Understanding the First Numbering of PostgreSQL Sequence

The sequence has a cache value.Default 1 for PostgreSQL.
Create this as 10 and each of the two sessions

select nextval('test_seq3'); 

and

SELECT* FROM test_seq3;

Try running to see how it works.
I think I can understand.

CREATE SEQUENCE test_seq3
    INCREMENT 1
    START2
    MINVALUE1
    CACHE 10
;

The sequence cache values can also be found in SQL below.

select 
 c. relowner::register as owner
, c. relnamespace::regnamespace as schema
, c.relname
, s.seqtypid::regtype
, s.seqstart, s.seqinclement, s.seqmax, s.seqmin, s.seqcache, s.seqcycle
, c. reltype::regtype
,array_to_string(relacl,',')relacl
from pg_sequences 
inner join pg_class con s.seqrelid = c.oid
where c.relname = 'test_seq3'
;

YumaInaura Yuma Inaura's comments helped solve the problem.

The sequence has the flag is_called and until the first call is_called=false, it is understood that 1 is returned exceptionally, and is_called=true is returned regularly from 2 after the second call.

Engineer Tips NotesPostgreSQL Sequence Operations

However, there is something to be careful about here.That is how to set the current value to 1.
For example, if the current value is set to 1, as shown above, the ID of the next allocated flower will be 2.
SELECT SETVAL('Flower_id_seq',1)
This is because one of the columns in the sequence, isis_called が, is ttrue. に
isis_called は is a flag that indicates whether llast_value が has already been called, which is true if called or false if not already called.
The sequence calls llast_value を at the time of creating the serial number because the current value must be known in order for the sequence to generate a serial numbers.In the early stages of , がis_called 」 is true.If isis_called が is false, it means that the sequence has not yet generated a serial number, that is, there is no data to be serialized yet.
In other words, even if the same current value ("last_value") is 1, it is divided into true and true of "is_called" and whether there is data with a value of 1 assigned.

Nextval Behavior of PostgreSQL Sequences

If you specify start with 1 in the create sequence, you will get 1 when you first do nextval().
So far, I wanted to reset the value, so I did setval('hoge',1) and then I did nextval('hoge') and got 2 back, and I was like, "Huh!?"
Looking at each condition, the value of is_called is false immediately after the create sequence, but true immediately after the set value.

SQL for Verification

 --Verifying
SELECT* FROM test_seq;

-- set
SELECT setval('test_seq',1,true);
-- numbering
select nextval('test_seq'); 

-- set
SELECT setval('test_seq',1,false);
-- numbering
select nextval('test_seq');