Re: pgstattupple vs pg_total_relation_size

On Wed, 30 Jan 2019 14:19:52 +0100
Tumasgiu Rossini <rossini(dot)t(at)gmail(dot)com> wrote:

> According to the doc [1],
> pg_total_relation_size add toasted data *and* indexes to the mix.

*and* FSM *and* VM.

> Any index, unique constraint, or primary key on your table ?
>
> [1]
> https://www.postgresql.org/docs/current/functions-admin.html#FUNCTIONS-ADMIN-DBSIZE
>
> Le mer. 30 janv. 2019 à 11:42, Mariel Cherkassky <
> mariel(dot)cherkassky(at)gmail(dot)com> a écrit :
>
> > 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)
> >
> >
> >
> >
> >

--
Jehan-Guillaume de Rorthais
Dalibo