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POTo normalize or not to normalize user_ids
 

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    <p>In my Rails application, I have a variety of database tables that contain user data. Some of these tables have a lot of rows (as many as 500,000 rows per user in some cases) and are queried frequently. Whenever I query any table for anything, the user_id of the current user is somewhere in the query - either directly, if the table has a direct relation with the user, or through a join, if they are related through some other tables.</p> <p>Should I denormalize the user_id and include it in every table, for faster performance?</p> <hr> <p>Here's one example:</p> <ul> <li>Address belongs to user, and has a user_id</li> <li>Envelope belongs to user, and has a user_id</li> <li>AddressesEnvelopes joins an Address and an Envelope, so it has envelope_id and address_id -- it doesn't have user_id, but could get to it through either the envelope or the address (which must belong to the same user).</li> </ul> <p>One common expensive query is to select all the AddressesEnvelopes for a particular user, which I could accomplish by joining with either Address or Envelope, even though I don't need anything from those tables. Or I could just duplicate the user id in this table.</p> <hr> <p>Here's a different scenario:</p> <ul> <li>Letter belongs to user, and has a user_id</li> <li>Recepient belongs to Letter, and has a letter_id</li> <li>RecepientOption belongs to Recepient, and has a recepient_id</li> </ul> <p>Would it make sense to duplicate the user_id in both Recepient and RecepientOption, even though I could always get to it by going up through the associations, through Letter?</p> <hr> <p>Some notes:</p> <ul> <li>There are never any objects that are shared between users. An entire hierarchy of related objects always belongs to the same user.</li> <li>The user owner of objects never changes.</li> <li>Database performance is important because it's a data intensive application. There are many queries and many tables.</li> </ul> <hr> <p>So should I include user_id in every table so I can use it when creating indexes? Or would that be bad design?</p>
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