BUG #13868: Strange performance of type conversion in jsonb operations

The following bug has been logged on the website:

Bug reference:      13868
Logged by:          Xtra Coder
Email address:      xtracoder@gmail.com
PostgreSQL version: 9.5.0
Operating system:   Windows
Description:

I suspect these 3 questions arise from some kind of bugs, because actual
result seems somewhat weird to me:

* why implicit type cast int=jsonb works, whereas explicit type cast via
::int or  cast(jsonb as int) says: 'cannot cast type jsonb to integer'

* why assignment int=text has same performance as int=int - the first one
should have some significant performance penalty for parcing of text to int

* why assignment int=jsonb->'int_field' is significantly slower than
int=jsonb->>'int_field' - to my mind it should be opposite because value is
already stored as int in jsonb

To check if i'm missing something i've asked question at stackoverflow, but
received now answer:
http://stackoverflow.com/questions/34716098/strange-performance-of-postgresql-type-conversion-in-jsonb-operations

Copy-pasting entire question here just for reference:
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
-

I'm thinking about implementation of 'global consts' via jsonb stored in a
table. To check what can be expected in terms of performance I've created
some simple perf test, which revealed same questions that I have no clear
answer.

- why implicit type cast `int=jsonb` works, whereas explicit type cast via
`::int` or `cast(jsonb as int)` says: '_cannot cast type jsonb to integer_'

- why assignment `int=text` has same performance as `int=int` - _the first
one should have some significant performance penalty for parcing of text to
int_

- why assignment `int=jsonb->'int_field'` is significantly slower than
`int=jsonb->>'int_field'` - _to my mind it should be opposite because value
is already stored as int in jsonb_

PS: extraction of JSONB from DB is rather quick - reading 10K jsonb takes 1
sec per 100K cycles. After that I will need rather excessive access to
dictionary of `name->int` values and looking for the most performant
implementation.

The test:

    DO LANGUAGE plpgsql $$
    DECLARE
      json_struct jsonb;
      json_primitive jsonb;
      str_val text;
      int_val int;
      int_val2 int;
      count int;
    BEGIN
      count = 0;
      json_struct = '{"val": 11}';
      json_primitive = 11;
      str_val = 11;

      int_val = json_struct->'val';
      raise notice 'int_val = %', int_val;

      int_val = json_struct->>'val';
      raise notice 'int_val = %', int_val;

      LOOP
        --int_val = count;                      -- 90ms
        --int_val2 = int_val;                   -- 111ms = +21ms

        --int_val = json_primitive;             -- 130ms = +40ms
        --int_val = json_struct->'val';         -- 160ms = +70ms

        --json_primitive = json_struct->'val';  -- 120ms = +30ms
        --int_val = str_val;                    -- 111ms = +21ms
        --int_val = json_struct->>'val';        -- 125ms = +35ms

        --int_val = cast(json_primitive as int); -- ERROR:  cannot cast type
jsonb to integer
        --int_val = json_primitive::int;        -- ERROR:  cannot cast type
jsonb to integer
        --int_val = json_primitive::text::int;  -- 140ms = +50ms

        count = count + 1;
        EXIT WHEN count > 100000;
      END LOOP;
    END; $$