Postgres 11

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Help with an interesting Postgres question: Why isn't an Index Only Scan used on a partition accessed via the parent table?.
create table t (id int, pid int , name name , dt date) partition by range(dt);
create table t1 partition of t for values from ('2020-01-01') to ('2020-02-01');
alter table t1  add constraint uk1 unique  (id,pid);
create table t2 partition of t for values from ('2020-02-01') to ('2020-03-01');
alter table t2  add constraint uk2 unique  (id,pid);
create table t4 partition of t for values from ('2020-03-01') to ('2020-04-01');
alter table t4  add constraint uk3 unique  (id,pid);
create table t3 partition of t for values from ('2020-04-01') to ('2020-05-01');
alter table t3  add constraint uk4 unique  (id,pid);




--SELECT conname FROM pg_constraint WHERE conrelid = m_table1::regclass::oid and contype = 'u' into conn_name;


CREATE or replace FUNCTION insert_trigger()
    RETURNS trigger
    LANGUAGE 'plpgsql'
    COST 100
    VOLATILE NOT LEAKPROOF
AS $BODY$
DECLARE
      conn_name text;
      
	  c_table TEXT;
	  t_schema text;
	  c_table1 text;
	  m_table1 text;
	
    BEGIN
	
      c_table1 := TG_TABLE_NAME;
      t_schema := TG_TABLE_SCHEMA;
      m_table1 := t_schema||'.'||TG_TABLE_NAME;
      SELECT conname FROM pg_constraint WHERE conrelid = TG_TABLE_NAME ::regclass::oid and contype = 'u' into conn_name;
    
    execute 'insert into '|| m_table1 || ' values ' || new.* || ' on conflict on constraint ' || conn_name || ' do nothing -- or somthing';
 
    
    RETURN null;
    end;
    
$BODY$;


	
CREATE TRIGGER insert
    BEFORE INSERT
    ON t4
    FOR EACH ROW
    WHEN (pg_trigger_depth() < 1)
    EXECUTE FUNCTION insert_trigger();
    CREATE TRIGGER insert
    BEFORE INSERT
    ON t3
    FOR EACH ROW
    WHEN (pg_trigger_depth() < 1)
    EXECUTE FUNCTION insert_trigger();
    CREATE TRIGGER insert
    BEFORE INSERT
    ON t2
    FOR EACH ROW
    WHEN (pg_trigger_depth() < 1)
    EXECUTE FUNCTION insert_trigger();
    CREATE TRIGGER insert
    BEFORE INSERT
    ON t1
    FOR EACH ROW
    WHEN (pg_trigger_depth() < 1)
    EXECUTE FUNCTION insert_trigger();
	
insert into t values (1,2,'raj','2020-01-01');

insert into t values (1,2,'raj','2020-01-01');
ERROR:  column "raj" does not exist
LINE 1: insert into public.t1 values (1,2,raj,2020-01-01) on conflic...
                                          ^
QUERY:  insert into public.t1 values (1,2,raj,2020-01-01) on conflict on constraint uk1 do nothing -- or somthing
CONTEXT:  PL/pgSQL function insert_trigger() line 17 at EXECUTE


select * from t;