Re: pgstattupple vs pg_total_relation_size

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)