Insert if not exists, else return id in postgresql

Question

I have a simple table in PostgreSQL that has three columns:

• id serial primary key
• key varchar
• value varchar

I have already see

Answer 1

And you can store value returned to variables in form of ... RETURNING field1, field2,... INTO var1, var2,...

RETURNING will normally return a query which would return Error 'query has no destination for result data' if you call it in plpgsql without using its returned result set.