Re: jsonb nesting level edge case

On Sun, Mar 18, 2018 at 09:42:14PM +0100, Dmitry Dolgov wrote:
> Hi,
>
> I've just realized, that looks like there is one edge-case in the current jsonb
> implementation, that can be quite confusing, and I couldn't find any related
> discussion about it. From what I see there is no limit for how many nested
> levels can be in a jsonb field, and e.g. when a jsonb is created from a string
> it basically means that we're limited only by `check_stack_depth` (in the
> following case it's about recursive `parse_object`/`parse_object_field`). So
> you can create a jsonb with quite many nesting levels:
>
> =# insert into test_jsonb values((
> (select '{' || string_agg('"field' || s.id || '"', ': {')
> from generate_series(1, 10000) as s(id))
> || ': "some_value"' ||
> (select string_agg('}', '') from generate_series(1, 10000)))::jsonb);
>
> INSERT 0 1
> Time: 29.129 ms
>
> But at the same time `jsonb_set` apparently has a different recursion schema,
> and reaches max_stack_depth faster (in this case it's about recursive
> `setPath`/`setPathObject`). It means that you can create a document, but you
> can't update its value using function, that was specified for that (so you
> probably need to override the entire jsonb to actually update something):
>
> =# update test_jsonb set data = jsonb_set(data,
> (select array_agg('field' || s.id) from generate_series(1,
> 10000) as s(id)),
> '"other_value"');
>
> ERROR: 54001: stack depth limit exceeded
> HINT: Increase the configuration parameter "max_stack_depth"
> (currently 2048kB), after ensuring the platform's stack depth limit is
> adequate.
> LOCATION: check_stack_depth, postgres.c:3163
> Time: 17.143 ms
>
> Is it something significant enough to worry about? Just to mention, in some
> other databases there is just a limit for number of nested levels for a
> document (e.g. in MongoDB Bson, MySQL binary json it's exactly 100).

I think our current behavior is the best we can do. We are limited
only by configured stack memory, which people can increase. I don't see
how we could improve it unless we reduced jsonb_set()'s stack memory
use.

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EnterpriseDB http://enterprisedb.com

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