On 2013-02-19 09:12:02 -0500, Robert Haas wrote: > On Tue, Feb 19, 2013 at 9:00 AM, Andres Freund <and...@2ndquadrant.com> wrote: > >> So the other way that we could do this is to use something that's the > >> same size as a TOAST pointer but has different content - the > >> seemingly-obvious choice being va_toastrelid == 0. > > > > Unfortunately that would mean you need to copy the varatt_external (or > > whatever it would be called) to aligned storage to check what it > > is. Thats why I went the other way. > > How big a problem is that, though?
There are quite some places where we test the actual type of a Datum inside tuptoaster.c. Copying it to local storage everytime might actually be noticeable performancewise. Besides the ugliness of needing a local variable, copying the data and only testing afterwards... > >> I'd be a little > >> reluctant to do it the way you propose because we might, at some > >> point, want to try to reduce the size of toast pointers. If you have > >> a tuple with many attributes, the size of the TOAST pointers > >> themselves starts to add up. It would be nice to be able to have 8 > >> byte or even 4 byte toast pointers to handle those situations. If we > >> steal one or both of those lengths to mean "the data is cached in > >> memory somewhere" then we can't use those lengths in a smaller on-disk > >> representation, which would seem a shame. > > > > I agree. As I said above, having the type overlayed into the lenght was > > and is a bad idea, I just haven't found a better one thats compatible > > yet. > > Except inventing typlen=-3 aka "toast2" or something. But even that > > wouldn't help getting rid of existing pg_upgraded tables. Besides being > > a maintenance nightmare. > > > > The only reasonable thing I can see us doing is renaming > > varattrib_1b_e.va_len_1be into va_type and redefine VARSIZE_1B_E into a > > switch that maps types into lengths. But I think I would put this off, > > except placing a comment somewhere, until its gets necessary. > > I guess I wonder how hard we think it would be to insert such a thing > when it becomes necessary. How much stuff is there out there that > cares about the fact that that length is a byte? You mean whether we could store the length in 6 bytes and use two for the type? That should probably work as well. But I don't see much advantage in that given that all those sizes ought to be static. Redefining VARSIZE_1B_E as indicated above should be fairly easy, there aren't many callsites that touch stuff at such low level. Greetings, Andres Freund -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
