Maximum len of data fit into the tuple

Hello all,

I try to write large objects in one table support for postgres and have
following problem:

I should calculate maximum amount of data I can put into the bytea field.
Table scheme is:

create table pg_largeobject {
loid oid,
lastbyte int4,
data bytea
);

If I will assume that oid == int4 I will have 4 * 2 + 4 (size of vl_len in bytea) = 12.
Plus 36 bytes of header (as noted in FAQ). It will be 48 bytes... But I still get
error that tuple is out too big... And that my size is 8188, but should be 8140.

It would be great if someone will clarify the way I should do such calculations.

--
Sincerely Yours,
Denis Perchine

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