Key value json output and key value loader to emulate left joins

[dcmoura]

Allowing to load dictionaries (e.g. key/value from json files), and in the query do dictionaries look-ups. This would cover a kind of LEFT JOIN with an equality condition. e.g.

SELECT adict[id] ... 

If id does not exist in adict , a NULL value should be returned.

[dcmoura]

Consider

• [x] creating a dict_agg aggregation function and saving output as json
• [x] adding a command line option to load the json object into memory.