PostgreSQL: Defining a primary key on a large database

Question

I am planing a database to store lots of text. (blog posts, news articles, etc.) The database needs to have the title, content (50k characters max), date, link and language fields. The same content can't occur on one link. Old content (older then 30 days, for example) will be deleted.

Now, the problem is the primary key. I could just set a automatically incrementing (SERIAL type) field and use it as a primary key. But, it seems stupid and a waste of disc space, because the field wouldn't serve any purpose but to be a primary key. (and the field could eventually run out, or not?) And there's always the other performance issue: the content of each new row inserted needs to be checked for duplicates. So the other solution for the primary key I've come up with would be to calculate a sha256 hash of content+link value and then put that in a new 'hash' column and use that as a primary key. Two birds with one stone. Of course, the problem with that are hash collisions. Is it a big threat?

I don't have any experience with PostgreSQL, and very little experience with DBMS's generally, so I would appreciate a second opinion before a create a database with the performance characteristics of a snail on the highway (horrible comparison).

Please help me out here if you have any experience with large databases. Is setting a 64 character string as a primary key field a good idea in my situation? (because I'm under the impression that generally this is avoided)

Answer 1

Just did this exact test for a rather medium-large DB (200GB+), bigserial won by quite a large margin. It was faster to generate, faster to join, less code, smaller footprint. Because of the way postgres stores it, a bigint is negligible compared to a normal int. You'll run out of storage space from your content long before you ever have to worry about overflowing the bigint. Having done the computed hash vs bigint - surrogate bigint all the way.

Answer 2

I would choose to use a surrogate key, ie. a key that isn't part of the business data of your application. The additional space requirements of an additional 64-bit integer when you're dealing with upto 50 kilobytes of text per record is negligible. You will actually be using less space as soon as you're starting using this key as a foreign key in other tables.

Using a hash of the data stored in a record is a very bad candidate for a primary key, should the data on which the hash is based ever change. You will then have changed the primary key as well, resulting in updates all over the place if you have relations from other tables to this one.

PS. A similar question has been asked and answered here before.

Here's another nice write-up about the topic: http://www.agiledata.org/essays/keys.html

Answer 3

You'd have to have an awful lot of records before your primary key integer ran out.

The integer will be faster for joins than a 64 character string primary key would be. Also it is much easier for people writing queries to deal with.

If a collision is ever possible, you can't use the hash as your primary key. Primary keys must be guarnateed to be unique by definintion.

I've seen hundreds of production databases for different corporations and government entities and not one used a hash primary key. Think there might be a reason?

But, it seems stupid and a waste of disc space, because the field wouldn't serve any purpose but to be a primary key.

Since a surrogate primary key should always be meaningless except as a primary key, I'm not sure what your objection would be.