Efficiently looping through a VARIADIC in Postgres - postgresql

Question

I have the following table in Postgres
Which would typically be populated like below
id day visits passes
1 Monday {11,13,19} {13,17}
2 Tuesday {7,9} {11,13,19}
3 Wednesday {2,5,21} {21,27}
4 Thursday {3,11,39} {21,19}`
In order to get the visit or passes ids over a range of days I have written the following function
CREATE OR REPLACE FUNCTION day_entries(p_column TEXT,VARIADIC ids int[]) RETURNS bigint[] AS
$$
DECLARE result bigint[];
DECLARE hold bigint[];
BEGIN
FOR i IN 1 .. array_upper(ids,1) LOOP
execute format('SELECT %I FROM days WHERE id = $1',p_column) USING ids[i] INTO hold;
result := unnest(result) UNION unnest(hold);
END LOOP;
RETURN result;
END;
$$
LANGUAGE 'plpgsql';
which works with a subsequent call to day_entries('visits',1,2,3) returning
{11,9,19,21,5,13,2,7}
While it does the job I am concerned that based on my one day old knowledge of writing Postgres functions I have worked in one or more inefficiences into the process. Can the function be made easier in some way?
The other issue is more a curiosity than a problem - the order of elements in the result appears to have no bearing to the order of visits entries in the three rows that are touched. Although this is not an issue as far as I am concerned I am curious to know why it happens.

Answer 1

You can do the unnesting and aggregating in a single statement, no need for a loop. And you can use the ANY operator with the array to select all matching rows.
CREATE OR REPLACE FUNCTION day_entries(p_column TEXT, variadic p_ids int[])
RETURNS bigint[] AS
$$
DECLARE
result bigint[];
BEGIN
execute
format('SELECT array(select unnest(%I) from days WHERE id = any($1))', p_column)
USING p_ids -- pass the whole array as a parameter
INTO result;
RETURN result;
END;
$$
LANGUAGE plpgsql;
Not related to your questions, but I think you are going down the wrong road with that design. While arrays might look intriguing to beginners at the beginning, they should only be used rarely.
And if you find yourself unnesting and aggregating things back and forth, this is a strong indication that something could be improved.
I would split your table up in two tables, one that stores the "day" information and one that stores visits and passes in the same table with a column distinguishing the two. Then finding visits is as simple as adding a where ... = 'visit' rather than having to cope with (slow and error prone) dynamic SQL.
Without knowing more details, I would probably create the tables like this:
create table days
(
id integer not null primary key,
day character varying(9) not null
);
create table event
(
day_id integer not null references days,
event_id integer not null,
event_type varchar(10) not null check (event_type in ('visit', 'pass'))
);
event_id might even be a foreign to key to another table you haven't shown us - again something you can't really do with de-normalized tables.
Getting all visits for specific days, is then as simple as:_
select event_id
from event
where day_id in (1,2)
and event_type = 'visit';
Or if you do need that as an array:
select array_agg(event_id)
from event
where day_id in (1,2)
and event_type = 'visit';
Online example