Re: int1? types?

On Fri, 10 Oct 2003 16:53:55 -0700, elein <elein@varlena.com> wrote:
>I don't think that you can create a genuine one byte datatype.
>The resulting type would probably be four bytes long, even if
>you create a one byte by-value data type.

Column values are not *expanded* to multiples of four bytes, they are
*aligned* according to their datatype (cf. pg_type.typalign).

Not counting heap tuple headers, we get the following offsets and
lengths:

    CREATE TABLE a (
        c1 "char" NOT NULL,    -- offset 0
        c2 "char" NOT NULL,    -- offset 1
        c3 "char" NOT NULL,    -- offset 2
        c4 "char" NOT NULL     -- offset 3
    );      -- size = 4

    CREATE TABLE b (
        c1 bool NOT NULL,      -- offset 0
        c2 int2 NOT NULL,      -- offset 2
        c3 bool NOT NULL,      -- offset 4
        c4 int  NOT NULL,      -- offset 8
        c5 bool NOT NULL,      -- offset 12
        c6 char(1) NOT NULL    -- offset 16
    );      -- size = 24

Here c6 consists of a four byte length followed by one data byte
(unless the character needs a multibyte representation), the length
has to be aligned on a four byte boundary and the whole row is padded
to a multiple of MAXALIGN, typically four on a 32 bit machine.  So we
have three padding bytes before c6 and three padding bytes after c6.

    CREATE TABLE bb (
        c6 char(1) NOT NULL,   -- offset 0
        c1 bool NOT NULL,      -- offset 5
        c3 bool NOT NULL,      -- offset 6
        c5 bool NOT NULL,      -- offset 7
        c4 int  NOT NULL,      -- offset 8
        c2 int2 NOT NULL       -- offset 12
    );      -- size = 16

Servus
 Manfred