PostgreSQL – How to Use Default Value of Data Type as Column Default

A simple restriction of the domain of the SQL data type by a constraint is the most natural way. You have two options

1. adding a constraint to the column of your table(s) in question
2. creating a domain for each data type you want to be tailored to fit your given limitations of the programming language

An example of the first options looks like

ALTER TABLE table_name 
  ADD CONSTRAINT constraint_name 
CHECK (column_name > -128 AND column_name < 127);

(change the table, constraint and column name as needed, and adjust the range of the checked bounds)

The second option is a two step process. First add a new domain to your DB (in this example the domain type is named c_byte8) with the command

CREATE DOMAIN c_byte8 NUMERIC CONSTRAINT c_byte8_constraint
 CHECK (VALUE > -128 AND VALUE < 127);

Second create your table with the appropriate types, i.g. c_byte8 instead of NUMERIC as the attribute's type. E.g.

CREATE TABLE test_table_1 (id NUMERIC, byte_value c_byte8);

Please keep in mind that the first solution will require you to add constraints to each attribute you have defined, or you will define in the future. It makes your DB-create script less readable, because of the repetition of the same constraint condition every time you use the restricted domain. A second downside of this approach is, that you can easily miss some columns of your schema when you add those constraints to an existing schema.

An upside of the first solution is that you can extend an existing schema without much hassle.

As for the second approach, the plus is a more readable DB creation script, as is the same with the table definitions of a running production DB, but changing an existing production DB is more complicated.

The documentation states the size of datatypes here.

0 (the number) is stored as 1 byte. Other single digit numbers will be stored in 2 bytes (one for the exponent, one for the mantissa). You can test this yourself by creating a test table and using the VSIZE() function on test data (doc link).

A CHAR(n) will be stored in n bytes.

I'd always store numbers in a NUMBER. Unless you're dealing with trillions of rows, the space saving will not be significant enough.