# Interesting question on how ferret indexes work

**URL:** <https://www.ruby-forum.com/t/interesting-question-on-how-ferret-indexes-work/115051>\
**Category:** Ferret\
**Created:** [September 20, 2007, 6:10am UTC](https://www.ruby-forum.com/t/interesting-question-on-how-ferret-indexes-work/115051 "2007-09-20T06:10:45Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![snowmaker](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/a183cd/32.png) [@snowmaker](https://www.ruby-forum.com/u/snowmaker)\
**Post date:** [September 20, 2007, 6:10am UTC](https://www.ruby-forum.com/t/interesting-question-on-how-ferret-indexes-work/115051/1 "2007-09-20T06:10:45Z")

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I have a question about how Ferret handles intersection queries. Say I  
do an intersection search on two fields, so I do ‘ruby author:(jared)’  
(no quotes)

Does Ferret do some kind of fancy index intersection to efficiently  
retrieve only items that meet both queries? Or does it just find all  
the ruby’s, then prune away everything that doesn’t match  
author:(jared)? Does it run both searches independently, then take the  
intersection?

Help understanding this would be much appreciated.

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**Author:** ![snowmaker](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/a183cd/32.png) [@snowmaker](https://www.ruby-forum.com/u/snowmaker)\
**Post date:** [September 20, 2007, 11:22pm UTC](https://www.ruby-forum.com/t/interesting-question-on-how-ferret-indexes-work/115051/2 "2007-09-20T23:22:55Z")

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On Thu, Sep 20, 2007 at 06:10:45AM +0200, Jared Friedman wrote:

> I have a question about how Ferret handles intersection queries. Say I  
> do an intersection search on two fields, so I do ‘ruby author:(jared)’  
> (no quotes)
> 
> Does Ferret do some kind of fancy index intersection to efficiently  
> retrieve only items that meet both queries? Or does it just find all  
> the ruby’s, then prune away everything that doesn’t match  
> author:(jared)? Does it run both searches independently, then take the  
> intersection?

that’s really an interesting question, and I’m not really sure about the  
answer.

> From looking at the ConjunctionScorer class of an old version of Ferret  
> (0.3.2, which was still mostly Ruby), I’d say it first collects the  
> document ids matching each single clause, and then steps through them  
> collecting those that have been matched by all clauses.

Jens

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