Re: querying within json

Tim Dudgeon wrote
> On 30/10/2014 15:22, Giuseppe Broccolo wrote:
>> Hi Tim,
>>
>> 2014-10-30 15:55 GMT+01:00 Tim Dudgeon <

> tdudgeon.ml@

>  
> > <mailto:

> tdudgeon.ml@

> >>:
>>
>>     Any advice on how to best query for values within json (using
>>     9.4). I have numeric fields within the json and want to include
>>     terms for those fields.
>>
>>     I've found that something like this works:
>>
>>     select * from atable where (json_col->>'numeric_prop')::numeric <
>> 100;
>>
>>     But whilst that works:
>>     1. seems to have unnecessary casts? The numeric _prop item is of
>>     numeric type, but its getting retrieved as text and then cast to
>>     numeric and then compared
>>
>>
>> The right operand type of the ->> oeprator is text when ->> is used to 
>> get a json object field. So the cast to numeric is needed.
> 
> Needed to work, yes. But if my reading of the docs is right then numeric 
> types within json as supposed to be treated as Postgres numeric type?
> See table 8.23 here: 
> http://www.postgresql.org/docs/9.4/static/datatype-json.html
> So this would mean a cast from numeric to text and then back to numeric?
> There is no way to ask for a json 'field' in its actual data type so 
> avoiding the cast?

While the semantic definition of json-number and postgres-numeric are made
to be similar (allowed range of values mostly, plus when creating json from
a numeric the system knows to store the data as a number instead of as text)
there is currently no way to directly go from the internal json-number
representation to postgres-numeric.

On a technical side-note: "The right operand type of the ->> oeprator [sic]
is text when ->> is used to 
get a json object field. So the cast to numeric is needed." doesn't make
sense.  The fact that the right operand is text has no bearing on whether a
cast to numeric is required.  The fact that the operator "json->>text"
returns text does.  Note that since an operator cannot return different
types dependent upon the value being returned if you really wanted to try
and directly return a numeric you would need a different operator/function.


>>
>> CREATE INDEX idxgin_numeric_prop ON atable USING 
>> gin((json_col->'numeric_prop'));
> 
> I can add the index, but no evidence of it being used when I run a query 
> like this:
> select * from atable where (json_col->>'numeric_prop')::numeric < 100;

The index is storing text while the expression has been cast to numeric so,
no, if what is shown above is exactly what you did then you would not be
using the index.





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