java - MongoDB distinct too big 16mb cap -

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i have mongodb collection. simply, has 2 columns: user , url. has 39274590 rows. key of table {user, url}.

using java, try list distinct urls:

  mongodbmanager db = new mongodbmanager( "website", "userlog" );   return db.getdistinct("url");

but receive exception:

exception in thread "main" com.mongodb.commandresult$commandfailure: command failed [distinct]:  { "serverused" : "localhost/127.0.0.1:27017" , "errmsg" : "exception: distinct big, 16mb cap" , "code" : 10044 , "ok" : 0.0}

how can solve problem? there plan b can avoid problem?

thanks.

take @ answer

1) easiest way via aggregation framework. takes 2 "$group" commands: first 1 groups distinct values, second 1 counts of distinct values

2) if want map/reduce can. two-phase process: in first phase build new collection list of every distinct value key. in second count() on new collection.

note cannot return result of map/reduce inline, because potentially overrun 16mb document size limit. can save calculation in collection , count() size of collection, or can number of results return value of mapreduce().


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