On 2013-10-28 13:41:46 -0400, Tom Lane wrote: > Andres Freund <and...@2ndquadrant.com> writes: > > On 2013-10-28 12:42:28 -0400, Tom Lane wrote: > >> Meh. If you don't include a function pointer you will still need the OID > >> of the datatype or the decompression function, so it's not like omitting > >> it is free. > > > That's what I thought at first too - but I am not sure it's actually > > true. The reason we need to include the toastrelid in varatt_externals > > (which I guess you are thinking of, like me) is that we need to be able > > to resolve "naked" Datums to their original value without any context. > > But at the locations where we'd need to call the memory > > representation->disk conversion function we should have a TupleDesc with > > type information, so we could lookup the needed information there. > > I don't think that's a safe assumption at all. We need to be able to do > flattening anywhere PG_DETOAST_DATUM() can be called.
I am not sure we want things to work along those lines. I'd rather make PG_DETOAST_DATUM pass along such in-memory Datums unchanged and require any funtion that wants to poke into into the Datum in detail to know about the different representations. That will require a bit more widespread changes in functions using those types natively, but it will make it more realistic to use the optimization across much of the code that detoasts Datums generally. > In any case, my point here is largely that I don't want to add a catalog > lookup to the operation. This whole proposal is basically about trading > greater short-term memory usage to gain speed, so griping about an extra 4 > or so bytes per value seems to me to be missing the point completely. > Or to put it even more bluntly: if you've not realized that the extra > palloc overhead of an out-of-line instance of the datum will swamp what > we're talking about here, you need to realize that. I am not arguing against this at all though. Greetings, Andres Freund -- Andres Freund http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
