Postgres 13

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CREATE SEQUENCE tag_seq START WITH 1;

CREATE TABLE tag 
    (id TEXT NOT NULL DEFAULT 'tag_'||nextval('tag_seq'::regclass), 
    name TEXT);

CREATE SEQUENCE

CREATE TABLE

INSERT INTO tag (name) VALUES ('foo'),('bar');    

INSERT 0 2

SELECT * FROM tag;

id
name

tag_1
foo

tag_2
bar

SELECT 2