Postgres 16

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select version();

version

PostgreSQL 16.0 (Debian 16.0-1.pgdg120+1) on x86_64-pc-linux-gnu, compiled by gcc (Debian 12.2.0-14) 12.2.0, 64-bit

SELECT 1

create TABLE testing_table (
  -- Keep in mind the name of this field
  field1 int4
, s1 text
, constraint pk_testing_table_field1 PRIMARY key (field1)
)

CREATE TABLE

create or replace function insert_testing_table(p_field1 int4, p_s1 text)
  returns testing_table
  language sql 
as $$
    insert into testing_table(field1, s1) 
         values (p_field1, p_s1)
        on conflict (field1)
        do update 
              set s1 = excluded.s1
        returning *;
$$;

CREATE FUNCTION

select * from insert_testing_table(1, 'aaa');

field1
s1

1
aaa

SELECT 1

select * from insert_testing_table(2, 'yyy')

field1
s1

2
yyy

SELECT 1

select * from insert_testing_table(1, 'bbb')

field1
s1

1
bbb

SELECT 1

select * 
  from testing_table;

field1
s1

2
yyy

1
bbb

SELECT 2