On 11/1/18, Nasby, Jim <nas...@amazon.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
