Designate groups of consecutive equal values in an ordered dataset - postgresql

Question

I'm trying to get the output to look like below. The problem is that I can't do a first_value or RANK because when I partition by event and order by time, then it doesn't break them up in that order. I need them to order by time first and then partition each time.

Answer 1

One of the known solutions
Use lag() to mark rows when event changes and cumulative sum() to designate groups, e.g.:
with my_table(event, time) as (
values
('A', '12:01'),
('A', '12:02'),
('B', '12:03'),
('A', '12:04'),
('A', '12:05'),
('B', '12:06'),
('B', '12:07'),
('A', '12:08')
)
select
event,
time,
sum(change) over (order by time) as "desired row number"
from (
select
event,
time,
(event is distinct from lag(event) over (order by time))::int as change
from my_table
) s
event | time | desired row number
-------+-------+--------------------
A | 12:01 | 1
A | 12:02 | 1
B | 12:03 | 2
A | 12:04 | 3
A | 12:05 | 3
B | 12:06 | 4
B | 12:07 | 4
A | 12:08 | 5
(8 rows)
Custom aggregate
It would be nice to have the function:
select *, group_number(event) over (order by time)
from my_table;
This can be done with the custom aggregate:
create type group_number_internal as (number int, lag text);
create or replace function group_number_transition(group_number_internal, anyelement)
returns group_number_internal language sql strict as $$
select
case
when $2::text is distinct from $1.lag then $1.number+ 1
else $1.number
end,
$2::text
$$;
create or replace function group_number_final(group_number_internal)
returns int language sql as $$
select $1.number
$$;
create aggregate group_number(anyelement) (
sfunc = group_number_transition,
stype = group_number_internal,
finalfunc = group_number_final,
initcond = '(0, null)'
);
Test it in rextester.