$__timeGroup with fill parameter panics with "integer divide by zero" for interval below 1s

[mrittler]

What happened:
When using $__timeGroup with a fill parameter (null, previous or 0) and an interval below 1s the query execution panics with the following error:
lvl=eror msg="executeQuery panic" logger=tsdb.postgres error="runtime error: integer divide by zero" stack="[sql_engine.go:215 panic.go:1052 panic.go:200 resample.go:93 sql_engine.go:357]"

The division by zero is happening in the following line:

grafana/pkg/tsdb/sqleng/resample.go

Line 93 in 43c81dd

startUnixTime := qm.TimeRange.From.Unix() / int64(qm.Interval.Seconds()) * int64(qm.Interval.Seconds())

What you expected to happen:
I would expect that the macro also works for intervals below 1s because the $__interval variable delivers values in the millisecond range if zoomed in far enough (see screen recording below) and the minimum time interval that can be specified via data source settings can also be in millisecond range (see: https://grafana.com/docs/grafana/latest/datasources/postgres/#min-time-interval).

This happens when using query builder:

...and also when using $__timeGroup/$__timeGroupAlias in the text editor directly:

How to reproduce it (as minimally and precisely as possible):

• use a SQL data source (I use TimescaleDB via PostgreSQL data source, but I guess all SQL data sources are affected by that issue)
• go to data source settings and change the minimum time interval to 1ms (default is 10s)
  
• group by time, change fill parameter to NULL, previous or 0 and zoom in far enough or set interval parameter to a value below 1s (see screen recording)
  

Environment:

• Grafana version: v8.3.2 (afb9e8e)
• Data source type & version: timescaledb:2.5.1-pg14 (via PostgreSQL plugin)
• OS Grafana is installed on: Docker grafana/grafana-oss:8.3.2
• User OS & Browser: Mozilla/5.0 (X11; Linux x86_64; rv:95.0) Gecko/20100101 Firefox/95.0
• Grafana plugins: PostgreSQL

[codeincarnate]

We're in the midst of converting SQL datasources away from Angular and this seems like a good follow up to that effort.

[gabor]

i was able to reproduce it on grafana 10.1.5, with the following sql:
(let's assume the table is named tbl, the time column is named t, and the value-column is named v)

make sure you use format=time series

run this query:

SELECT
  $__timeGroup(t,$__interval,NULL),
  avg(v)
FROM tbl
WHERE
  $__timeFilter(t)
GROUP BY 1
ORDER BY 1

set query options / min interval to 1ms

and keep zooming in until the query options / interval value shows less than 1s (it will probably show 500ms.

[gabor]

originally i had a comment here that has proven incorrect later.
the misunderstanding was the result-type of extract(epoch from column).. i assumed it's an integer, but in fact it's a numeric, so it can have fractional parts, in other words, extract(epoch from t) - floor(extract(epoch from t)) is not always zero.