SQL CHECK constraint to prevent date overlap

Question

I have a table that describes which software versions were installed on a machine at various times:

machine_id::integer, version::text, datefrom::timestamp, dateto::timestamp

I'd like to do a constraint to ensure that no date ranges overlap, i.e. it is not possible to have multiple software versions installed on a machine at the same time.

How can this be achieved in SQL? I am using PostgreSQL v8.4.

Answer 1

Do you really want a CHECK costraint, like mentioned in the title? That is not possible, since CHECK constrains can only work one row at a time. There might be a way to do this using triggers, though...

Answer 2

In PostgreSQL 8.4 this can only be solved with triggers. The trigger will have to check on insert/update that no conflicting rows exist. Because transaction serializability doesn't implement predicate locking you'll have to do the necessary locking by yourself. To do that SELECT FOR UPDATE the row in the machines table so that no other transaction could be concurrently inserting data that might conflict.

In PostgreSQL 9.0 there will be a better solution to this, called exclusion constraints (somewhat documented under CREATE TABLE). That will let you specify a constraint that date ranges must not overlap. Jeff Davis, the author of that feature has a two part write-up on this: part 1, part 2. Depesz also has some code examples describing the feature.