Let's say around 249MB (23 bytes per row, according to that page) for the columns you mention, so that leaves 234MB unexplained.I can see 44 bytes per page header (given 5 columns, so 20 bytes ItemIdData, with 24 bytes PageHeaderData). Given page size of 8kb that would imply page headers of about 3.3MB over a 611MB table.
Of course I got that slightly wrong: ItemIdData is for each row, not for each column; an extra 4 bytes for each row makes the per-row space 290MB, leaving 167MB unexplained.
In answer to other questions: the data was inserted in bulk, so there have been no updates; even so I have run a VACUUM FULL just in case and it makes no difference.
I'm assuming the remaining 167MB is related to the alignment requirements, although that does seem quite a lot at 15 bytes per row (perhaps I'm just unfortunate with the data sizes resulting in poor alignment).
I guess the answer to my question is that there is no answer to my question; pg just does use a massive (especially in relation to thin but tall tables) proportion of diskspace for its own purposes.
Thanks again for all the responses.
Geoff
