postgres generate sql statement with dynamic fields - postgresql

Question

im building a package to generate HTML code that later will be sent by email.
the data query has be dynamic as it will have all days of the month.
in the end query should be something like
select day_20230101, day_20230102, ...,day_20230131,avg_cap, rank1,... from v_html_report;
and then execute in the FOR to generate the HTML. The problem I'm facing is that the list of the columns is being seen as an entire string. when executing the loop the result_rec.day_20230101 doesnt exist but exist the result_rec.column_list.
is there a way to execute for example the query from a string in the FOR or the query seen this as fields and not a single field?
Below my Procedure:
create or replace procedure proc_html_gen ()
language plpgsql
as $$
DECLARE
days_month int;
column_list text ;
result_rec record;
html_header text ;
html_result text ;
html_days text ;
html_sql text ;
begin
SELECT
date_part(
'days',
((to_date((select value from parameters rtp where id='html_report_month' and id2='execute_month')||'01','YYYYMMDD')) + interval '1 month - 1 day')
) into days_month;
update html_gen rtp
set aux1=days_month
where html_type='html_report';
SELECT to_char(generate_series(01,days_month ),'00') into column_list;
select string_agg(concat('days_',days_list),',') from (
SELECT concat(
(select value from parameters rtp where id='html_report' and id2='execute_month')
,trim(to_char(generate_series(01,days_month),'00'))) as days_list
) a into column_list;
update html_gen rtp
set aux2=column_list
where html_type='html_report';
html_sql := ' select ' || column_list || ', avg_cap,rank1, rank2, rank3, rank4, rank5, rank6, rank7, rank8, rank9, rank10 from v_monthly_polygon_peak_cap_table' ;
FOR result_rec IN execute html_sql
LOOP
select string_agg(days_list,'') into html_days
from (
select concat('<td>',concat(1,trim(to_char(generate_series(01,31),'00'))),'</td>') as days_list
) a ;
html_result := concat(html_result,'<tr>',html_days,'<td> ' || result_rec.avg_cap || '</td></tr>');
END LOOP;
commit;
end;$$
update the query with the solution
now im facing a new problem. how to get the field name dynamically?
like below im getting an error. days_ doesnt exist. i need to append the number of the day in the end of the variable.
html_result := result_rec.days_|| trim(to_char(generate_series(01,31),'00')))
Best Regards,
Macieira