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From Erik Hatcher <e...@ehatchersolutions.com>
Subject Re: Paging results
Date Wed, 29 Mar 2006 12:08:39 GMT

On Mar 29, 2006, at 5:09 AM, Marios Skounakis wrote:
> I am executing searches that return between 2000 and 10000  
> documents and sorting the results by relevance (or sometimes  
> alphabetically).
>
> In every query, I need to discard some of the results based on  
> their docId. I have a list of the docIds that need to be discarded  
> in an array. The size of the list is usually about 100.
>
> I generally perform paging, so I don't need to display all the  
> results, but only those of the current page (e.g. results 100 to 120)
>
> Currently, after getting the Hits object from the Searcher, I loop  
> over the documents, retrieve their docId, and see if it is in the  
> discard list. If not, I put the document in a validResult  
> collection. When I have read enough valid results to be able to  
> show the current page, I stop. E.g., in the above example, I would  
> read until I had 120 valid results.
>
> The problem with this implementation is that I have to read the  
> docId for the results that precede the current page, in order to  
> determine if they are in the invalid list. So, when showing the  
> pages near the last page, I have to read the entire result list. I  
> don't like this because this means that the computation required to  
> display a page is not constant but depends on the page's position.  
> Anyway, what do the experts think? Is this implementation  
> prohibitively expensive?

Yes, that implementation is going to be prohibitively expensive.  The  
recommended way to employ a search constraint as you described is to  
use a Filter.  You could write a custom Filter that would use your  
discarded docId array, and turn off those documents from the BitSet,  
and turn on all the rest.  Then you will use the search(Query,  
Filter) method signature for searching.

> (As a sidenote, when calling hits.doc(i), does Lucene retrieve the  
> whole document, or just a pointer to it, and retrieves the data  
> when doing hits.doc(i).getField...?)

It retrieves the whole document currently.

> An alternative would be to extend the query to exclude the ids in  
> the discard list. How would adding 100 exclusion clauses to the  
> query impact the query's performance? Are there any studies on  
> search speed in relation to the number of query clauses?

Adding them to the query would certainly work but a Filter is a  
cleaner and faster way for your scenario.

	Erik


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