Faceting and Multifaceting syntax in Solr 1.4

Introduction

So you've installed Solr 1.4 and you've managed to get some data indexed. Now you're ready to have some fun with faceting. Faceting is basically just filtering the results of a search without effecting the relevance score. Sites such as ebay use faceting to help narrow down the results of a generic search like TV, to give you options such as 32" wide-screen LCD etc.

 

It makes for a very pleasant user experience. It's one of the main reasons to use Solr and Solr makes this process very easy. There are actually 3 types of faceting. In this post I'm only going to talk about "field faceting". Once you've mastered "field faceting", the other 2 types ("query faceting" and "date faceting") are very easy and the basic Solr Wiki will be enough for you to get going.

Field Faceting

Now when you're indexing data in Solr, it's best to use the "text" field type, as this applies lots of filters which will help sort out plurals, remove white space etc. However when it comes to faceting, the values returned will be the values after the filters have been applied. Therefore it's best to create new versions of any fields you wish to facet.

You can use the copyfield keyword to make this process very easy.

For example if you have a field called county

Now, when you index county, Solr will use the text field for searching and the string field for displaying the facet results.

Lets get faceting

First up you'll need to turn on the faceting as it's turned off by default.

http://localhost:8080/solr/select/?q=*%3A*&rows=0&facet=on

To facet on a field, all you have to do is use the facet.field=fieldname

http://localhost:8080/solr/select/?q=*%3A*&rows=0&facet=on&facet.field=fcounty

You can apply extra keywords to: limit the results

http://localhost:8080/solr/select/?q=*%3A*&rows=0&facet=on&facet.field=fcounty&facet.limit=3

or even page the results

http://localhost:8080/solr/select/?q=*%3A*&rows=0&facet=on&facet.field=fcounty&facet.limit=5&&facet.offset=5

  

You can sort the results by number descending (Default)

http://localhost:8080/solr/select/?q=*%3A*&rows=0&facet=on&facet.field=fcounty&facet.sort=count

or alphabetical

http://localhost:8080/solr/select/?q=*%3A*&rows=0&facet=on&facet.field=fcounty&facet.sort=lex

 

You can make it so Solr only brings back facets with a minimum number of results:

http://localhost:8080/solr/select/?q=*%3A*&rows=0&facet=on&facet.field=fcounty&facet.mincount=10

  

for more information see solr wiki

Multifaceting

On a website i've worked on we used multifaceting, Solr has a bit of a clunky syntax to support this, but it works and works well.. so who cares :) Lets say I want to apply a filter (fq) on Kent as a county

http://localhost:8080/solr/select/?q=*%3A*&rows=0&facet=on&facet.field=fcounty&fq=fcounty:Kent

 

When you apply the above filter you can see that the other counties get a facet count of 0. This is single faceting and works well. To get multifaceting to work you need to tell solr to exclude the filter (fq) when it works out the facet counts. To do this you can use the ex and tag key words. When you do this you'll notice that the counts for the other facet's come back now, and the filter has still been applied to the results (numFound is still 11).

http://localhost:8080/solr/select/?q=*%3A*&version=2.2&start=0&rows=0&indent=on&facet=on&facet.field={!ex=fcounty}fcounty&fq={!tag=fcounty}fcounty:Kent

 

You can apply as many filter queries as you want now by ORing them together. So if i wanted to filter on Kent OR Cambridgeshire, you can do that as follows:

http://localhost:8080/solr/select/?q=*%3A*&version=2.2&start=0&rows=0&indent=on&facet=on&facet.field={!ex=fcounty}fcounty&fq={!tag=fcounty}fcounty:Kent%20OR%20Cambridgeshire

See how the numFound has gone up from 11 to 18

Conclusion

Now using Solr you can create a really rich user experience with searches and give ebay a run for it's money. Enjoy.