Tracking amount of bytes written to temp tables - postgresql

Question

With PostgreSQL 9.5, I would like to track the total amount of bytes written (since DB cluster start) to:
WAL
temp files
temp tables
For 1.:
select
pg_size_pretty(archived_count * 16*1024*1024) temp_bytes,
(now() - stats_reset)::text uptime
from pg_stat_archiver;
For 2.:
select
(now() - stats_reset)::text uptime,
pg_size_pretty(temp_bytes) temp_bytes
from pg_stat_database where datname = 'mydb';
How do I get 3.?
In response to a comment below, I did some tests to check where temp tables are actually written.
First, the DB parameter temp_buffers is at 8GB on this cluster:
select pg_size_pretty(setting::bigint*8192) from pg_settings
where name = 'temp_buffers';
-- "8192 MB"
Lets create a temp table:
drop table if exists foo;
create temp table foo as
select random() from generate_series(1, 1000000000);
-- Query returned successfully: 1000000000 rows affected, 10:22 minutes execution time.
Check the PostgreSQL backend pid and OID of the created temp table:
select pg_backend_pid(), 'pg_temp.foo'::regclass::oid;
-- 46573;398695055
Check the RSS size of the backend process
~$ grep VmRSS /proc/46573/status
VmRSS: 9246276 kB
As can be seen, this is only slightly above the 8GB set with temp_buffers.
The data inserted into the temp table is however immediately written, and it is written to the normal tablespace directories, not temp files:
select * from pg_relation_filepath('pg_temp.foo')
-- "base/16416/t3_398695055"
Here is the number of files and amount written:
with temp_table_files as
(
select * from pg_ls_dir('base/16416/') fn
where fn like 't3_398695055%'
)
select
count(*) as cnt,
pg_size_pretty(sum((pg_stat_file('base/16416/' || fn)).size)) as size
from temp_table_files;
-- 34;"34 GB"
And finally verify that the set of temp files owned by this backend PID is indeed empty:
with temp_files_per_pid as
(
with temp_files as
(
select
temp_file,
(regexp_replace(temp_file, $r$^pgsql_tmp(\d+)\..*$$r$, $rr$\1$rr$, 'g'))::int as pid,
(pg_stat_file('base/pgsql_tmp/' || temp_file)).size as size
from pg_ls_dir('base/pgsql_tmp') temp_file
)
select pid, pg_size_pretty(sum(size)) from temp_files group by pid order by pid
)
select * from temp_files_per_pid where pid = 46573;
Returns nothing.
What is also "interesting", after dropping the temp table
DROP TABLE foo;
the RSS of the backend process does not reduce:
~$ grep VmRSS /proc/46573/status
VmRSS: 9254544 kB
Doing the following will also not free the RSS again:
RESET ALL;
DEALLOCATE ALL;
DISCARD TEMP;

Answer 1

What I know, there are not any special metric for temp tables. The temp tables uses session (process) memory to temp_buffers size (8MB by default). When these temp buffers are full, then temporary files are generated.