PostgreSQL: default constraint names

The standard names for indexes in PostgreSQL are:

{tablename}_{columnname(s)}_{suffix}

where the suffix is one of the following:

• pkey for a Primary Key constraint
• key for a Unique constraint
• excl for an Exclusion constraint
• idx for any other kind of index
• fkey for a Foreign key
• check for a Check constraint

Standard suffix for sequences is

• seq for all sequences

Proof of your UNIQUE-constraint:

NOTICE: CREATE TABLE / UNIQUE will create implicit index "example_a_b_key" for table "example"

The manual is pretty clear about this ("tableconstraint: This form adds a new constraint to a table using the same syntax as CREATE TABLE.")

So you can simply run:

ALTER TABLE example ADD UNIQUE (a, b);