Re: jsonb subscripting assignment performance - Mailing list pgsql-hackers

st 14. 4. 2021 v 9:57 odesílatel Dmitry Dolgov <9erthalion6@gmail.com> napsal:

> On Wed, Apr 14, 2021 at 09:20:08AM +0200, Pavel Stehule wrote:
> st 14. 4. 2021 v 7:39 odesílatel Joel Jacobson <joel@compiler.org> napsal:
>
> > Hi,
> >
> > commit 676887a3 added support for jsonb subscripting.
> >
> > Many thanks for working on this. I really like the improved syntax.
> >
> > I was also hoping for some performance benefits,
> > but my testing shows that
> >
> >    jsonb_value['existing_key'] = new_value;
> >
> > takes just as long time as
> >
> >    jsonb_value := jsonb_set(jsonb_value, ARRAY['existing_key'], new_value);
> >
> > which is a bit surprising to me. Shouldn't subscripting be a lot faster,
> > since it could modify the existing data structure in-place? What am I
> > missing here?
> >
>
> no - it doesn't support in-place modification. Only arrays and records
> support it.
>
>
> > I came to think of the this new functionality when trying to optimize some
> > PL/pgSQL code where the bottle-neck turned out to be lots of calls
> > to jsonb_set() for large jsonb objects.
> >
>
> sure - there is big room for optimization. But this patch was big enough
> without its optimization. And it was not clean, if I will be committed or
> not (it waited in commitfest application for 4 years). So I accepted
> implemented behaviour (without inplace update). Now, this patch is in core,
> and anybody can work on others possible optimizations.

Right, jsonb subscripting deals mostly with the syntax part and doesn't
change internal jsonb behaviour. If I understand the original question
correctly, "in-place" here means updating of e.g. just one particular
key within a jsonb object, since jsonb_set looks like an overwrite of
the whole jsonb. If so, then update will still cause the whole jsonb to
be updated, there is no partial update functionality for the on-disk
format. Although there is work going on to optimize this in case when
jsonb is big enough to be put into a toast table (partial toast
decompression thread, or bytea appendable toast).

Almost all and almost everywhere Postgres's values are immutable. There is only one exception - runtime plpgsql. "local variables" can hold values of complex values unboxed. Then the repeated update is significantly cheaper. Normal non repeated updates have the same speed, because the value should be unboxed and boxed. Outside plpgsql the values are immutable. I think this is a very hard problem, how to update big toasted values effectively, and I am not sure if there is a solution. TOAST value is immutable. It needs to introduce some alternative to TOAST. The benefits are clear - it can be nice to have fast append arrays for time series. But this is a very different topic.

Regards

Pavel