On 2016/05/19 2:48, Tom Lane wrote:
> 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().

I think I confused refcounting method of keeping things around with the
RelationClearRelation()'s method.  I now understand that you meant the
latter in your original message.

>> 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.

I see.

>> 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.

OK, I will try the RelationClearRelation()'s method of keeping partition
descriptor data around so that no repeated copying is necessary.


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