with task_count_by_bucket as (
  -- get the count of tasks by delay bucket. Tasks are grouped by their
  -- characteristics (type, status, policy, priority, current interval),
  -- then by delay buckets that are 1 hour wide between -24 and +24 hours,
  -- and 1 day wide outside of this range.
  -- A positive delay means the task execution is late wrt scheduling.
  select
    "type",
    status,
    "policy",
    priority,
    current_interval,
    (
      -- select the bucket widths
      case when delay between - 24 * 3600 and 24 * 3600 then
        (ceil(delay / 3600)::bigint) * 3600
      else
        (ceil(delay / (24 * 3600))::bigint) * 24 * 3600
      end
    ) as delay_bucket,
    count(*)
  from
    task
    join lateral (
      -- this is where the "positive = late" convention is set
      select
        extract(epoch from (now() - next_run)) as delay
    ) as d on true
    group by
      "type",
      status,
      "policy",
      priority,
      current_interval,
      delay_bucket
    order by
      "type",
      status,
      "policy",
      priority,
      current_interval,
      delay_bucket
),
delay_bounds as (
  -- get the minimum and maximum delay bucket for each task group. This will
  -- let us generate all the buckets, even the empty ones in the next CTE.
  select
    "type",
    status,
    "policy",
    priority,
    current_interval,
    min(delay_bucket) as min,
    max(delay_bucket) as max
  from
    task_count_by_bucket
  group by
    "type",
    status,
    "policy",
    priority,
    current_interval
),
task_buckets as (
  -- Generate all time buckets for all categories.
  select
    "type",
    status,
    "policy",
    priority,
    current_interval,
    delay_bucket
  from
    delay_bounds
    join lateral (
      -- 1 hour buckets
      select
        generate_series(- 23, 23) * 3600 as delay_bucket
      union
      -- 1 day buckets. The "- 1" is used to make sure we generate an empty
      -- bucket as lowest delay bucket, so prometheus quantile calculations
      -- stay accurate
      select
        generate_series(min / (24 * 3600) - 1, max / (24 * 3600)) * 24 * 3600 as delay_bucket
    ) as buckets on true
),
task_count_for_all_buckets as (
  -- This join merges the non-empty buckets (task_count_by_bucket) with
  -- the full list of buckets (task_buckets).
  -- The join clause can't use the "using (x, y, z)" syntax, as it uses
  -- equality and priority and current_interval can be null. This also
  -- forces us to label all the fields in the select. Ugh.
  select
    task_buckets."type",
    task_buckets.status,
    task_buckets."policy",
    task_buckets.priority,
    task_buckets.current_interval,
    task_buckets.delay_bucket,
    coalesce(count, 0) as count -- make sure empty buckets have a 0 count instead of null
  from
    task_buckets
  left join task_count_by_bucket
    on task_count_by_bucket."type" = task_buckets."type"
    and task_count_by_bucket.status = task_buckets.status
    and task_count_by_bucket. "policy" = task_buckets."policy"
    and task_count_by_bucket.priority is not distinct from task_buckets.priority
    and task_count_by_bucket.current_interval is not distinct from task_buckets.current_interval
    and task_count_by_bucket.delay_bucket = task_buckets.delay_bucket
),
cumulative_buckets as (
  -- Prometheus wants cumulative histograms: for each bucket, the value
  -- needs to be the total of all measurements below the given value (this
  -- allows downsampling by just throwing away some buckets). We use the
  -- "sum over partition" window function to compute this.
  -- Prometheus also expects a "+Inf" bucket for the total count. We
  -- generate it with a null lt value so we can sort it after the rest of
  -- the buckets.