fix(promql): default a stepless subquery to a one-minute step (#9481)

* fix(promql): default a stepless subquery to a one-minute step

`x[5m:]` evaluated the inner expression at the query step, which is 1s
for instant queries. Prometheus uses the evaluation interval, one minute
by default.

Signed-off-by: Dennis Zhuang <killme2008@gmail.com>

* test(promql): cover the default subquery step

Signed-off-by: Dennis Zhuang <killme2008@gmail.com>

* test(promql): cover the default subquery step in a range query

Signed-off-by: Dennis Zhuang <killme2008@gmail.com>

---------

Signed-off-by: Dennis Zhuang <killme2008@gmail.com>
(cherry picked from commit 845fc0af9c)
Signed-off-by: WenyXu <wenymedia@gmail.com>

src/query/src/promql/planner.rs

@@ -587,9 +587,10 @@ impl PromPlanner {
         };
 
         let current_interval = self.ctx.interval;
-        if let Some(step) = step {
-            self.ctx.interval = step.as_millis() as _;
-        }
+        self.ctx.interval = match step {
+            Some(step) => step.as_millis() as _,
+            None => DEFAULT_SUBQUERY_STEP_MS,
+        };
         ensure!(self.ctx.interval > 0, ZeroRangeSelectorSnafu);
         let current_start = self.ctx.start;
         let current_end = self.ctx.end;

src/query/src/promql/planner/test.rs

@@ -5682,14 +5682,6 @@ async fn subquery_rejects_invalid_range_and_zero_step() {
         matches!(&err, crate::promql::error::Error::ZeroRangeSelector { .. }),
         "{err}"
     );
-    // An omitted child step inherits the zero parent step.
-    let err = plan_subquery_offset_probe("sum_over_time(subquery_offset_offgrid[5s:])", 60, 60, 0)
-        .await
-        .unwrap_err();
-    assert!(
-        matches!(&err, crate::promql::error::Error::ZeroRangeSelector { .. }),
-        "{err}"
-    );
 }
 
 /// Rejects nonempty child windows whose derived millisecond bounds are unrepresentable.