Very slow sorting, even on small result sets

[Simon Wistow <si...@thegestalt.org>]

We've got a largish corpus (~94 million documents). We'd like to be able 
to sort on one of the string fields. However this takes an incredibly 
long time. A warming query for that field takes about ~20 minutes.

However most of the time the result sets are small since we use filters 
heavily - typically a result set is between 2 and 100 documents.

Yet sorting on the string field is still very, very slow.

Now, as I understand it sorting on a field requires building a 
FieldCache for every document no matter how many documents actually 
match the query.

Is there any way round that - is there any way to say "just sort the 
matched documents"?

We can probably work round this by sorting in application space but I 
wanted to double check that I'm not missing anything before I implement 
that.

thanks,

Simon