Amit Langote <langote_amit...@lab.ntt.co.jp> writes:
> On 2016/05/18 2:22, Tom Lane wrote:
>> The two ways that we've dealt with this type of hazard are to copy data
>> out of the relcache before using it; or to give the relcache the
>> responsibility of not moving a particular portion of data if it did not
>> change.  From memory, the latter applies to the tuple descriptor and
>> trigger data, but we've done most other things the first way.

After actually looking at the code, we do things that way for the
tupledesc, the relation's rules if any, and RLS policies --- see
RelationClearRelation().

> It seems that tuple descriptor is reference-counted; however trigger data
> is copied.  The former seems to have been done on performance grounds (I
> found 06e10abc).

We do refcount tuple descriptors, but we've been afraid to try to rely
completely on that; there are too many places that assume a relcache
entry's tupdesc is safe to reference.  It's not that easy to go over to
a fully refcounted approach, because that creates a new problem of being
sure that refcounts are decremented when necessary --- that's a pain,
particularly when a query is abandoned due to an error.

> So for a performance-sensitive relcache data structure, refcounting is the
> way to go (although done quite rarely)?

I'd be suspicious of this because of the cleanup problem.  The
don't-replace-unless-changed approach is the one that's actually battle
tested.

                        regards, tom lane


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