Is it possible to store a 1 byte number in Postgres?

Question

I have 7 8-bit integer values per record that I want to store in Postgres. Pg doesn\'t offer a single byte integer type, SMALLINT, or 2 bytes, being the smallest integer datatyp

Answer 1

"char"

This is a one-byte type in PostgreSQL that fits in the range of -128,127. From the docs,

The type "char" (note the quotes) is different from char(1) in that it only uses one byte of storage. It is internally used in the system catalogs as a simplistic enumeration type.

You can bias this towards [-128,127], by subtracting 128 from any input in the range of [0-255] before you write to the database, and adding it back on the output when you read from the database.

-- works
SELECT (-128)::"char", 127::"char";

-- generates out of range
SELECT (-128)::"char";
SELECT 128::"char";

-- Shifts to unsigned range.
-- If you're going to be using "char"
-- review the results of this query!
SELECT
  x::int AS "inputUnsigned",
  chr(x) AS "extendedASCII",
  -- this is the "char" types representation for that input.
  signed::"char" AS "charRepresentation",

  signed     AS "inputUnsignedToSigned",
  signed+128 AS "inputUnsignedToSignedToUnsigned"
FROM generate_series(1,255) AS gs(x)
-- Here we map the input in the range of [0,255] to [-128,127]
CROSS JOIN LATERAL ( VALUES (x::int-128) )
  AS v(signed);

Small excerpt of the output

 inputUnsigned | extendedASCII | charRepresentation | inputUnsignedToSigned | inputUnsignedToSignedToUnsigned 
---------------+---------------+--------------------+-----------------------+---------------------------------
....
           190 | ¾             | >                  |                    62 |                             190
           191 | ¿             | ?                  |                    63 |                             191
           192 | À             | @                  |                    64 |                             192
           193 | Á             | A                  |                    65 |                             193
           194 | Â             | B                  |                    66 |                             194
           195 | Ã             | C                  |                    67 |                             195
           196 | Ä             | D                  |                    68 |                             196
...

We use generate_series(1,255) only because chr(0) throws because you can't generate or output the ASCII NUL (PostgreSQL uses cstrings)

pguint Extension

Pguint is an extension that provides two one byte representations,

• int1 (signed)
• uint1 (unsigned)