How to create sequence which start from 1 in each day - sql

Question

Sequence should return values 1,2,3 etc starting for 1 for every day.
current_date should used for day determination.
For example, calling today first time it shoudl return 1, in second time 2 etc.
Tomorrow, first call shoud return again 1, second call 2 etc.
Postgres 9.1 is used.

Answer 1

Use a table to keep the sequence:
create table daily_sequence (
day date, s integer, primary key (day, s)
);
This function will retrieve the next value:
create or replace function daily_sequence()
returns int as $$
insert into daily_sequence (day, s)
select current_date, coalesce(max(s), 0) + 1
from daily_sequence
where day = current_date
returning s
;
$$ language sql;
select daily_sequence();
Be prepared to retry in case of an improbable duplicate key value error. If previous days' sequences are not necessary delete them to keep the table and the index as light as possible:
create or replace function daily_sequence()
returns int as $$
with d as (
delete from daily_sequence
where day < current_date
)
insert into daily_sequence (day, s)
select current_date, coalesce(max(s), 0) + 1
from daily_sequence
where day = current_date
returning s
;
$$ language sql;

Answer 2

You just need to think of cronjob as running a shell command at a specified time or day.
Shell Command for running cron job
psql --host host.domain.com --port 32098 --db_name databaseName < my.sql
You can then just add this to your crontab (I recommend you use crontab -e to avoid breaking things)
# It will run your command at 00:00 every day
# min hour wday month mday command-to-run
0 0 * * * psql --host host.domain.com --port 32098 --db_name databaseName < my.sql

Answer 3

It is quite interesting task.
Lets try to use additional sequence for the date and alternative function to get next value:
-- We will use anonymous block here because it is impossible to use
-- variables and functions in DDL directly
do language plpgsql $$
begin
execute 'create sequence my_seq_day start with ' || (current_date - '1900-01-01')::varchar;
end; $$;
-- Initialize sequence
select nextval('my_seq_day');
create sequence my_seq;
create or replace function nextval_daily(in p_seq varchar) returns bigint as $$
declare
dd bigint;
lv bigint;
begin
select current_date - '1900-01-01'::date into dd;
-- Here we should to retrieve current value from sequence
-- properties instead of currval function to make it session-independent
execute 'select last_value from '||p_seq||'_day' into lv;
if dd - lv > 0 then
-- If next day has come
-- Reset main sequens
execute 'alter sequence '||p_seq||' restart';
-- And set the day sequence to the current day
execute 'alter sequence '||p_seq||'_day restart with '||dd::varchar;
execute 'select nextval('''||p_seq||'_day'')' into lv;
end if;
return nextval(p_seq);
end; $$ language plpgsql;
Then use function nextval_daily instead of nextval.
Hope it was helpful.

Answer 4

I have came across with almost similar requirement.
Handled the logic from query rather than modifying the sequence.
used setval() to reset the sequence to 0 if its the first entry to the table for the day.
Else nextval() of the sequence.
Below is the sample query :
SELECT
CASE WHEN NOT EXISTS (
SELECT primary_key FROM schema.table WHERE date(updated_datetime) = #{systemDate} limit 1)
THEN
setval('scheam.job_seq', 1)
ELSE
nextval('scheam.job_seq')
END
UPDATE privilege is required for the user to execute setval.
GRANT UPDATE ON ALL SEQUENCES IN SCHEMA ur_schema TO user;