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copied!<p>This is an interesting question!</p> <p>The solution really depends on your expected use case. If each user has a list of posts they've tagged, and that is all the information you need, it will be expedient to list these as a field in the user's table (or in their blob if you're using a nosql backend - a viable option if this is your use case!). There will be no impact on transmission time since the list will be the same size either way, but in this solution you will probably save on lookup time, since you're only using one table and dbs will optimize to keep this information close together.</p> <p>On the other hand, if you have to be able to query a given post for all the users that have tagged it, then option two will be much better. In the former method, you'd have to query all users and see if each one had the post. In this option, you simply have to find all the relations and work from there. Presumably you'd have a <code>user</code> table, a <code>post</code> table and a <code>user_post</code> table with foreign keys to the first two tables. There are other ways to do this, but it necessitates maintaining multiple lists and cross checking each time, which is an expensive set of operations and error-prone.</p> <p>Note that the latter option shouldn't choke on 'millions' of connections, since the db should be optimized for this sort of quick read. (pro tip: index the proper columns!) Do be careful about any data massage, though. One unnecessary for-loop will kill your performance.</p>

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