db.scores.aggregate([{$match:{game:/^G[123]/}},{$count:"c"}])
{ "c" : 368061 }
db.scores.aggregate([{$match:{game:/^G[123]/}},{$group:{_id:0, size:{$sum:{$bsonSize:"$$ROOT"}}}}])
{ "_id" : 0, "size" : 30215379 }  /* 28GBs */
db.scores.aggregate([{$match:{game:/^G[123]/}}, {$group:{_id:"$game", top2: { $accumulator: { init: function() {  return null;   },  accumulateArgs: [ [1,2,3,4,5,"$player","$score"] ],                     accumulate: function(state, val) {    return state;     },                     merge: function(state1, state2) {                         return state1;  },      finalize: function(state) {   return state;   }   } }  } },{$count:"c"}])
{ "c" : 33 }
/* as soon as I pass a document as args */
db.scores.aggregate([{$match:{game:/^G[123]/}}, {$group:{_id:"$game", top2: { $accumulator: { init: function() {  return null;   },  accumulateArgs: [ {score:"$score"} ],                     accumulate: function(state, val) {    return state;     },                     merge: function(state1, state2) {                         return state1;  },      finalize: function(state) {   return state;   }   } }  } },{$count:"c"}])
Error: command failed: {
	"ok" : 0,
	"errmsg" : "Exceeded memory limit for $group, but didn't allow external sort. Pass allowDiskUse:true to opt in.",
	"code" : 292,
	"codeName" : "QueryExceededMemoryLimitNoDiskUseAllowed"
} : aggregate failed :
_getErrorWithCode@src/mongo/shell/utils.js:25:13
doassert@src/mongo/shell/assert.js:18:14
_assertCommandWorked@src/mongo/shell/assert.js:618:17
assert.commandWorked@src/mongo/shell/assert.js:708:16
DB.prototype._runAggregate@src/mongo/shell/db.js:266:5
DBCollection.prototype.aggregate@src/mongo/shell/collection.js:1012:12
@(shell):1:1
/* with limit */
db.scores.aggregate([{$match:{game:/^G[123]/}},{$limit:335000}, {$group:{_id:"$game", top2: { $accumulator: { init: function() {  return null;   },  accumulateArgs: [ {score:"$score"} ],                     accumulate: function(state, val) {    return state;     },                     merge: function(state1, state2) {                         return state1;  },      finalize: function(state) {   return state;   }   } }  } },{$count:"c"}])
{ "c" : 30 }
db.scores.aggregate([{$match:{game:/^G[123]/}},{$limit:340000}, {$group:{_id:"$game", top2: { $accumulator: { init: function() {  return null;   },  accumulateArgs: [ {score:"$score"} ],                     accumulate: function(state, val) {    return state;     },                     merge: function(state1, state2) {                         return state1;  },      finalize: function(state) {   return state;   }   } }  } },{$count:"c"}])
Error: command failed: {
	"ok" : 0,
	"errmsg" : "Exceeded memory limit for $group, but didn't allow external sort. Pass allowDiskUse:true to opt in.", ...

db.scores.aggregate({$collStats:{storageStats:{scale:1024*1024}}},{$project:{"storageStats.wiredTiger":0,"storageStats.indexDetails":0}}).pretty()
{
	"ns" : "agg.scores",
	"host" : "asyas-mbp-4.lan:27017",
	"localTime" : ISODate("2020-05-23T18:44:17.107Z"),
	"storageStats" : {
		"size" : 95,
		"count" : 1130151,
		"avgObjSize" : 88,
		"storageSize" : 31,
		"freeStorageSize" : 0,
		"capped" : false,
		"nindexes" : 2,
		"indexBuilds" : [ ],
		"totalIndexSize" : 51,
		"totalSize" : 82,
		"indexSizes" : {
			"_id_" : 19,
			"game_1_score_-1" : 32
		},
		"scaleFactor" : 1048576
	}
}