--> show rowcount of the test table:

db=# select count(*) from myjsonfile100k ;
 count  
--------
 790000
(1 row)

--> show size of the test table, and index:

db=# \dti+ myjsonfile100k* 
                                                       List of relations
 Schema |               Name                | Type  |  Owner   |     Table      | Persistence |  Size  |      Description      
--------+-----------------------------------+-------+----------+----------------+-------------+--------+-----------------------
 public | myjsonfile100k                    | table | aardvark |                | permanent   | 589 MB | 
 public | myjsonfile100k_js_pathops_gin_idx | index | aardvark | myjsonfile100k | permanent   | 135 MB | 
 public | myjsonfile100k_pkey               | index | aardvark | myjsonfile100k | permanent   | 17 MB  | 
(3 rows)


--> show table+index structure

db=# \d+ myjsonfile100k
                                                Table "public.myjsonfile100k"
 Column |  Type   | Collation | Nullable |                  Default                   | Storage  | Stats target | Description 
--------+---------+-----------+----------+--------------------------------------------+----------+--------------+-------------
 js     | jsonb   |           |          |                                            | extended |              | 
 id     | integer |           | not null | nextval('myjsonfile100k_id_seq'::regclass) | plain    |              | 
Indexes:
    "myjsonfile100k_pkey" PRIMARY KEY, btree (id)
    "myjsonfile100k_js_pathops_gin_idx" gin (js jsonb_path_ops)
Access method: heap


--> show single jsonb 'record' structure:

db=# select jsonb_pretty(js) from myjsonfile100k limit 1;
                                             jsonb_pretty                                              
-------------------------------------------------------------------------------------------------------
 {                                                                                                    +
     "bool": false,                                                                                   +
     "city": "Cebu City",                                                                             +
     "date": "1980-08-12",                                                                            +
     "enum": "generator",                                                                             +
     "array": [                                                                                       +
         "Eve",                                                                                       +
         "Chandra",                                                                                   +
         "Binny",                                                                                     +
         "Tabbatha",                                                                                  +
         "Anallese"                                                                                   +
     ],                                                                                               +
     "regEx": "helloooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo to you",+
     "random": 11,                                                                                    +
     "country": "Iran, Islamic Republic Of",                                                          +
     "lastname": "Seligman",                                                                          +
     "firstname": "Correy",                                                                           +
     "countryCode": "DE",                                                                             +
     "random float": 60.823,                                                                          +
     "array of objects": [                                                                            +
         {                                                                                            +
             "index": 0,                                                                              +
             "index start at 5": 5                                                                    +
         },                                                                                           +
         {                                                                                            +
             "index": 1,                                                                              +
             "index start at 5": 6                                                                    +
         },                                                                                           +
         {                                                                                            +
             "index": 2,                                                                              +
             "index start at 5": 7                                                                    +
         }                                                                                            +
     ],                                                                                               +
     "email from expression": "Correy.Seligman@yopmail.com",                                          +
     "email uses current data": "Correy.Seligman@gmail.com"                                           +
 }
(1 row)


--> crunch time:
-->     (NOTE that below I truncated the result rows with '...' for PM-display):

db=# select
    id
  , js->>'firstname' as fname
  , js->>'lastname'  as lname
  , *

from myjsonfile100k

where js @> '{"firstname": "Lilith"}' -- nevermind the crazy names,
  and js @> '{"lastname": "Trace"}'   -- randomly generated stuff
;

   id   | fname  | lname |                                                                                                                                                    
--------+--------+-------+------------------------------------------------------------------
 127245 | Lilith | Trace | {"bool": false, "city": "Kolkata", "date": "1995-05-14", "enum": ...
 393835 | Lilith | Trace | {"bool": true, "city": "Sarajevo", "date": "1987-06-07", "enum": ...
(2 rows)

Time: 0.846 ms

--> less than a millisecond