### Primary keys

When designing a table, you should always keep in mind how the table will be addressed later. In most cases, you will need an identifier in order to uniquely address a specific row; this identifier should be created as your primary key. A primary key needn't necessarily consist of a single column; it may contain as many columns as necessary to uniquely identify a row. If you need many columns (rule of thumb: more than 3), it might be a good idea to invent an additional column with a convenient data type, e.g. serial or bigserial, which holds the primary key.

Only in rare cases it doesn't make sense to use a primary key. That means, a missing primary key is quite a strong indicator for table not being designed completely; that's why a Guru Hint will arise if you create a table without primary key.

If you look at PostgreSQL's system tables, you will find that none of them has a primary key, so what's this about? Actually, all of these tables have one or two columns (usually OID only) which uniquely identifies each row, obeying the second rule for a primary key, not allowing zero, and being covered by an index for faster access. Usage of OIDs has historic reasons, and isn't really first choice for designing user tables. PostgreSQL still uses this for backwards compatibility, and while a newer approach would probably use explicit primary keys it won't be changed now any more.

As the case of system tables shows, the goal of uniqueness and fast access can be achieved with other approaches than a primary key. Still, for clarity of the data model, you're strongly encouraged to use primary keys for this purpose.