Is the primary key useful when is never used

Question

the question is about table2.id

Table1

id     -- Uniqueidentifier primary key

Table2

id        -- Uniqueidentifier primary key
id_table1 -- Duplicate foreign key
value     -- varchar or other

The possible queries are :

select table1.id, table2.value 
from table1
inner join table2 ON table2.id_table1 = table1.id
where table1.id = 'foo'

or

select value
from table2
where table2.id_table1 = 'foo'

Are the performance changed with or without the existence of table2.id. Even if queries doesn't use it?

If these are all the queries, why bother adding the id column to the second table? just use the combination of `id_table1` and `value` as a primary key. If you already have a clustered unique index on that combination, it will not effect performance, only storage size. — Zohar Peled, Jun 26 '16 at 08:44
http://stackoverflow.com/questions/45399/advantages-and-disadvantages-of-guid-uuid-database-keys — Ivan Starostin, Jun 26 '16 at 09:43
@IvanStarostin While an interesting reading, the link you provided has no barring on the question itself. Whether the OP should use an int or guid is a totally different question then the one posted. — Zohar Peled, Jun 26 '16 at 11:13
@ZoharPeled I know. At first I wanted to add _"you are asking wrong question"_ to my comment with that link. I guess he wouldn't ask anything without having troubles. And primary key column itself does not provide any troubles while `guid+clustered` combination _sometimes_ does. Not sure if idea of `guid+something` as clustered pk is much better. — Ivan Starostin, Jun 26 '16 at 11:18

score 0 · Answer 1 · answered Jun 26 '16 at 11:38

I think the answer is yes. Having columns and indexes you will never use will have a negative performance impact although maybe not much depending on your exact situation.

I think though that what you really need is a better understanding of primary keys and indexes. In particular I recommend you research the debate on surrogate keys. You will find plenty of info on the topic.

Is the primary key useful when is never used

1 Answers1