Re: PostgreSQL Limits and lack of documentation about them.

On 11/1/18, Nasby, Jim <nasbyj(at)amazon(dot)com> wrote:
> Hmm… 18 bytes doesn’t sound right, at least not for the Datum. Offhand I’d
> expect it to be the small (1 byte) varlena header + an OID (4 bytes). Even
> then I don’t understand how 1600 text columns would work; the data area of a
> tuple should be limited to ~2000 bytes, and 2000/5 = 400.

The wording in the docs (under Physical Storage) is "Allowing for the
varlena header bytes, the total size of an on-disk TOAST pointer datum
is therefore 18 bytes regardless of the actual size of the represented
value.", and as I understand it, it's

header + toast table oid + chunk_id + logical size + compressed size.

This is one area where visual diagrams would be nice.

-John Naylor