pgstattupple vs pg_total_relation_size

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Hey,
I'm using postgresql 9.6.11. I wanted to ask something about the functions I mentioned in the title : 
I created the next table : 
postgres=# \d students;
      Table "public. students  "
  Column  |  Type   | Modifiers
----------+---------+-----------
 id| integer |
 name| text    |
 age| integer |
 data     | jsonb   |

I inserted one row. When I query the table`s size with pg_total_relation_size I see that the data occupies 2 pages : 

postgres=# select pg_total_relation_size(' students  ');
 pg_total_relation_size
------------------------
                  16384
(1 row)

postgres=# select pg_relation_size(' students  ');
 pg_relation_size
------------------
             8192
(1 row)

When I used pgstattuple : 
postgres=# select * from pgstattuple('pg_toast.pg_toast_1187222');
 table_len | tuple_count | tuple_len | tuple_percent | dead_tuple_count | dead_tuple_len | dead_tuple_percent | free_space | free_percent
-----------+-------------+-----------+---------------+------------------+----------------+--------------------+------------+--------------
         0 |           0 |         0 |             0 |                0 |              0 |                  0 |          0 |            0
(1 row)

postgres=# select * from pgstattuple('students');
 table_len | tuple_count | tuple_len | tuple_percent | dead_tuple_count | dead_tuple_len | dead_tuple_percent | free_space | free_percent
-----------+-------------+-----------+---------------+------------------+----------------+--------------------+------------+--------------
      8192 |           1 |      1221 |          14.9 |                0 |              0 |                  0 |       6936 |        84.67
(1 row)

Which means, the toasted table is empty and you can see that the row I inserted should occupy only one page(8K in my system).

Then, why the pg_total_relation_size shows another page ?(16KB in total)





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