Let's say I have two Tables, called Person, and Couple, where each Couple record stores a pair of Person id's (also assume that each person is bound to *at most* another different person).
I am planning to support a lot of queries where I will ask for Person records that are not married yet. Do you guys think it's worthwhile to add a 'partnerId' field to Person? (It would be set to null if that person is not married yet)
I am hesitant to do this because the partnerId field is something that is computable - just go through the Couple table to find out. The performance cost for creating new couple will also increase because I have to do this extra book keeping.

I hope that it doesn't sound like I am asking two different questions here, but I felt that this is relevant. Is it a good/common idea to include extra fields that are redundant (computable/inferable by joining with other tables), but will make your query a lot easier to write and faster? 
Thanks!
['https://Stackoverflow.com/questions/26371381', 'https://Stackoverflow.com', 'https://Stackoverflow.com/users/4143394/']
A better option is to keep the data normalized, and utilize a view (indexed, if supported by your rdbms). This gets you the convenience of dealing with all the relevant fields in one place, without denormalizing your data.

Note: Even if a database doesn't support indexed views, you'll likely still be better off with a view as the indexes on the underlying tables can be utilized.
Is there always a zero to one relationship between Person and Couples? i.e. a person can have zero or one partner? If so then your `Couple` table is actually redundant, and your new field is a better approach.

The only reason to split `Couple` off to another table is if one Person can have many partners.

When someone gets a partner you either write one record to the `Couple` table or update one record in the `Person` table. I argue that your `Couple` table is redundant here. You haven't indicated that there is any extra info on the `Couple` record besides the link, and it appears that there is only ever zero or one Couple record for every `Person` record.