRFeature *poResult;

    if( poSrcFeature == NULL )
        return NULL;

    poResult = TranslateFeature( poSrcFeature );
    poResult->SetFID( nFID );
    
    delete poSrcFeature;

    return poResult;
}

/************************************************************************/
/*                          GetSpatialFilter()                          */
/************************************************************************/

OGRGeometry *OGRGenSQLResultsLayer::GetSpatialFilter() 

{
    return NULL;
}

/************************************************************************/
/*                            GetLayerDefn()                            */
/************************************************************************/

OGRFeatureDefn *OGRGenSQLResultsLayer::GetLayerDefn()

{
    return poDefn;
}

/************************************************************************/
/*                         CreateOrderByIndex()                         */
/*                                                                      */
/*      This method is responsible for creating an index providing      */
/*      ordered access to the features according to the supplied        */
/*      ORDER BY clauses.                                               */
/*                                                                      */
/*      This is accomplished by making one pass through all the         */
/*      eligible source features, and capturing the order by fields     */
/*      of all records in memory.  A quick sort is then applied to      */
/*      this in memory copy of the order-by fields to create the        */
/*      required index.                                                 */
/*                                                                      */
/*      Keeping all the key values in memory will *not* scale up to     */
/*      very large input datasets.                                      */
/************************************************************************/

void OGRGenSQLResultsLayer::CreateOrderByIndex()

{
    swq_select *psSelectInfo = (swq_select *) pSelectInfo;
    OGRField *pasIndexFields;
    int      i, nOrderItems = psSelectInfo->order_specs;
    long     *panFIDList;

    if( nOrderItems == 0 )
        return;

    ResetReading();

/* -------------------------------------------------------------------- */
/*      Allocate set of key values, and the output index.               */
/* -------------------------------------------------------------------- */
    nIndexSize = poSrcLayer->GetFeatureCount();

    pasIndexFields = (OGRField *) 
        CPLCalloc(sizeof(OGRField), nOrderItems * nIndexSize);
    panFIDIndex = (long *) CPLCalloc(sizeof(long),nIndexSize);
    panFIDList = (long *) CPLCalloc(sizeof(long),nIndexSize);

    for( i = 0; i < nIndexSize; i++ )
        panFIDIndex[i] = i;

/* -------------------------------------------------------------------- */
/*      Read in all the key values.                                     */
/* -------------------------------------------------------------------- */
    OGRFeature *poSrcFeat;
    int         iFeature = 0;

    while( (poSrcFeat = poSrcLayer->GetNextFeature()) != NULL )
    {
        int iKey;

        for( iKey = 0; iKey < nOrderItems; iKey++ )
        {
            swq_order_def *psKeyDef = psSelectInfo->order_defs + iKey;
            OGRFieldDefn *poFDefn;
            OGRField *psSrcField, *psDstField;

            psDstField = pasIndexFields + iFeature * nOrderItems + iKey;

            if ( psKeyDef->field_index >= iFIDFieldIndex)
            {
                if ( psKeyDef->field_index < iFIDFieldIndex + SPECIAL_FIELD_COUNT )
                {
                    switch (SpecialFieldTypes[psKeyDef->field_index - iFIDFieldIndex])
                    {
                      case SWQ_INTEGER:
                        psDstField->Integer = poSrcFeat->GetFieldAsInteger(psKeyDef->field_index);
                        break;

                      default:
                        psDstField->String = CPLStrdup( poSrcFeat->GetFieldAsString(psKeyDef->field_index) );
                        break;
                    }
                }
                continue;
            }
            
            poFDefn = poSrcLayer->GetLayerDefn()->GetFieldDefn( 
                psKeyDef->field_index );

            psSrcField = poSrcFeat->GetRawFieldRef( psKeyDef->field_index );

            if( poFDefn->GetType() == OFTInteger 
                || poFDefn->GetType() == OFTReal )
                memcpy( psDstField, psSrcField, sizeof(OGRField) );
            else if( poFDefn->GetType() == OFTString )
            {
                if( poSrcFeat->IsFieldSet( psKeyDef->field_index ) )
                    psDstField->String = CPLStrdup( psSrcField->String );
                else
                    memcpy( psDstField, psSrcField, sizeof(OGRField) );
            }
        }

        panFIDList[iFeature] = poSrcFeat->GetFID();
        delete poSrcFeat;

        iFeature++;
    }

    CPLAssert( nIndexSize == iFeature );

/* -------------------------------------------------------------------- */
/*      Quick sort the records.                                         */
/* -------------------------------------------------------------------- */
    SortIndexSection( pasIndexFields, 0, nIndexSize );

/* -------------------------------------------------------------------- */
/*      Rework the FID map to map to real FIDs.                         */
/* -------------------------------------------------------------------- */
    for( i = 0; i < nIndexSize; i++ )
        panFIDIndex[i] = panFIDList[panFIDIndex[i]];

    CPLFree( panFIDList );

