Uses RPostgres as backend. nodbi creates or uses
a PostgreSQL table, with columns _id and json created and used
by package nodbi, applying SQL functions as per
https://www.postgresql.org/docs/current/functions-json.html
to the json column.
Each row in the table represents a JSON document.
Any root-level _id is extracted from the document(s) and used
for column _id, otherwise a UUID is created as _id.
The table is indexed on _id. A custom plpgsql function
is used for docdb_update().
The order of variables in data frames returned by docdb_get()
and docdb_query() can differ from their order the input to
docdb_create().
For a benchmark, see https://github.com/ropensci/nodbi#benchmark