How hll works in postgresql

hll-in-postgresql.sql

drop table if exists s_user_table;
create table s_user_table (
       id int,
       user_id int
);


drop table if exists s_user_cnt_table;
create table s_user_cnt_table (
       user_id int,
       cnt hll
);


insert into s_user_table select id, id from generate_series(1, 987654) as id;

insert into s_user_cnt_table select user_id % 1000, hll_add_agg(hll_hash_integer(user_id)) from s_user_table group by 1;

with A(user_id, cnt) as (
     select user_id, hll_cardinality(cnt) from s_user_cnt_table limit 10),

     B(user_id, cnt) as (
     select user_id % 1000, count(id) from s_user_table where user_id % 1000 in (select user_id from A) group by 1)

select A.user_id, A.cnt, B.cnt, abs(A.cnt - B.cnt) / B.cnt from A, B where A.user_id = B.user_id;