SQL select all rows per group after a condition is met - sql

Question

I would like to select all rows for each group after the last time a condition is met for that group. This related question has an answer using correlated subqueries.
In my case I will have millions of categories and hundreds of millions/billions of rows. Is there a way to achieve the same results using a more performant query?
Here is an example. The condition is all rows (per group) after the last 0 in the conditional column.
category | timestamp | condition
--------------------------------------
A | 1 | 0
A | 2 | 1
A | 3 | 0
A | 4 | 1
A | 5 | 1
B | 1 | 0
B | 2 | 1
B | 3 | 1
The result I would like to achieve is
category | timestamp | condition
--------------------------------------
A | 4 | 1
A | 5 | 1
B | 2 | 1
B | 3 | 1

Answer 1

If you want everything after the last 0, you can use window functions:
select t.*
from (select t.*,
max(case when condition = 0 then timestamp end) over (partition by category) as max_timestamp_0
from t
) t
where timestamp > max_timestamp_0 or
max_timestamp_0 is null;
With an index on (category, condition, timestamp), the correlated subquery version might also perform quite well:
select t.*
from t
where t.timestamp > all (select t2.timestamp
from t t2
where t2.category = t.category and
t2.condition = 0
);

Answer 2

You might want to try window functions:
select category, timestamp, condition
from (
select
t.*,
min(condition) over(partition by category order by timestamp desc) min_cond
from mytable t
) t
where min_cond = 1
The window min() with the order by clause computes the minimum value of condition over the current and following rows of the same category: we can use it as a filter to eliminate rows for which there is a more recent row with a 0.
Compared to the correlated subquery approach, the upside of using window functions is that it reduces the number of scans needed on the table. Of course this computing also has a cost, so you'll need to assess both solutions against your sample data.