I’ve been spending the last few days at work trying to improve our search performance, and have been banging my head against the dismax query target and parser in Solr. For those not familiar with the Dismax, its a simplified parser for Solr that eliminates the complexity from the Standard query parser. Instead of search terms like “field_name:value” you can simple enter “value”, but you can no longer search for a specific term in a specific field.
Our search index has grown in the last few months by 20% and our JVM and Solr setups were beginning to groan under the weight of the data. I went through a few rounds of JVM tuning, which reduced garbage collection time to less than 2%, and with some Solr configuration options managed to bring our typical query back under 5 seconds. This felt like a major win, until I adjusted the query.
Looking at our query parameters on search I noticed we were using the “fq” parameter to specify the id of the particular site we were looking for. These queries were taking anywhere from 5-15 seconds across our 360GB index, and I suspected that we were pulling in data to the JVM only to filter it away. The garbage collection graphs seemed to indicate this as well, since we had a very slow growing heap, and our eden space was emptying very quickly even with 20G allocated to it. When I changed from dismax to the standard target and specified the site id, I noticed my search time went from 5 seconds to .06 seconds, so started reading, and came across an article on nested queries. My idea was that this would allow me to apply a constraint to the initial set of data returned, using the standard search target, and then perform a full text search using dismax and achieve the same results.
Original Query (grossly simplified):
Becomes the following nested query:
Original Query Time : 5 seconds
Nested Query Time : 87 milliseconds
Both return identical results. So, if performing a query against a large index and you want to use dismax, you should try using a nested search. You’re likely see much better performance, particularly if you’re filtering based on a facet. And this gives you a relatively easy way to specify the value of a field, and still want to use a dismax query.
Another good article on Dismax can be found here.