Re: RFC: Restructuring pg_aggregate

Tom Lane wrote:
> Christopher Kings-Lynne <chriskl(at)familyhealth(dot)com(dot)au> writes:
> > The real problem here is the fact that there are now missing attnos in
> > pg_attribute. Either that's handled or we renumber the attnos - which is
> > also quite hard?
>
> Updating pg_attribute per se is not so hard --- just store new copies of
> all the rows for the table. However, propagating the changes into other
> places could be quite painful (I'm thinking of column numbers in stored
> constraints, rules, etc).
>
> It seems to me that reducing the column to NULLs already gets you the
> majority of the space savings. I don't think there is a case to be made
> that getting back that last bit is worth the pain involved, either in
> implementation effort or direct runtime costs (do you really want a DROP
> COLUMN to force an immediate rewrite of the whole table?)

That is an excellent point about having to fix all the places that refer
to attno. In fact, we have been moving away from attname references to
attno references for a while, so it only gets worse. Tom is also
correct that setting it to NULL removes the problem of disk space usage
quite easily.

That only leaves the problem of having gaps in the pg_attribute for that
relation, and as I remember, that was the problem for Hiroshi's DROP
COLUMN change, but at this point, after years of delay with no great
solution on the horizon, we may as well just get this working.

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