Re: querying within json - Mailing list pgsql-sql

On 30/10/2014 15:22, Giuseppe Broccolo wrote:

Hi Tim,

2014-10-30 15:55 GMT+01:00 Tim Dudgeon <tdudgeon.ml@gmail.com>:

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?

 

2. its not going to use any index on the json_col jsonb column.

The usage of an index is mostly ruled by the 'selectivity' of the query. Anyway, if querying for particular items within the key is common (as 'numeric_prop' in your example), defining an index like this may be worthwhile:

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;

Tim

Regards,

--

Giuseppe Broccolo - 2ndQuadrant Italy
PostgreSQL Training, Services and Support
giuseppe.broccolo@2ndQuadrant.it | www.2ndQuadrant.it