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Craig Kerstiens

Diving into Postgres JSON operators and functions

Just as PostgreSQL 9.3 was coming out I had a need to take advantage of the JSON datatype and some of the operators and functions within it. The use case was pretty simple, run a query across a variety of databases, then take the results and store them. We explored doing something more elaborate with the columns/values, but in the end just opted to save the entire result set as JSON then I could use the operators to explore it as desired.

Here’s the general idea in code (using sequel):

result = r.connection { |c| c.fetch(self.query).all }
mymodel.results = result.to_json

As the entire dataset was stored as some compressed JSON I needed to do a bit of manipulation to get it back into a form that was workable. Fortunately all the steps were fairly straightforward.

First you want to unnest each result from the json array, in my case this looked like:

SELECT json_array_elements(result)

The above will unnest all of the array elements so I have an individual result as JSON. A real world example would look something similar to:

SELECT json_array_elements(result) 
FROM query_results 

{“column_name”:“data_in_here”} {“column_name_2”:“other_data_in_here”} (2 rows)

From here based on the query I would want to get some specific value. In this case I’m going to search for the text key column_name_2:

SELECT json_array_elements(result)->'column_name_2' 
FROM query_results 

(1 rows)

One gotcha I encountered was when I wanted to search for some value or exclude some value… Expecting I could just compare the result of the above in a where statement I was sadly mistaken because the equals operator didn’t translate. My first attempt at fixing this was to cast in this form:

SELECT json_array_elements(result)->'column_name_2'::text

The sad part is because of the operator the cast doesn’t get applied as I’d expect. Instead you’ll want to do:

SELECT (json_array_elements(result)->'column_name_2')::text

Of course theres plenty more you can do with the JSON operators in the new Postgres 9.3. If you’ve already got JSON in your application give them a look today. And while slightly worse, if you’ve got JSON stored in a text field simply cast it with ::json to begin using the operators.

If you’re looking for a deeper resource on Postgres I recommend the book The Art of PostgreSQL. It is by a personal friend that has aimed to create the definitive guide to Postgres, from a developer perspective. If you use code CRAIG15 you’ll receive 15% off as well.