When I want to remove all objects from my mongoDB collection comments I do this with this command:
mongo $MONGODB_URI --eval 'db.comments.deleteMany({});'
However, this is super slow when there are millions of records inside the collection.
In a relational db like Postgres I'd simply copy the structure of the collection, create a comments2 collection, drop the comments collection, and rename comments2 to comments.
Is this possible to do in MongoDB as well? Or are there any other tricks to speed up the progress?
Thanks, the answers inspired my own solution. I forgot that MongoDB doesn't have a schema like a relationalDB.
So what I did is this:
mongodump --host=127.0.0.1 --port=7001 --db=coral --collection=comments --query='{"id": "doesntexist"}' --out=./dump
This will create a folder ./dump with the contents comments.bson (empty) and comments.metadata.json
comments collectionmongo mongodb://127.0.0.1:7001/coral --eval 'db.comments.drop();'
new_comments.json (different from comments.bson)mongoimport --uri=mongodb://127.0.0.1:7001/coral --file=new_comments.json --collection comments --numInsertionWorkers 12
This is way faster than first adding the indexes, and then importing.
mongorestore --uri=mongodb://127.0.0.1:7001/coral --dir dump/coral --nsInclude coral.comments --numInsertionWorkersPerCollection 12
Note that --numInsertionWorkers speeds up to process by dividing the work over 12 cpus.
How many cpus do you have can be found on OSx with:
sysctl -n hw.ncpu
db.cities.aggregate([{ $match: {} }, { $out: "collection2" }]) in case you can login to the mongo prompt and simply drop the previous collection. Otherwise, the approach you have posted is the one.
mongoexport.exe /host: /port: /db:test /collection:collection1 /out:collection1.json mongoimport.exe /host: /port: /db:test /collection:collection2 /file:collection1.json
Thanks, Neha
For mongodb version >=4.0 you can do this via db.comments.renameCollection("comments2") ,but it is kind of resource intensive operation and for bigger collections better you do mongodump/mongorestore. So the best action steps are:
mongodump -d x -c comments -out dump.bson
>use x
>db.comments.drop()
mongorestore -d x -c comments2 dump.bson
Plese, note deleteMany({}) is even more resource intensive operation since it will create oplog single entry for every document you delete and propagate to all replicaSet members.