Question

Given: Connection is Safe=True so Update’s return will contain update information.

Say I have a documents that look like:

[{'a': [1]}, {'a': [2]}, {'a': [1,2]}]

And I issue:

coll.update({}, {'$addToSet': {'a':1}}, multi=True)

The result would be:

{u'connectionId': 28,
 u'err': None,
 u'n': 3,
 u'ok': 1.0,
 u'updatedExisting': True
}

Even when come documents already have that value. To avoid this I could issue a command.

coll.update({'a': {'$ne': 1}}, {'$push': {'a':1}}, multi=True)

What’s the Time Complexity Comparison for $addToSet vs. $push with a $ne check ?

Answer 1

Looks like $addToSet is doing the same thing as your command: $push with a $ne check. Both would be O(N)

https://github.com/mongodb/mongo/blob/master/src/mongo/db/ops/update_internal.cpp

if speed is really important then why not use a hash:

instead of:

{'$addToSet': {'a':1}}
{'$addToSet': {'a':10}}

use:

{$set: {'a.1': 1}
{$set: {'a.10': 1}