/* -------------------------------------------------------------------- */
/*      Free the key field values.                                      */
/* -------------------------------------------------------------------- */
    for( int iKey = 0; iKey < nOrderItems; iKey++ )
    {
        swq_order_def *psKeyDef = psSelectInfo->order_defs + iKey;
        OGRFieldDefn *poFDefn;

        if ( psKeyDef->field_index >= iFIDFieldIndex &&
            psKeyDef->field_index < iFIDFieldIndex + SPECIAL_FIELD_COUNT )
        {
            /* warning: only special fields of type string should be deallocated */
            if (SpecialFieldTypes[psKeyDef->field_index - iFIDFieldIndex] == SWQ_STRING)
            {
                for( i = 0; i < nIndexSize; i++ )
                {
                    OGRField *psField = pasIndexFields + iKey + i * nOrderItems;
                    CPLFree( psField->String );
                }
            }
            continue;
        }

        poFDefn = poSrcLayer->GetLayerDefn()->GetFieldDefn( 
            psKeyDef->field_index );

        if( poFDefn->GetType() == OFTString )
        {
            for( i = 0; i < nIndexSize; i++ )
            {
                OGRField *psField = pasIndexFields + iKey + i * nOrderItems;
                
                if( psField->Set.nMarker1 != OGRUnsetMarker 
                    || psField->Set.nMarker2 != OGRUnsetMarker )
                    CPLFree( psField->String );
            }
        }
    }

    CPLFree( pasIndexFields );
}

/************************************************************************/
/*                          SortIndexSection()                          */
/*                                                                      */
/*      Sort the records in a section of the index.                     */
/************************************************************************/

void OGRGenSQLResultsLayer::SortIndexSection( OGRField *pasIndexFields, 
                                              int nStart, int nEntries )

{
    if( nEntries < 2 )
        return;

    swq_select *psSelectInfo = (swq_select *) pSelectInfo;
    int      nOrderItems = psSelectInfo->order_specs;

    int nFirstGroup = nEntries / 2;
    int nFirstStart = nStart;
    int nSecondGroup = nEntries - nFirstGroup;
    int nSecondStart = nStart + nFirstGroup;
    int iMerge = 0;
    long *panMerged;

    SortIndexSection( pasIndexFields, nFirstStart, nFirstGroup );
    SortIndexSection( pasIndexFields, nSecondStart, nSecondGroup );

    panMerged = (long *) CPLMalloc( sizeof(long) * nEntries );
        
    while( iMerge < nEntries )
    {
        int  nResult;

        if( nFirstGroup == 0 )
            nResult = -1;
        else if( nSecondGroup == 0 )
            nResult = 1;
        else
            nResult = Compare( pasIndexFields 
                               + panFIDIndex[nFirstStart] * nOrderItems, 
                               pasIndexFields 
                               + panFIDIndex[nSecondStart] * nOrderItems );

        if( nResult < 0 )
        {
            panMerged[iMerge++] = panFIDIndex[nSecondStart++];
            nSecondGroup--;
        }
        else
        {
            panMerged[iMerge++] = panFIDIndex[nFirstStart++];
            nFirstGroup--;
        }
    }

    /* Copy the merge list back into the main index */

    memcpy( panFIDIndex + nStart, panMerged, sizeof(long) * nEntries );
    CPLFree( panMerged );
}

/************************************************************************/
/*                              Compare()                               */
/************************************************************************/

int OGRGenSQLResultsLayer::Compare( OGRField *pasFirstTuple,
                                    OGRField *pasSecondTuple )

{
    swq_select *psSelectInfo = (swq_select *) pSelectInfo;
    int  nResult = 0, iKey;

    for( iKey = 0; nResult == 0 && iKey < psSelectInfo->order_specs; iKey++ )
    {
        swq_order_def *psKeyDef = psSelectInfo->order_defs + iKey;
        OGRFieldDefn *poFDefn;

        if( psKeyDef->field_index >= iFIDFieldIndex )
            poFDefn = NULL;
        else
            poFDefn = poSrcLayer->GetLayerDefn()->GetFieldDefn( 
                psKeyDef->field_index );
        
        if( (pasFirstTuple[iKey].Set.nMarker1 == OGRUnsetMarker 
             && pasFirstTuple[iKey].Set.nMarker2 == OGRUnsetMarker)
            || (pasSecondTuple[iKey].Set.nMarker1 == OGRUnsetMarker 
                && pasSecondTuple[iKey].Set.nMarker2 == OGRUnsetMarker) )
            nResult = 0;
        else if ( poFDefn == NULL )
        {
            switch (SpecialFieldTypes[psKeyDef->field_index - iFIDFieldIndex])
            {
              case SWQ_INTEGER:
                if( pasFirstTuple[iKey].Integer < pasSecondTuple[iKey].Integer )
                    nResult = -1;
                else if( pasFirstTuple[iKey].Integer > pasSecondTuple[iKey].Integer )
                    nResult = 1;
                break;
              case SWQ_STRING:
                nResult = strcmp(pasFirstTuple[iKey].String,
                                 pasSecondTuple[iKey].String);
                break;

              default:
                CPLAssert( FALSE );
                nResult = 0;
            }
        }
        else if( poFDefn->GetType() == OFTInteger )
        {
            if( pasFirstTuple[iKey].Integer < pasSecondTuple[iKey].Integer )
                nResult = -1;
            else if( pasFirstTuple[iKey].Integer 
                     > pasSecondTuple[iKey].Integer )
                nResult = 1;
        }
        else if( poFDefn->GetType() == OFTString )
            nResult = strcmp(pasFirstTuple[iKey].String,
                             pasSecondTuple[iKey].String);
        else if( poFDefn->GetType() == OFTReal )
        {
            if( pasFirstTuple[iKey].Real < pasSecondTuple[iKey].Real )
                nResult = -1;
            else if( pasFirstTuple[iKey].Real > pasSecondTuple[iKey].Real )
                nResult = 1;
        }

        if( psKeyDef->ascending_flag )
            nResult *= -1;
    }

    return nResult;
}