Truncate a collection

To truncate a collection and keep the indexes use

 db.<collection>.remove({})

You can efficiently drop all data and indexes for a collection with db.collection.drop(). Dropping a collection with a large number of documents and/or indexes will be significantly more efficient than deleting all documents using db.collection.remove({}). The remove() method does the extra housekeeping of updating indexes as documents are deleted, and would be even slower in a replica set environment where the oplog would include entries for each document removed rather than a single collection drop command.

Example using the mongo shell:

var dbName = 'nukeme';
db.getSiblingDB(dbName).getCollectionNames().forEach(function(collName) {
    // Drop all collections except system ones (indexes/profile)
    if (!collName.startsWith("system.")) {
        // Safety hat
        print("WARNING: going to drop ["+dbName+"."+collName+"] in 5s .. hit Ctrl-C if you've changed your mind!");
        sleep(5000);
        db[collName].drop();
    }
})

It is worth noting that dropping a collection has different outcomes on storage usage depending on the configured storage engine:

WiredTiger (default storage engine in MongoDB 3.2 or newer) will free the space used by a dropped collection (and any associated indexes) once the drop completes.
MMAPv1 (default storage engine in MongoDB 3.0 and older) will not free up preallocated disk space. This may be fine for your use case; the free space is available for reuse when new data is inserted.

If you are instead dropping the database, you generally don't need to explicitly create the collections as they will be created as documents are inserted.

However, here is an example of dropping and recreating the database with the same collection names in the mongo shell:

var dbName = 'nukeme';

// Save the old collection names before dropping the DB
var oldNames = db.getSiblingDB(dbName).getCollectionNames();

// Safety hat
print("WARNING: going to drop ["+dbName+"] in 5s .. hit Ctrl-C if you've changed your mind!")
sleep(5000)

db.getSiblingDB(dbName).dropDatabase();

// Recreate database with the same collection names
oldNames.forEach(function(collName) {
    db.getSiblingDB(dbName).createCollection(collName);
})

the below query will delete all records in a collections and will keep the collection as is,

db.collectionname.remove({})

There is no equivalent to the "truncate" operation in MongoDB. You can either remove all documents, but it will have a complexity of O(n), or drop the collection, then the complexity will be O(1) but you will loose your indexes.

Truncate a collection

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