How to Generate Subsequences in PostgreSQL Using Sequences and Triggers?

This article details how to generate subsequences in PostgreSQL using sequences and triggers. The scenario involves a table with a composite primary key (id, seq), requiring a custom solution to create a unique sequence for each id.

The solution involves creating two tables and implementing triggers:

Table Structure:

• things: This table stores main entities, with id as the primary key.
• stuff: This table stores subsequences, containing id (foreign key referencing things), seq (the subsequence number), and notes. The primary key is the combination of id and seq.

Triggers and Functions:

1. things table trigger (make_thing_seq): This trigger is executed after each insertion into the things table. It creates a new sequence named thing_seq_{id} for each new id.

2. stuff table trigger (fill_in_stuff_seq): This trigger is executed before each insertion into the stuff table. It retrieves the next value from the appropriate sequence (thing_seq_{id}) based on the inserted id and assigns it to the seq column.

Example Implementation:

The following code demonstrates the table creation and trigger implementation:

<code class="language-sql">-- Create the tables
CREATE TABLE things (
    id serial primary key,
    name text
);

CREATE TABLE stuff (
    id integer references things,
    seq integer NOT NULL,
    notes text,
    primary key (id, seq)
);

-- Trigger in things table to create new sequences
CREATE OR REPLACE FUNCTION make_thing_seq() RETURNS trigger
    LANGUAGE plpgsql
    AS $$
begin
  EXECUTE format('CREATE SEQUENCE IF NOT EXISTS thing_seq_%s', NEW.id); -- Added IF NOT EXISTS
  RETURN NEW;
end;
$$;

CREATE TRIGGER make_thing_seq AFTER INSERT ON things FOR EACH ROW EXECUTE PROCEDURE make_thing_seq();

-- Trigger in stuff table to use correct sequence
CREATE OR REPLACE FUNCTION fill_in_stuff_seq() RETURNS trigger
    LANGUAGE plpgsql
    AS $$
begin
  NEW.seq := nextval(format('thing_seq_%s', NEW.id));
  RETURN NEW;
end;
$$;

CREATE TRIGGER fill_in_stuff_seq BEFORE INSERT ON stuff FOR EACH ROW EXECUTE PROCEDURE fill_in_stuff_seq();</code>

This setup ensures that inserting data into things and stuff automatically generates unique subsequences for each id, maintaining the integrity of the composite primary key. The IF NOT EXISTS clause has been added to the make_thing_seq function to prevent errors if a sequence for a given ID already exists.

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