On 03/13/2013 12:51 PM, Gavin Flower wrote:
On 14/03/13 02:02, Andrew Dunstan wrote:
On 03/13/2013 08:17 AM, Robert Haas wrote:
On Fri, Mar 8, 2013 at 4:42 PM, Andrew Dunstan <and...@dunslane.net>
wrote:
So my order of preference for the options would be:
1. Have the JSON type collapse objects so the last instance of a
key wins
and is actually stored
2. Throw an error when a JSON type has duplicate keys
3. Have the accessors find the last instance of a key and return that
value
4. Let things remain as they are now
On second though, I don't like 4 at all. It means that the JSON type
things a value is valid while the accessor does not. They
contradict one
another.
You can forget 1. We are not going to have the parser collapse
anything.
Either the JSON it gets is valid or it's not. But the parser isn't
going to
try to MAKE it valid.
Why not? Because it's the wrong thing to do, or because it would be
slower?
What I think is tricky here is that there's more than one way to
conceptualize what the JSON data type really is. Is it a key-value
store of sorts, or just a way to store text values that meet certain
minimalist syntactic criteria? I had imagined it as the latter, in
which case normalization isn't sensible. But if you think of it the
first way, then normalization is not only sensible, but almost
obligatory. For example, we don't feel bad about this:
rhaas=# select '1e1'::numeric;
numeric
---------
10
(1 row)
I think Andrew and I had envisioned this as basically a text data type
that enforces some syntax checking on its input, hence the current
design. But I'm not sure that's the ONLY sensible design.
I think we've moved on from this point, because a) other
implementations allow duplicate keys, b) it's trivially easy to make
Postgres generate such json, and c) there is some dispute about
exactly what the spec mandates.
I'll be posting a revised patch shortly that doesn't error out but
simply uses the value for the later key lexically.
cheers
andrew
How about adding a new function with '_strict' added to the existing
name, with an extra parameter 'coalesce' - or using other names, if
considered more appropriate!
That way slower more stringent functionality can be added where
required. This way, the existing function need not be changed.
If coalesce = true,
then: the last duplicate is used
else: an error is returned when the new key is a duplicate.
For good or ill, we now already have a json type that will accept
strings with duplicate keys, and generator functions which can now
generate such strings. If someone wants functions to enforce a stricter
validity check (e.g. via a check constraint on a domain), or to convert
json to a canonical version which strips out prior keys of the same name
and their associated values, then these should be relatively simple to
implement given the parser API in the current patch. But they aren't
part of the current patch, and I think it's way too late to be adding
such things. I have been persuaded by arguments made upthread that the
best thing to do is exactly what other well known json-accepting
implementations do (e.g. V8), which is to accept json with duplicate
keys and to treat the later key/value as overriding the former
key/value. If I'd done that from the start nobody would now be talking
about this at all.
cheers
andrew
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