On 09/27/2014 08:00 AM, Stephen Frost wrote:
Andrew, all,

* Andrew Dunstan (and...@dunslane.net) wrote:
I should have been paying a bit more attention to the recent work on
adding an ignore_nulls option to row_to_json(). Here are some
belated thought. I apologize to Pavel and Stephen for not having
commented earlier.
No problem at all and thanks for continuing to think about it!  We
certainly still have quite a bit of time til 9.5 to get this right.

I think this is really a bandaid, and it will fail to catch lots of
cases. Several examples:
As discussed on IRC- I agree.  I tend to think of JSON objects as
relatively simple hstore-like structures and so hadn't considered the
complex structure case (as I'm guessing Pavel hadn't either).

I think a much more comprehensive solution would be preferable. What
I have in mind is something like

     json_strip_null_fields(json) -> json

and a similar function for jsonb.
Right, this makes sense to me.

These would operate recursively. There is a downside, in that they
would be required to reprocess the json/jsonb. But adding an option
like this to all the json generator functions would be seriously
ugly, especially since they are mostly aggregate functions or
variadic functions. At least in the jsonb case the cost of
reprocessing is likely to be fairly low.
Yeah, I don't see adding this option to all json generator functions as
making a lot of sense but rather just to the select few things which it
really makes sense for and then having a function which can be used by
users to do the same for results from other operations.


I guess I'm questioning the wisdom of keeping it for row_to_json given that it doesn't operate recursively anyway (and making it do so would be difficult and ugly).

The counter argument for this is that nested composites and arrays of composites are relatively rare in records, so providing a fast non-recursive stripping of nulls for the common case is reasonable.

If we're going to keep this, I think we also need to provide it (non-recursive) for json_agg via an optional second argument. This should be a fairly simple change: just steer the result via composite_to_json if it's a record, rather than to datum_to_json.



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