Fix sub-second intervals in hierarchical caggs

Previously we used date_part("epoch", interval) and integer division
internally to determine whether the top cagg's interval is a
multiple of its parent's.
This led to precision loss and wrong results
in the case of intervals with sub-second components.

Fixed by using the `ts_interval_value_to_internal` function to convert
intervals to appropriate integer representation for division.

Fixes #5277

CHANGELOG.md

@@ -29,6 +29,7 @@ We recommend that you upgrade at the next available opportunity.
 * #5367 Fix column name handling in old-style continuous aggregates
 * #5378 Fix multinode DML HA performance regression
 * #5384 Fix Hierarchical Continuous Aggregates chunk_interval_size
+* #5304 Fix sub-second intervals in hierarchical caggs
 
 **Thanks**
 * @justinozavala for reporting an issue with PL/Python procedures in the background worker

tsl/src/continuous_aggs/create.c

@@ -1151,11 +1151,10 @@ get_bucket_width(CAggTimebucketInfo bucket_info)
 				bucket_info.interval->day = bucket_info.interval->month * DAYS_PER_MONTH;
 				bucket_info.interval->month = 0;
 			}
-			Datum epoch = DirectFunctionCall2(interval_part,
-											  PointerGetDatum(cstring_to_text("epoch")),
-											  IntervalPGetDatum(bucket_info.interval));
-			/* Cast float8 to int8. */
-			width = DatumGetInt64(DirectFunctionCall1(dtoi8, epoch));
+
+			/* Convert Interval to int64 */
+			width =
+				ts_interval_value_to_internal(IntervalPGetDatum(bucket_info.interval), INTERVALOID);
 			break;
 		}
 		default: