NOT DEFERRABLE versus DEFERRABLE INITIALLY IMMEDIATE

Question

I read this about the SQL keyword DEFERRABLE in Database Systems - The Complete Book.

The latter [NOT DEFERRABLE] is defau

Answer 1

I'm very late to the party but I wanted to add that -- as of December 2018 -- only two databases I know of (there may be more) offer some level of implementation of this standard SQL feature:

Database    NOT DEFERRABLE  DEFERRABLE           DEFERRABLE 
                            INITIALLY IMMEDIATE  INITIALLY DEFERRED
----------  --------------  -------------------  ------------------
Oracle      N/A *1          Yes (default)        Yes
PostgreSQL  Yes (default)   Yes                  Yes
DB2         -               -                    -
SQL Server  -               -                    -
MySQL       -               -                    -
MariaDB     -               -                    -
SAP Sybase  -               -                    -
HyperSQL    -               -                    -
H2          -               -                    -
Derby       -               -                    -

*1 Even though Oracle 12c accepts the NOT DEFERRABLE constraint state, it actually ignores it and makes it work as DEFERRABLE INITIALLY IMMEDIATE.

As you see, Oracle does not implement the first type (NOT DEFERRABLE), and that's why developers using Oracle (the OP in this case) may get confused and consider the first two types equivalent.

Interestingly enough Oracle and PostgreSQL have a different default type. Maybe it has performance implications.