Adding a constraint concurrently
A table constraint is a guarantee that must be satisfied by all of the rows in the table. Therefore, adding a constraint to a table is a two-phase procedure – first, the constraint is created, and second, the existing rows are validated. Both happen in the same transaction, and the table will be locked according to the type of constraint for the whole duration.
For example, if we add a Foreign Key to a table, we will lock the table to prevent all write transactions against it. This validation could run for an hour in some cases and prevent writes for all that time.
This recipe demonstrates another case – that it is possible to split those two phases into multiple transactions since this allows validation to occur with a lower lock level than what's required to add the constraint, reducing the effect of locking on the table.