Re: jsonb concatenate operator's semantics seem questionable

On Sun, May 17, 2015 at 8:37 AM, Dmitry Dolgov <9erthalion6(at)gmail(dot)com> wrote:
> And I agree with thoughts above, that both concatenation modes ("simple" and
> "deep") definitely can be useful. I can try to figure out how much work that
> would be to modify the IteratorConcat function (or adapt Ilya's solution)

I tend to think of it like this: jsonb more or less supports two use cases.

Firstly, it supports the hstore use case, with a heterogeneous object
stored in each row -- a hodge-podge of different attributes, which can
be used to do something EAV-like, where there are application end user
defined attributes, say. This is unlikely to involve any nesting,
because we're only storing attributes of one entity (the row). This
isn't the use of jsonb that people got excited about, and I think it's
less important, although it does matter. The existence operator (which
operates at the least nested level) is firmly about this use case. And
for that matter, so is the new remove capability/minus operator thing
(which also operates at the least nested level). Fine.

The second use case is the "document database" use case, which is
where jsonb is really compelling. Think of storing more or less fixed
structure documents from a third party web API. Containment works in a
nested fashion in support of that. And as I pointed out, not having
the concatenate operator work in a nested fashion hobbles this use
case. How are users supposed to write an SQL query that update's a
single field in a nested object? That's obviously what they expect
from this.

I think it's misguided to make the concatenate operator target the
hstore use case - if you have that use case, you're unlikely to have
any nesting by convention anyway, and so it doesn't matter to you.
Besides which, as I said, the "document database" use case is the one
most users actually care about these days.

--
Peter Geoghegan