Re: Re: patch: fix performance problems with repated decomprimation of varlena values in plpgsql

Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Wed, Jan 19, 2011 at 12:10 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>>> Yeah. Many-times-repeated detoasting is really bad, and this is not
>>> the only place in the backend where we have this problem. :-(

>> Yeah, there's been some discussion of a more general solution, and I
>> think I even had a trial patch at one point (which turned out not to
>> work terribly well, but maybe somebody will have a better idea someday).

> I'm pretty doubtful that there's going to be a general solution to
> this problem - I think it's going to require gradual refactoring of
> problem spots.

Do you remember the previous discussion? One idea that was on the table
was to make the TOAST code maintain a cache of detoasted values, which
could be indexed by the toast pointer OIDs (toast rel OID + value OID),
and then PG_DETOAST_DATUM might give back a pointer into the cache
instead of a fresh value. In principle that could be done in a fairly
centralized way. The hard part is to know when a cache entry is not
actively referenced anymore ...

regards, tom lane