Let say you have a collection of 10,000 documents and I make a find query with a the option limit(50). How will mongoDb choose which 50 documents to return.
Will it auto-sort them(maybe by their creation date) or not?
Will the query return the same documents every time it is called? How does the limit option work in mongodb?
Does mongoDB limit the documents after they are returned or as it queries them. Meaning will mongoDB query all documents the limit the results to 50 documents or will it query the 50 documents only?
The first 50 documents of the result set will be returned.
If you do not sort the documents (or if the order is not well-defined, such as sorting by a field with values that occur multiple times in the result set), the order may change from one execution to the next.
Will it auto-sort them(maybe by their creation date) or not?
No.
Will the query return the same documents every time it is called?
The query may produce the same results for a while and then start producing different results if, for example, another document is inserted into the collection.
Meaning will mongoDB query all documents the limit the results to 50 documents or will it query the 50 documents only?
Depends on the query. If an index is used, only the needed documents will be read from the storage engine. If a sort stage is used in the query execution, all documents will be read from storage, sorted, then the required number will be returned and the rest discarded.
Related
I have a mongodb collection called employeeInformation, in which I have two documents:
{"name1":"tutorial1"}, {"name2":"tutorial2"}
When I do db.employeeInformation.find(), I get both these documents displayed. My question is - is there a query that I can run to confirm that the collection contains only those two specified documents? I tried db.employeeInformation.find({"name1":"tutorial1"}, {"name2":"tutorial2"}) but I only got the id corresponding to the first object with key "name1". I know it's easy to do here with 2 documents just by seeing the results of .find(), but I want to ensure that in a situation where I insert multiple (100's) of documents into the collection, I have a way of verifying that the collection contains all and only those 100 documents (note I will always have the objects themselves as text). Ideally this query should work in mongoatlas console/interface as well.
db.collection.count()
will give you number of inserts once you have inserted the document.
Thanks,
Neha
Let's say I have a collection mycollection that has 1,000,000 records.
How many records will this query return?
const query = firestore.collection('mycollection').get()
I couldn't find that in docs.
There is no default limit. The query you're showing is asking for all of the documents in mycollection. For large collections, you will need to impose a limit in order to avoid excessive costs and running out of memory.
From firebase.google.com documentation:
By default, a query retrieves all documents that satisfy the query in
ascending order by document ID. You can specify the sort order for
your data using orderBy(), and you can limit the number of documents
retrieved using limit().
I want to delete all the collections from my db which are not used for long time. Is there any why i can check when the particular collection was last used?
It depends what you mean by 'last used'. If you mean the last time a document was inserted into the collection then you could do this by converting the ObjectId of the last inserted document into a date. The following query should return the date the last document was inserted:
db.<collection_name>.findOne({},{_id:1})._id.getTimestamp()
the findOne query will return documents in natural order, therefore if you input no query criteria ('{}') then it will return the most recently inserted document. You can then get the _id field and call the getTimestamp() function
I'm not sure if there is any way to reliably tell when a collection was last queried. If you're running your database with profiling enabled then there might be entries in the db.system.profile collection, or in the oplog.
I am trying to add and retrieve documents from collection.I went through mongodb manual and didn't find ways to add documents in descending order by date. Is it possible to add documents to collection in descending order by date while inserting the document as i don't want to query and return the top 50 documents.
TIA.
MongoDB does not guarantee the retrieval order of documents. Even if you insert them in descending order, there is no guarantee the records will be returned in this order. As suggested by BatScream you could add an descending index on your date field and the order your result when querying the data.
My collection name is trial and data size is 112mb
My query is,
db.trial.find()
and i have added limit up-to 10.
db.trial.find.limit(10).
but the limit is not working.the entire query is running.
Replace
db.trial.find.limit(10)
with
db.trial.find().limit(10)
Also you mention that the entire database is being queried? Run this
db.trial.find().limit(10).explain()
It will tell you how many documents it looked at before stopping the query (nscanned). You will see that nscanned will be 10.
The .limit() modifier on it's own will only "limit" the results of the query that is processed, so that works as designed to "limit" the results returned. In a raw form though with no query you should just have the n scanned as the limit you want:
db.trial.find().limit(10)
If your intent is to only operate on a set number of documents you can alter this with the $maxScan modifier:
db.trial.find({})._addSpecial( "$maxScan" , 11 )
Which causes the query engine to "give up" after the set number of documents have been scanned. But that should only really matter when there is something meaningful in the query.
If you are actually trying to do "paging" then you are better of using "range" queries with $gt and $lt and cousins to effectively change the range of selection that is done in your query.