Question

Let’s say we are having a table with this definition:

range (
  id bigint primary key,
  colourId int references colour(id),
  smellId int references smell(id),
  from bigint,
  to bigint
)

This table is actually a reduced view over enormously big table:

item (
  id bigint primary key,
  colourId int references colour(id),
  smellId int references smell(id),
  CONSTRAINT item_colour_smell_unique UNIQUE (colour, smell, id)
)

I would like to translate item_colour_smell_unique constraint in the range table. It should watch overlaps of ranges [from, to] while taking account of colourId and smellId column values.

Answer 1

Note that any trigger-based solution is inherently unsafe from race conditions, e.g. when two concurrent transactions insert a row with conflicting ranges, neither of them will see the other conflicting row, due to the “isolation” ACID property (only commited data can be seen).

Some solutions:

• Use procedures with explicit locking of the table to force serialization of inserts.
• Split the [from, to] range into [from, from+1, …, to-1, to] and insert a row for each. This way you can use a simple UNIQUE INDEX on the “range” table.

PostgreSQL developer Jeff Davis has been writing about this lately and will implement range conflict constraints in PostgreSQL 8.5