Coalesce in PostgreSQL trigger does not fire on upddate

(This question is also exposed here:
https://stackoverflow.com/questions/60117123/coalesce-in-postgresql-trigger-does-not-fire-on-upddate
)

Hi Everyone!

In the context of a database for a paleontological collection, I have this
table definition:

CREATE TABLE taxon (
id integer DEFAULT NEXTVAL('taxon_oid_seq') PRIMARY KEY,
taxon varchar(100) UNIQUE NOT NULL,
reino varchar(50) NOT NULL,
phylum varchar(100) ,
subphylum varchar(100) ,
classe varchar(100) ,
subclasse varchar(100) ,
superordem varchar(100) ,
ordem varchar(100) ,
subordem varchar(100) ,
infraordem varchar(100) ,
familia varchar(100) ,
subfamilia varchar(100) ,
genero varchar(100) ,
especie varchar(100) ,
subespecie varchar(100) );

The taxon field is to be automatically filled with the lowest level to
which it was possible to determine a given species taxonomy. In order to
achieve that I have this trigger:

CREATE OR REPLACE FUNCTION get_taxon() RETURNS TRIGGER LANGUAGE
plpgsql AS $BODY$ BEGIN
NEW.taxon := coalesce(NEW.subespecie, NEW.especie, NEW.genero, NEW.subfamilia,
NEW.familia, NEW.infraordem, NEW.subordem,
NEW.ordem, NEW.superordem,
NEW.subclasse, NEW.classe, NEW.subphylum,
NEW.phylum, NEW.reino); RETURN NEW; END; $BODY$
VOLATILE;
CREATE TRIGGER update_taxon
BEFORE INSERT OR UPDATE ON taxonFOR EACH ROW EXECUTE PROCEDURE get_taxon();

However this trigger only fires on INSERT, nothing happens if an UPDATE is
made. How can have this trigger also firing in the case of an UPDATE?

Thanks,
Andre

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