Re: jsonb concatenate operator's semantics seem questionable

On 17/05/15 16:04, Andrew Dunstan wrote:
>
> On 05/16/2015 10:56 PM, Peter Geoghegan wrote:
>> Another thing that I noticed about the new jsonb stuff is that the
>> concatenate operator is based on the hstore one. This works as
>> expected:
>>
>> postgres=# select '{"a":1}'::jsonb || '{"a":2}';
>> ?column?
>> ----------
>>   {"a": 2}
>> (1 row)
>>
>> However, the nesting doesn't "match up" -- containers are not merged
>> beyond the least-nested level:
>>
>> postgres=# select '{"a":{"nested":1}}'::jsonb || '{"a":{"also
>> nested":2}}';
>>           ?column?
>> ---------------------------
>>   {"a": {"also nested": 2}}
>> (1 row)
>>
>> This feels wrong to me. When jsonb was initially introduced, we took
>> inspiration for the *containment* ("operator @> jsonb") semantics from
>> hstore, but since jsonb is nested it worked in a nested fashion. At
>> the top level and with no nested containers there was no real
>> difference, but we had to consider the behavior of more nested levels
>> carefully (the containment operator is clearly the most important
>> jsonb operator). I had envisaged that with the concatenation of jsonb,
>> concatenation would similarly behave in a nested fashion. Under this
>> scheme, the above query would perform nested concatenation as follows:
>>
>> postgres=# select '{"a":{"nested":1}}'::jsonb || '{"a":{"also
>> nested":2}}'; -- does not match actual current behavior
>>           ?column?
>> ---------------------------
>>   {"a": {"nested":1, "also nested": 2}}
>> (1 row)
>>
>> Now, I think it's good that the minus operator ("operator - text" and
>> friends) discussed on the nearby thread accepts a text (or int)
>> argument and remove string elements/pairs at the top level only. This
>> works exactly the same as existence (I happen to think that removing
>> elements/pairs at a nested level is likely to be more trouble than
>> it's worth, and so I don't really like the new "jsonb - text[]"
>> operator much, because it accepts a Postgres (not JSON) array of texts
>> that constitute a path, which feels odd). So I have no issue with at
>> least the plain minus operators' semantics. But I think that the
>> concatenate operator's current semantics are significantly less useful
>> than they could be, and are not consistent with the overall design of
>> jsonb.
>>
>
> Historical note: I think it's based on the nested hstore work, not on
> current hstore, but Dmitry can answer on that.
>
> I didn't dismiss this because it was a bad idea, but because it was too
> late in the process. If there is a consensus that we need to address
> this now then I'm happy to reopen that, but given the recent amount of
> angst about process I'm certainly not going to make such a decision
> unilaterally.
>
> Personally, I think there is plenty of room for both operations, and I
> can see use cases for both. If I were designing I'd leave || as it is
> now and add a + operation to do a recursive merge. I'm not sure how much
> work that would be. Not huge but not trivial either.
>

Agreed, if you look at jquery for example, the extend() method by 
default behaves like our current || and you have to specify that you 
want deep merge if you want the behavior described by Peter. So there is 
definitely point for both, at this time we just support only one of 
them, that's all.


--  Petr Jelinek                  http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training &
Services