AmoghAtreya opened a new pull request, #42329:
URL: https://github.com/apache/superset/pull/42329
### SUMMARY
Fixes #42254
When applying a time grain to a pure `DATE` column in PostgreSQL,
`DATE_TRUNC` implicitly converts the column value to a `TIMESTAMP WITH TIME
ZONE` evaluated at `00:00:00`. Downstream processing and client timezone
conversions then shift the value, causing plain calendar dates (e.g.,
`2026-03-30`) to display as the previous day (e.g., `2026-03-29`).
This PR overrides `get_timestamp_expr` in `PostgresBaseEngineSpec` to wrap
time grain truncation in `CAST(... AS DATE)` whenever the source column is a
pure `DATE` type. This preserves calendar date semantics across PostgreSQL and
dialect-derived engines (Redshift, Greenplum, etc.) without altering genuine
`TIMESTAMP` or `DATETIME` time-series behavior.
### BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF
* **Before SQL:** `DATE_TRUNC('day', event_date)` → returns `TIMESTAMP` →
triggers timezone offset shift.
* **After SQL:** `CAST(DATE_TRUNC('day', event_date) AS DATE)` → stays
`DATE` → no timezone shift.
### TESTING INSTRUCTIONS
1. Add a PostgreSQL table containing a column with type `DATE`.
2. Connect the dataset to Superset in a non-UTC timezone.
3. Select the `DATE` column with a `Day` time grain in Explore View.
4. Verify the generated SQL wraps the expression in `CAST(... AS DATE)` and
that the calendar date remains unshifted.
5. Added unit tests covering `DATE` vs `DATETIME` columns in
`tests/unit_tests/db_engine_specs/test_postgres.py`.
### ADDITIONAL INFORMATION
- [x] Has associated issue: #42254
- [ ] Required feature flags:
- [ ] Changes UI
- [ ] Includes DB Migration (follow approval process in
[SIP-59](https://github.com/apache/superset/issues/13351))
- [ ] Migration is atomic, supports rollback & is backwards-compatible
- [ ] Confirm DB migration upgrade and downgrade tested
- [ ] Runtime estimates and downtime expectations provided
- [ ] Introduces new feature or API
- [ ] Removes existing feature or API
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