How can I import a JSON file into PostgreSQL?

You can feed the JSON into a SQL statement that extracts the information and inserts that into the table. If the JSON attributes have exactly the name as the table columns you can do something like this:

with customer_json (doc) as (
   values 
    ('[
      {
        "id": 23635,
        "name": "Jerry Green",
        "comment": "Imported from facebook."
      },
      {
        "id": 23636,
        "name": "John Wayne",
        "comment": "Imported from facebook."
      }
    ]'::json)
)
insert into customer (id, name, comment)
select p.*
from customer_json l
  cross join lateral json_populate_recordset(null::customer, doc) as p
on conflict (id) do update 
  set name = excluded.name, 
      comment = excluded.comment;

New customers will be inserted, existing ones will be updated. The “magic” part is the json_populate_recordset(null::customer, doc) which generates a relational representation of the JSON objects.

The above assumes a table definition like this:

create table customer 
(
  id        integer primary key,
  name      text not null,
  comment   text
);

If the data is provided as a file, you need to first put that file into some table in the database. Something like this:

create unlogged table customer_import (doc json);

Then upload the file into a single row of that table, e.g. using the \copy command in psql (or whatever your SQL client offers):

\copy customer_import from 'customers.json' ....

Then you can use the above statement, just remove the CTE and use the staging table:

insert into customer (id, name, comment)
select p.*
from customer_import l
  cross join lateral json_populate_recordset(null::customer, doc) as p
on conflict (id) do update 
  set name = excluded.name, 
      comment = excluded.comment;