JSON facet performance for aggregations

[Mikhail Ibraheem]

Hi,

I am trying to do aggregation with JSON faceting but performance is very bad 
for one of the requests:

json.facet={  

   studentId:{  

      type:terms,

      limit:-1,

      field:"studentId",

                  facet:{

                  x:"sum(grades)"

                  }

   }

}

 

This request finishes in 250 seconds, and we can't paginate for this service 
for functional reason so we have to use limit:-1, and the cardinality of the 
studentId is 7500.

 

If I try the same with flat facet it finishes in 3 seconds :  
stats=true&facet=true&stats.field={!tag=piv1 
sum=true}grades&facet.pivot={!stats=piv1}studentId

 

We are hoping to use one approach json or flat for all our services. JSON facet 
performance is better for many case.

 

Please advise on why the performance for this is so bad and if we can improve 
it. Also what is the default algorithm used for json facet.

 

Thanks

Mikhail