/// <summary>
/// WrapParallel builds a parallel copy of the query stage for each source in ConcatenatingTable sources.
/// It is used to allow running optimized query stages and running in parallel when multiple tables are involved.
/// </summary>
/// <remarks>
/// WrapParallel can only be used by verbs where the output when run on the concatenated inputs rows from many sources
/// produces the same output as running in parallel on each source and then concatenating the result rows.
/// </remarks>
/// <param name="source">IXTable to wrap</param>
/// <param name="builder">Wrapping function</param>
/// <returns>Wrapped IXTable</returns>
public static IXTable WrapParallel(this IXTable source, XqlParser parser, Func <IXTable, IXTable> builder)
{
ConcatenatedTable cSource = source as ConcatenatedTable;
if (cSource != null)
{
Position currentPosition = parser.CurrentPosition;
return(ConcatenatedTable.Build(cSource.Sources.Select((s) =>
{
parser.RewindTo(currentPosition);
return builder(s);
})));
}
return(builder(source));
}