feat: Implement date_part scalar function

[devanbenz]

Implement date_part(part, expression)

Arguments

- part: Part of the date to return. The following date parts are supported:
   - year
    - quarter (emits value in inclusive range [1, 4] based on which quartile of the year the date is in)
    - month
    - week (week of the year)
    - day (day of the month)
    - hour
    - minute
    - second
    - millisecond
    - microsecond
    - nanosecond
    - dow (day of the week where Sunday is 0)
    - doy (day of the year)
    - epoch (seconds since Unix epoch)
    - isodow (day of the week where Monday is 0)
- expression: Time expression to operate on. Must be time VarRef

Example usage:

SELECT * FROM some_measurement
WHERE time >= now() - 10d AND time <= now() AND (date_part(time, dow) != 0 AND date_part(time, dow) != 6)

SELECT value, date_part('hour', time) FROM some_measurement

Please see #27001 (comment) for additional information on date_part limitations in 1.x.

[gwossum]

What is date_part_time? I don't see any tests for it.

[devanbenz]

It's used to create a reference to time since time is an auxiliary field https://github.com/influxdata/influxdb/pull/27005/files#diff-609a7e16be956ed6386e1a4a4efadf600b7d4de7dcfea27330dc692d1e901dc8R930-R944 I'm going to create some ValueMapper tests for this.

[devanbenz]

@gwossum I can add tests for this but it would likely require exporting

influxdb/query/select.go

Line 881 in 362217b

type valueMapper struct {

and testing it. We don't currently have any valueMapper specific tests. It's basically just a struct filled with maps so we would likely just be testing go's map functionality, which may not be worth the effort?

[davidby-influx]

Some changes from the first pass. Will review again after changes.

[davidby-influx]

In the spec, it says:

expression: Time expression to operate on. Can be a constant, column, or function.

Here it looks like you are prohibiting anything not the time column. Which is correct?

[devanbenz]

I need to modify our version of the spec. I think it should only operate on the time column. For v3 they can decide if they would like to add more features and allow it to work on constants.

[devanbenz]

I just changed the spec in the PR description to match: #27001 I recall we had a conversation yesterday during standup about the usefulness of constant timestamps. I verified that for other functions in influxdb 1.x (math functions predominately) we don't allow constants, so implementing a constant here would be non-trivial.

[davidby-influx]

Some refactoring, and a question about which IteratorOptions we should be looking at.

[davidby-influx]

LGTM.

Does date_part work in a GROUP BY clause? That's a common usage in SQL (e.g., are Mondays busier than Wednesdays?)

[devanbenz]

LGTM.

Does date_part work in a GROUP BY clause? That's a common usage in SQL (e.g., are Mondays busier than Wednesdays?)

It does not currently. Since we only support time() in the GROUP BY clause at the moment I would need to plumb through a lot of existing code to get date_part working. I just went through all our GROUP BY code, I would need to modify our internal iterators to have specialized date_part keys in the iteration and use that as a grouping.

I would expect to spend a few more days to get it working.

[sanderson]

I think it would be important to spend the time making date_part work in the GROUP BY clause or atleast being able to group by column aliases. For example:

SELECT  
  mean(temp)
WHERE
  time >= '2025-01-01T00:00:00Z'
  time < '2026-01-01T00:00:00Z'
GROUP BY
  date_part('month', time),
  date_part('year', time) AS year,
  location

OR

SELECT
  date_part('month', time) AS month,
  date_part('year', time) AS year,
  location,
  mean(temp)
WHERE
  time >= '2025-01-01T00:00:00Z'
  time < '2026-01-01T00:00:00Z'
GROUP BY
  month,
  year,
  location

Or something similar. I realize these both somewhat break InfluxQL conventions, but I think filtering and grouping by these values are the two most common use cases for this function. This will also make filtering/grouping by calendar units (week/month/year/etc.) possible in InfluxQL; things that have been requested for a long time.

[devanbenz]

@sanderson I agree that it would be useful in the GROUP BY clause.

[devanbenz]

I now have time to work on the GROUP BY portion of this PR. Setting to draft mode until I finish.