Would it be possible to select random rows with a little preference for a specific column? - postgresql

Question

I would like to get a random selection of records from my table but I wonder if it would be possible to give a better chance for items that are newly created. I also have pagination so this is why I'm using setseed
Currently I'm only retrieving items randomly and it works quite well, but I need to give a certain "preference" to newly created items.
Here is what I'm doing for now:
SELECT SETSEED(0.16111981), RANDOM();
I don't know what to do and I can't figure what can be a good solution without being an absolute performance disaster.

Answer 1

Firstly I want to explain how we can select random records on a table. On PostgreSQL, we can use random() function in the order by statement. Example:
select * from test_table
order by random()
limit 1;
I am using limit 1 for selecting only one record. But, using this method our query performance will be very bad for large size tables (over 100 million data)
The second way, you can manually be selecting records using random() if the tables are had id fields. This way is very high performance.
Let's firstly write our own randomize function for using it's easily on our queries.
CREATE OR REPLACE FUNCTION random_between(low integer, high integer)
RETURNS integer
LANGUAGE plpgsql
STRICT
AS $function$
BEGIN
RETURN floor(random()* (high-low + 1) + low);
END;
$function$;
This function returns a random integer value in the range of our input argument values. Then we can write a query using our random function. Example:
select * from test_table
where id = (select random_between(min(id), max(id)) from test_table);
This query I tested on the table has 150 million data and gets the best performance, Duration 12 ms. In this query, if you need many rows but not one, then you can write where id > instead of where id=.
Now, for your little preference, I don't know your detailed business logic and condition statements which you want to set to randomizing. I can write for you some sample queries for understanding the mechanism. PostgreSQL has not a function for doing this process, so randomize data using preferences. We must write this logic manually. I created a sample table for testing our queries.
CREATE TABLE test_table (
id serial4 NOT NULL,
is_created bool NULL,
action_date date NULL,
CONSTRAINT test_table_pkey PRIMARY KEY (id)
);
CREATE INDEX test_table_id_idx ON test_table USING btree (id);
For example, I want to set more preference only to data which are action dates has a closest to today. Sample query:
select
id,
is_created,
action_date,
(extract(day from (now()-action_date))) as dif_days
from
test.test_table
where
id > (select random_between(min(id), max(id)) from test.test_table)
and
(extract(day from (now()-action_date))) = random_between(0, 6)
limit 1;
In this query this (extract(day from (now()-action_date))) as dif_days query will returned difference between action_date and today. On the where clause firstly I select data that are id field values greater than the resulting randomize value. Then using this query (extract(day from (now()-action_date))) = random_between(0, 6) I select from this resulting data only which data are action_date equals maximum 6 days ago (maybe 4 days ago or 2 days ago, mak 6 days ago).
Сan wrote many logic queries (for example set more preferences using boolean fields: closed are opened and etc.)