Re: json(b)_array_elements use causes very large memory usage when also referencing entire json document

Andres Freund <andres@anarazel.de> writes:
> Hm. We've a bunch of places where we free detoasted data, just out of
> efficiency concerns. And since the jsonb functions are quite likely to
> detoast a lot, it doesn't seem unreasonable to do so for the most likely
> offenders. I mean if you've a bit more complex expression involving a
> few fields accessed, freeing in the accesses will reduce maximum memory
> usage by quite a bit. I'm not suggesting to work towards leak free, just
> towards reducing the lifetime of a few potentially large allocations.

Dunno, for the common case of not-so-large values, this would just be
a net loss.  pfree'ing a value we can afford to ignore till the next
per-tuple context reset is not a win.

Maybe we need some kind of PG_FREE_IF_LARGE_COPY macro?  Where it
would kick in for values over 8K or thereabouts?

(You might argue that any value that got detoasted at all would be
large enough to be worth worrying about; but I think that falls down
because we folded short-header unshorting into the detoast mechanism.)

			regards, tom lane