Is it possible to use nextval of a sequence as default value of a field in a table in infomrix database?

Question

I have a table named T1 with a field id(int8) and a sequence named seq_id.

Can I use seq_id.nextval as a default value for id in informix database?

Or another way, can I use a trigger to update id before insert with seq_id.nextval?

Answer 1

In Informix you cannot use a sequence NEXTVAL as a default value for a column.

One option is to turn the column into a BIGSERIAL.

Other option is with an insert trigger. I did not find a way to directly assign the sequence NEXTVAL in the trigger definition, but it can be done by using a stored procedure.

CREATE SEQUENCE seq_id 
    INCREMENT BY 1 START WITH 1 
    MINVALUE 0 
    NOCYCLE CACHE 10 
    ORDER; 

CREATE TABLE t1
(
      id BIGINT NOT NULL
    , val1 CHAR(4)
); 

I found 2 ways to use the procedure. A generic procedure that returns the sequence NEXTVAL and a trigger procedure, that assigns the sequence NEXTVAL to the id.

Using a generic procedure:

CREATE FUNCTION spl_get_seq_id()
RETURNING BIGINT AS seq_id_next;
    DEFINE seq_id_next BIGINT;
    LET seq_id_next = seq_id.NEXTVAL;
    RETURN seq_id_next;
END FUNCTION;

CREATE TRIGGER t1_ti
    INSERT ON t1 REFERENCING NEW AS new_ins
    FOR EACH ROW
    (
        EXECUTE FUNCTION spl_get_seq_id() INTO id
    );

Inserting a few values into the table and checking the result:

INSERT INTO t1( id, val1 ) VALUES ( 1000, 'AAAA' );
INSERT INTO t1( val1 ) VALUES ( 'AAAB' );
INSERT INTO t1( id ) VALUES ( 1 );
INSERT INTO t1( id, val1 ) VALUES ( NULL::BIGINT, 'AAAD' );

SELECT * FROM t1;

id val1

 1 AAAA
 2 AAAB
 3
 4 AAAD

Using a trigger procedure:

DROP TRIGGER t1_ti;

CREATE PROCEDURE t1_ti_spl_get_seq_id()
REFERENCING NEW AS new_values FOR t1;
    LET new_values.id = seq_id.NEXTVAL;
END PROCEDURE;

CREATE TRIGGER t1_ti
    INSERT ON t1
    FOR EACH ROW
    (
        EXECUTE PROCEDURE t1_ti_spl_get_seq_id() WITH TRIGGER REFERENCES
    );

Inserting a few values into the table and checking the result:

INSERT INTO t1( id, val1 ) VALUES ( 1000, 'AAAE' );
INSERT INTO t1( val1 ) VALUES ( 'AAAF' );
INSERT INTO t1( id ) VALUES ( 1 );
INSERT INTO t1( id, val1 ) VALUES ( NULL::BIGINT, 'AAAH' );

SELECT * FROM t1;

id val1

 1 AAAA
 2 AAAB
 3
 4 AAAD
 5 AAAE
 6 AAAF
 7
 8 AAAH

I used BIGINT for the id column, but it should work for INT8 (as to why did I use it, seems there is some benefits: Counters and codes: BIGINT, INT8, INTEGER, and SMALLINT).

EDIT 1:

In response to your comment, you can try a condition on the trigger, based on the session user. This will only work if the cdc software is using a dedicated user. In this example, cdc_agent is the user that the cdc software uses in Informix.

DATABASE db1;

GRANT CONNECT TO cdc_agent;
GRANT CONNECT TO myuser;

CREATE SEQUENCE seq_id 
    INCREMENT BY 2 START WITH 2 
    MINVALUE 0 
    NOCYCLE CACHE 10 
    ORDER; 

GRANT SELECT ON seq_id TO cdc_agent;
GRANT SELECT ON seq_id TO myuser;

CREATE TABLE t1
(
      id BIGINT NOT NULL
    , val1 CHAR(4)
);

GRANT ALL ON t1 TO cdc_agent;
GRANT ALL ON t1 TO myuser;

CREATE PROCEDURE t1_ti_spl_get_seq_id()
REFERENCING NEW AS new_values FOR t1;
    LET new_values.id = seq_id.NEXTVAL;
END PROCEDURE;

GRANT EXECUTE ON t1_ti_spl_get_seq_id TO cdc_agent;
GRANT EXECUTE ON t1_ti_spl_get_seq_id TO myuser;  

CREATE TRIGGER t1_ti
    INSERT ON t1 REFERENCING NEW AS new_ins
    FOR EACH ROW WHEN ( USER <> "cdc_agent" )
    (
        EXECUTE PROCEDURE t1_ti_spl_get_seq_id() WITH TRIGGER REFERENCES
    );

Inserting a few values into the table and checking the result:

-- with user "cdc_agent"
INSERT INTO t1( id, val1 ) VALUES ( 11, 'AAAA' );
INSERT INTO t1( id, val1 ) VALUES ( 13, 'AAAC' );
-- with user "myuser"
INSERT INTO t1( id, val1 ) VALUES ( 1, 'AAAB' );
INSERT INTO t1( id, val1 ) VALUES ( 3, 'AAAD' );

SELECT * FROM t1;   

  id val1

  11 AAAA
  13 AAAC
   2 AAAB
   4 AAAD