Limiting the occurence of a value in PostgreSQL

Question

Using PostgreSQL 11.7 and I have to model some real-world constraints. Effectively, I'm modeling a graph and an edge, of course, has two nodes. Each node can have zero, one, or two edges. Thus, I need a constraint similar to:

node UUID NOT NULL CHECK (SELECT true FROM this_table WHERE node = NEW.node HAVING COUNT(*) <= 2)

Except that this syntax is illegal. I also have the following problem since this applies to four of our tables:

• I don't know the table name up front
• I don't know the column name up front
• I don't know the allowed number of occurrences up front

I almost have it working, but I can't figure out how to extract the value of the NEW.node. I have written a trigger that's close, but no cigar:

CREATE DOMAIN uint2 AS int4 CHECK(VALUE >= 0 AND VALUE < 65536);

CREATE OR REPLACE FUNCTION limit_count() RETURNS TRIGGER AS $$
DECLARE
    total       integer;
    this_table  text := TG_TABLE_NAME::regclass::text;
    this_column text := TG_ARGV[0]; -- first argument (triggers cannot define arguments directly)
    this_limit uint2 := TG_ARGV[1]; -- second argument
BEGIN
    IF cardinality(TG_ARGV) <> 2 THEN
        RAISE EXCEPTION 'limit_count() trigger expects a column name and a limit';
    END IF;

    EXECUTE FORMAT('SELECT COUNT(%I) FROM %I WHERE %1$I = $1', this_column, this_table)
       USING NEW.quote_literal(this_column)
       INTO total;

    IF total + 1 > this_limit THEN
        RAISE EXCEPTION '%.% has % occurences, but must not have more than %', this_table, this_column, total, this_limit;
    END IF;

    RETURN NEW;
END;
$$ LANGUAGE plpgsql;

CREATE TRIGGER fibre_route_edge_start_node_limit_trigger BEFORE INSERT OR UPDATE ON fibre_route_edge
    FOR EACH ROW EXECUTE PROCEDURE limit_count('start_node', 2);
CREATE TRIGGER fibre_route_edge_end_node_limit_trigger BEFORE INSERT OR UPDATE ON fibre_route_edge
    FOR EACH ROW EXECUTE PROCEDURE limit_count('end_node', 2);

The above fails with:

ERROR:  schema "new" does not exist
LINE 1: SELECT NEW.quote_literal(this_column)
               ^
QUERY:  SELECT NEW.quote_literal(this_column)

Happy to solve this problem without a trigger, but I'm kinda stumped right now. And examples I've seen using hstore and other tools have been incomprehensible to me.

Answer 1

You can't access the fields of the new record dynamically using names stored in a variable.

The only way I can think of, is to convert the NEW record into a JSON value, then you can access the value "dynamically" through the name:

this_column_value := (to_jsonb(new) ->> this_column)::int; 
EXECUTE FORMAT('SELECT COUNT(*) FROM %I WHERE %I = $1', this_table, this_column)
  USING this_column_value
  INTO total;