sdf-jkl opened a new issue, #22033:
URL: https://github.com/apache/datafusion/issues/22033
### Is your feature request related to a problem or challenge?
Working on #18319 I noticed that `date_trunc` defines its own
`DateTruncGranularity` enum.
https://github.com/apache/datafusion/blob/2c7af17529533cd2458b6b227a9128294e1fc209/datafusion/functions/src/datetime/date_trunc.rs#L56-L67
`date_part` solves the same parsing problem differently — it delegates to
`arrow_cast::parse::IntervalUnit::from_str`:
https://github.com/apache/datafusion/blob/2c7af17529533cd2458b6b227a9128294e1fc209/datafusion/functions/src/datetime/date_part.rs#L230-L244
This leads to a user-visible inconsistency between the two functions. For
example, `date_part` accepts "mon" | "mons" | "month" | "months", while
`date_trunc` only accepts "month".
`IntervalUnit`, however, is the wrong enum for either of these functions —
it exists to parse Postgres INTERVAL literals (INTERVAL '1 year 2 months'),
which is why it has `Century` / `Decade` but no `Quarter` (Postgres rejects
INTERVAL '1 QUARTER'). The right counterpart is
`arrow_arith::temporal::DatePart`, which represents the date-part field
namespace, includes `Quarter`, and is #[non_exhaustive]:
https://github.com/apache/arrow-rs/blob/8091f3f17b2de355f7c47e7a0907000d308f8f3e/arrow-arith/src/temporal.rs#L38-L79
### Describe the solution you'd like
I propose:
- Add a `FromStr` impl for `arrow_arith::DatePart` (with the Postgres alias
map currently inside IntervalUnit::from_str plus the date-part-only field
names).
- Migrate `date_part`, `date_trunc`, and the Spark variants (`date_trunc`,
`trunc`, `time_trunc`, `date_part`) to the new parser. The unparser sites in
sql/src/unparser/utils.rs can also use it as an input canonicalizer.
### Describe alternatives you've considered
_No response_
### Additional context
_No response_
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