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Databases Mongo Basics Go Further With Mongo Doing More With Queries in Mongo

Posted by davidlichtsteiner
davidlichtsteiner
7,314 Points

Order of operations

Does it matter if i use .limit(2).skip(sort).sort({title: 1}) or sort({title: 1}).limit(2).skip(2)?

2 Answers

David Kanwisher
David Kanwisher
9,751 Points
David Kanwisher
David Kanwisher
9,751 Points

I ran the queries in several different orders and it did not appear to make a difference

> db.posts.find({}, {title: true}).sort({title: -1})  /* show all in collection */
{ "_id" : ObjectId("58f1b04e312bac794e553c60"), "title" : "This is the updated title!" }
{ "_id" : ObjectId("58f1b04e312bac794e553c63"), "title" : "Parenting 101" }
{ "_id" : ObjectId("58f1b04e312bac794e553c61"), "title" : "I love the holidays" }
{ "_id" : ObjectId("58f1b04e312bac794e553c62"), "title" : "How to workout" }

> db.posts.find({}, {title: true}).sort({title: -1}).limit(2).skip(2)
{ "_id" : ObjectId("58f1b04e312bac794e553c61"), "title" : "I love the holidays" }
{ "_id" : ObjectId("58f1b04e312bac794e553c62"), "title" : "How to workout" }

> db.posts.find({}, {title: true}).sort({title: -1}).skip(2).limit(2)
{ "_id" : ObjectId("58f1b04e312bac794e553c61"), "title" : "I love the holidays" }
{ "_id" : ObjectId("58f1b04e312bac794e553c62"), "title" : "How to workout" }

> db.posts.find({}, {title: true}).skip(2).limit(2).sort({title: -1})
{ "_id" : ObjectId("58f1b04e312bac794e553c61"), "title" : "I love the holidays" }
{ "_id" : ObjectId("58f1b04e312bac794e553c62"), "title" : "How to workout" }

> db.posts.find({}, {title: true}).limit(2).skip(2).sort({title: -1})
{ "_id" : ObjectId("58f1b04e312bac794e553c61"), "title" : "I love the holidays" }
{ "_id" : ObjectId("58f1b04e312bac794e553c62"), "title" : "How to workout" }

> db.posts.find({}, {title: true}).limit(2).sort({title: -1}).skip(2)
{ "_id" : ObjectId("58f1b04e312bac794e553c61"), "title" : "I love the holidays" }
{ "_id" : ObjectId("58f1b04e312bac794e553c62"), "title" : "How to workout" }
Daniel Phillips
Daniel Phillips
26,940 Points
Daniel Phillips
Daniel Phillips
26,940 Points

If you're asking about the order of the sort(), limit() and skip() methods, then It matters.

Consider these records (simplified - just titles) : { title: 'C' }, { title: 'B' }, { title: 'D' }, { title: 'A' }

If we want to get this for page 1: { title: 'A' }, { title: 'B' } and this for page 2: { title: 'C' }, { title: 'D' } then for the first page we need to do .sort({title: 1}).limit(2).skip(0). This will sort our full list of records, then take the first 2.

For the second page, we would use .sort({title: 1}).limit(2).skip(2). This finds our records, sorts them, then gives us 2 after skipping the first 2 (so take index 2 and 3 of the sorted records).

If instead for the first page we do: .limit(2).skip(0).sort({title: 1}) then what we are asking for is "find my records, but just 2, don't skip any, then sort them by title". We would get back { title: 'B' }, { title: 'C' }. This isn't what we expected

Then for the second page if we do: .limit(2).skip(2).sort({title: 1}) then what we are asking for is "find my records, but just 2, and skip the first 2, then sort them by title". We would get back { title: 'A' }, { title: 'D' } This also incorrect.