$skip and $limit in aggregation framework
MongodbAggregation FrameworkMongodb Problem Overview
When I read the document I found the following notes:
> When a $sort immediately precedes a $limit in the pipeline, the $sort operation only maintains the top n results as it progresses, where n is the specified limit, and MongoDB only needs to store n items in memory. This optimization still applies when allowDiskUse is true and the n items exceed the aggregation memory limit.
If I'm right about this, it applies only when I use the $sort and $limit together like
db.coll.aggregate([
...,
{$sort: ...},
{$limit: limit},
...
]);
However, I think most of the time we would have
db.coll.aggregate([
...,
{$sort: ...},
{$skip: skip},
{$limit: limit},
...
]);
Question 1: Does it mean the rule above doesn't apply if I use $skip here?
I ask this question because theoretically MongoDB can still calculate the top n records and enhance performance by sorting only top n records. I didn't find any document about this though. And if the rule doesn't apply,
Question 2: Do I need to change my query to the following to enhance performance?
db.coll.aggregate([
...,
{$sort: ...},
{$limit: skip + limit},
{$skip: skip},
{$limit: limit},
...
]);
EDIT: I think explains my use case would make the question above makes more sense. I'm using the text search feature provided by MongoDB 2.6 to look for products. I'm worried if the user inputs a very common key word like "red", there will be too many results returned. Thus I'm looking for better ways to generate this result.
EDIT2: It turns out that the last code above equals to
db.coll.aggregate([
...,
{$sort: ...},
{$limit: skip + limit},
{$skip: skip},
...
]);
Thus I we can always use this form to make the top n rule apply.
Mongodb Solutions
Solution 1 - Mongodb
Since this is a text search query we are talking about then the most optimal form is this:
db.collection.aggregate([
{
"$match": {
"$text": { "$search": "cake tea" }
}
},
{ "$sort": { "score": { "$meta": "textScore" } } },
{ "$limit": skip + limit },
{ "$skip": skip }
])
The rationale on the memory reserve from the top "sort" results will only work within it's own "limits" as it were and this will not be optimal for anything beyond a few reasonable "pages" of data.
Beyond what is reasonable for memory consumption, the additional stage will likely have a negative effect rather than positive.
These really are the practical limitations of the text search capabilities available to MongoDB in the current form. But for anything more detailed and requiring more performance, then just as is the case with many SQL "full text" solutions, you are better off using an external "purpose built" text search solution.
Solution 2 - Mongodb
I found that it seems the sequence of limit and skip is immaterial. If I specify skip before limit, the mongoDB will make limit before skip under the hood.
> db.system.profile.find().limit(1).sort( { ts : -1 } ).pretty()
{
"op" : "command",
"ns" : "archiprod.userinfos",
"command" : {
"aggregate" : "userinfos",
"pipeline" : [
{
"$sort" : {
"updatedAt" : -1
}
},
{
"$limit" : 625
},
{
"$skip" : 600
}
],
},
"keysExamined" : 625,
"docsExamined" : 625,
"cursorExhausted" : true,
"numYield" : 4,
"nreturned" : 25,
"millis" : 25,
"planSummary" : "IXSCAN { updatedAt: -1 }",
/* Some fields are omitted */
}
What happens if I swtich $skip and $limit? I got the same result in terms of keysExamined and docsExamined.
> db.system.profile.find().limit(1).sort( { ts : -1 } ).pretty()
{
"op" : "command",
"ns" : "archiprod.userinfos",
"command" : {
"aggregate" : "userinfos",
"pipeline" : [
{
"$sort" : {
"updatedAt" : -1
}
},
{
"$skip" : 600
},
{
"$limit" : 25
}
],
},
"keysExamined" : 625,
"docsExamined" : 625,
"cursorExhausted" : true,
"numYield" : 5,
"nreturned" : 25,
"millis" : 71,
"planSummary" : "IXSCAN { updatedAt: -1 }",
}
I then checked the explain result of the query. I found that totalDocsExamined is already 625 in the limit stage.
> db.userinfos.explain('executionStats').aggregate([ { "$sort" : { "updatedAt" : -1 } }, { "$limit" : 625 }, { "$skip" : 600 } ])
{
"stages" : [
{
"$cursor" : {
"sort" : {
"updatedAt" : -1
},
"limit" : NumberLong(625),
"queryPlanner" : {
"winningPlan" : {
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"updatedAt" : -1
},
"indexName" : "updatedAt_-1",
}
},
},
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 625,
"executionTimeMillis" : 22,
"totalKeysExamined" : 625,
"totalDocsExamined" : 625,
"executionStages" : {
"stage" : "FETCH",
"nReturned" : 625,
"executionTimeMillisEstimate" : 0,
"works" : 625,
"advanced" : 625,
"docsExamined" : 625,
"inputStage" : {
"stage" : "IXSCAN",
"nReturned" : 625,
"works" : 625,
"advanced" : 625,
"keyPattern" : {
"updatedAt" : -1
},
"indexName" : "updatedAt_-1",
"keysExamined" : 625,
}
}
}
}
},
{
"$skip" : NumberLong(600)
}
]
}
And surprisingly, I found switching the $skip and $limit results in the same explain result.
> db.userinfos.explain('executionStats').aggregate([ { "$sort" : { "updatedAt" : -1 } }, { "$skip" : 600 }, { "$limit" : 25 } ])
{
"stages" : [
{
"$cursor" : {
"sort" : {
"updatedAt" : -1
},
"limit" : NumberLong(625),
"queryPlanner" : {
/* Omitted */
},
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 625,
"executionTimeMillis" : 31,
"totalKeysExamined" : 625,
"totalDocsExamined" : 625,
/* Omitted */
}
}
},
{
"$skip" : NumberLong(600)
}
]
}
As you can see, even though I specified $skip before $limit, in the explain result, it's still $limit before $skip.
Solution 3 - Mongodb
Answer: $skip before $limit
The order of $skip and $limit definitely matters, for aggregations at least. I just tried this, I don't know how it was missed, maybe it has changed since the op but I thought I would share.
I agree with @vkarpov15's comment in this conversation
> In aggregate, $limit limits the number of documents sent to the next aggregation state, and $skip skips the first N documents, so if $skip is after $limit and $skip >= $limit, you won't get any results. In short, this is expected behavior in MongoDB.