Why sort document by id is slower with $match than not in mongodb? - mongodb

Question

So, I tried to query
db.collection('collection_name').aggregate([
{
$match: { owner_id: '5be9b2f03ef77262c2bd49e6' }
},
{
$sort: { _id: -1 }
}])
the query above takes up 20s
but If I tried to query
db.collection('collection_name').aggregate([{$sort : {_id : -1}}])
it's only take 0.7s
Why does it the one without $match is actually faster than without match ?
update :
when I try this query
db.getCollection('callbackvirtualaccounts').aggregate([
{
$match: { owner_id: '5860457640b4fe652bd9c3eb' }
},
{
$sort: { created: -1 }
}
])
it's only takes 0.781s
Why sort by _id is slower than by created field ?
note : I'm using mongodb v3.0.0

Answer 1

db.collection('collection_name').aggregate([
{
$match: { owner_id: '5be9b2f03ef77262c2bd49e6' }
},
{
$sort: { _id: -1 }
}])
This collection probably won't be having and index on owner_id; Try using below mentioned index creation query and rerun your previous code.
db.collection('collection_name').createIndexes({ owner_id:1}) //Simple Index
or
db.collection('collection_name').createIndexes({ owner_id:1,_id:-1}) //Compound Index
**Note:: If you don't know how to compound index yet, you can create simple indexes individually on all keys which are used either in match or sort and that should be making query efficient as well.

Answer 2

The query speed depends upon a lot of factors. The size of collection, size of the document, indexes defined on the collection (and used in the queries and properly), the hardware components (like CPU, RAM, network) and other processes running at the time the query is running.
You have to tell what indexes are defined on the collection being discussed for further analysis. The command will retrieve them: db.collection.getIndexes()
Note the unique index on the _id field is created by default, and cannot be modified or deleted.
(i)
But If I tried to query: db.collection.aggregate( [ { $sort : { _id : -1 } } ] ) it's
only take 0.7s.
The query is faster because there is an index on the _id field and it is used in sort process. Aggregation queries use indexes with sort stage and when this sort happens early in the pipeline. You can verify if the index is used or not by generating a query plan (use explain with executionStats mode). There will be an index scan (IXSCAN) in the generated query plan.
(ii)
db.collection.aggregate([
{
$match: { owner_id: '5be9b2f03ef77262c2bd49e6' }
},
{
$sort: { _id: -1 }
}
])
The query above takes up 20s.
When I try this query it's only takes 0.781s.
db.collection.aggregate([
{
$match: { owner_id: '5860457640b4fe652bd9c3eb' }
},
{
$sort: { created: -1 }
}
])
Why sort by _id is slower than by created field ?
Cannot come to any conclusions with the available information. In general, the $match and $sort stages present early in the aggregation query can use any indexes created on the fields used in the operations.
Generating a query plan will reveal what the issues are.
Please run the explain with executionStats mode and post the query plan details for all queries in question. There is documentation for Mongodb v3.0.0 version on generation query plans using explain: db.collection.explain()