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mongodbdrop

Faster way to remove all entries from mongodb collection by dropping collection and recreating schema

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?

Solution

  • 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:

    1. dump an empty collection + the indexes of the collection

    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

    2. Drop the comments collection

    mongo mongodb://127.0.0.1:7001/coral --eval 'db.comments.drop();'

    3. Import new data 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.

    4. Add indexes back

    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