Re: semi-variable length type

James Harper <james.harper@bendigoit.com.au> writes:
> I want to make a float(n) type that emulates the mssql float type. The
> storage requirements are documented as 4 bytes for 1 <= n <=24, and 8
> bytes for 25 <= n <= 53.

Haven't we got that already?

regression=# create table t1 (f1 float(5), f2 float(30));
CREATE TABLE
regression=# \d t1
           Table "public.t1"
 Column |       Type       | Modifiers
--------+------------------+-----------
 f1     | real             |
 f2     | double precision |

Other than the fact that we don't remember whether you asked for 5 bits
or 24, I think this meets the spec requirements.

> If I understand correctly, my options for
> emulating this in postgres are: 1. declare as variable length. Storage
> is then 8 bytes (4 byte length + 4 byte storage), or 12 bytes (4 byte
> length + 8 byte storage).

Well, you could use a short varlena header (1 byte).  The type doesn't
need to be int-aligned, either, though whether that buys anything will
depend on context --- and you'll need to spend cycles realigning it,
if you want the code to work on non-Intel architectures.

            regards, tom lane