internal async Task <T> HeadAsync(bool single, bool orDefault)
{
// Optimized code path for First/Last/Single variants where we can be smart and only ask for 1 or 2 results from the db
// we can use the EXACT streaming mode with Limit = 1|2, and it will work if TargetBytes is 0
if ((this.TargetBytes ?? 0) != 0 || (this.Mode != FdbStreamingMode.Iterator && this.Mode != FdbStreamingMode.Exact))
{ // fallback to the default implementation
return(await FdbAsyncEnumerable.Head(this, single, orDefault, this.Transaction.Cancellation).ConfigureAwait(false));
}
var options = new FdbRangeOptions()
{
Limit = single ? 2 : 1,
TargetBytes = 0,
Mode = FdbStreamingMode.Exact,
Reverse = this.Reversed
};
//BUGBUG: do we need special handling if OriginalRange != Range ? (weird combinations of Take/Skip and Reverse)
var tr = this.Snapshot ? this.Transaction.Snapshot : this.Transaction;
var results = await tr.GetRangeAsync(this.Begin, this.End, options, 0).ConfigureAwait(false);
if (results.IsEmpty)
{ // no result
if (!orDefault)
{
throw new InvalidOperationException("The range was empty");
}
return(default(T));
}
if (single && results.Count > 1)
{ // there was more than one result
throw new InvalidOperationException("The range contained more than one element");
}
// we have a result
return(this.Transform(results.First));
}
