database - How to return a new value in a trigger from a cte in PostgreSQL -

from previous question in postgresql (9.3), how check sequence efficiently used , unused values in postgresql, have table chart_gap has list of unused chart numbers table charts:

create table chart_gap (chart_number integer); 

in trying understand triggers bit more, have added following trigger procedure taken more or less question:

create or replace function next_chart() returns trigger $body$ begin   -- check empty chart number   if new.chart_number null      ins (       select chart_number         chart_gap        pg_try_advisory_xact_lock(chart_number)       limit  1     )     delete chart_gap c     using  ins      i.chart_number = c.chart_number;      new.chart_number := select chart_number ins;   <--wrong!          end if;    return new; end; $body$ language plpgsql volatile; 

the trigger procedure tied charts file with:

create trigger next_chart   before insert   on charts   each row   execute procedure next_chart(); 

what attempting replace empty chart number field in charts whenever new chart record added. can done in trigger? (and correct syntax??)

in plpgsql function can use query returning ... ... follows:

create function next_chart() returns trigger $body$ begin   -- check empty chart number   if new.chart_number null     ins (       select chart_number         chart_gap        pg_try_advisory_xact_lock(chart_number)       limit  1     )     delete chart_gap c     using  ins      i.chart_number = c.chart_number     returning i.chart_number new.chart_number;   end if;    return new; end; $body$ language plpgsql volatile;