Fix refresh on empty CAgg with variable bucket

So far, we have not handled CAggs with variable buckets correctly. The
CAgg refresh on a hypertable without any data lead to the error message
"timestamp out of range". This patch fixes the problem by declaring
empty CAggs as up-to-date.

.unreleased/bugfix_6660

@@ -0,0 +1 @@
+Fixes: #6660 Fix refresh on empty CAgg with variable bucket

tsl/src/continuous_aggs/refresh.c

@@ -824,9 +824,18 @@ continuous_agg_refresh_internal(const ContinuousAgg *cagg,
 	if (refresh_window.end > invalidation_threshold)
 		refresh_window.end = invalidation_threshold;
 
-	/* Capping the end might have made the window 0, or negative, so
-	 * nothing to refresh in that case */
-	if (refresh_window.start >= refresh_window.end)
+	/* Capping the end might have made the window 0, or negative, so nothing to refresh in that
+	 * case.
+	 *
+	 * For variable with buckets we use a refresh_window.start value that is lower than the
+	 * -infinity value (ts_time_get_nobegin < ts_time_get_min). Therefore, the first check in the
+	 * following if statement is not enough. If the invalidation_threshold returns the min_value for
+	 * the data type, we end up with [nobegin, min_value] which is an invalid time interval.
+	 * Therefore, we have also to check if the invalidation_threshold is defined. If not, no refresh
+	 * is needed.  */
+	if ((refresh_window.start >= refresh_window.end) ||
+		(IS_TIMESTAMP_TYPE(refresh_window.type) &&
+		 invalidation_threshold == ts_time_get_min(refresh_window.type)))
 	{
 		emit_up_to_date_notice(cagg, callctx);
 

tsl/test/expected/cagg_invalidation.out

@@ -1226,3 +1226,26 @@ CALL refresh_continuous_aggregate('cond_10', 0, 200);
 WARNING:  invalid value for session variable "timescaledb.materializations_per_refresh_window"
 DETAIL:  Expected an integer but current value is "-".
 \set VERBOSITY terse
+-- Test refresh with undefined invalidation threshold and variable sized buckets
+CREATE TABLE timestamp_ht (
+  time timestamptz NOT NULL,
+  value float
+);
+SELECT create_hypertable('timestamp_ht', 'time');
+     create_hypertable     
+---------------------------
+ (9,public,timestamp_ht,t)
+(1 row)
+
+CREATE MATERIALIZED VIEW temperature_4h
+  WITH  (timescaledb.continuous) AS
+  SELECT time_bucket('4 hour', time), avg(value)
+    FROM timestamp_ht
+    GROUP BY 1 ORDER BY 1;
+NOTICE:  continuous aggregate "temperature_4h" is already up-to-date
+CREATE MATERIALIZED VIEW temperature_4h_2
+  WITH  (timescaledb.continuous) AS
+  SELECT time_bucket('4 hour', time, 'Europe/Berlin'), avg(value)
+    FROM timestamp_ht
+    GROUP BY 1 ORDER BY 1;
+NOTICE:  continuous aggregate "temperature_4h_2" is already up-to-date

tsl/test/sql/cagg_invalidation.sql

@@ -719,3 +719,23 @@ SET timescaledb.materializations_per_refresh_window='-';
 INSERT INTO conditions VALUES (140, 1, 1.0);
 CALL refresh_continuous_aggregate('cond_10', 0, 200);
 \set VERBOSITY terse
+
+-- Test refresh with undefined invalidation threshold and variable sized buckets
+CREATE TABLE timestamp_ht (
+  time timestamptz NOT NULL,
+  value float
+);
+
+SELECT create_hypertable('timestamp_ht', 'time');
+
+CREATE MATERIALIZED VIEW temperature_4h
+  WITH  (timescaledb.continuous) AS
+  SELECT time_bucket('4 hour', time), avg(value)
+    FROM timestamp_ht
+    GROUP BY 1 ORDER BY 1;
+
+CREATE MATERIALIZED VIEW temperature_4h_2
+  WITH  (timescaledb.continuous) AS
+  SELECT time_bucket('4 hour', time, 'Europe/Berlin'), avg(value)
+    FROM timestamp_ht
+    GROUP BY 1 ORDER BY 1;