Flush statistics during idle periods in parallel apply worker.

Parallel apply workers previously failed to report statistics while
waiting for new work in the main loop. This resulted in the stats from the
most recent transaction remaining unbuffered, leading to arbitrary
reporting delays—particularly when streamed transactions were infrequent.

This commit ensures that statistics are explicitly flushed when the worker
is idle, providing timely visibility into accumulated worker activity.

Author: Zhijie Hou <houzj.fnst@fujitsu.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Backpatch-through: 16, where it was introduced
Discussion: https://postgr.es/m/TYRPR01MB1419579F217CC4332B615589594202@TYRPR01MB14195.jpnprd01.prod.outlook.com

diff --git a/src/backend/replication/logical/applyparallelworker.c b/src/backend/replication/logical/applyparallelworker.c
index 4a2c67f9e40938aa9290b0e6cef50364fb8f6562..2495ad5609cf377e8b833140a33fac122a7bb8ec 100644 (file)
--- a/src/backend/replication/logical/applyparallelworker.c
+++ b/src/backend/replication/logical/applyparallelworker.c
@@ -814,6 +814,15 @@ LogicalParallelApplyLoop(shm_mq_handle *mqh)
 
                                if (rc & WL_LATCH_SET)
                                        ResetLatch(MyLatch);
+
+                               /*
+                                * Force stats reporting to avoid long delays. There can be long
+                                * idle gaps before the leader assigns the next transaction, and
+                                * the only opportunity to report stats during such gaps is
+                                * here.
+                                */
+                               if ((rc & WL_TIMEOUT) && !IsTransactionState())
+                                       pgstat_report_stat(true);
                        }
                }
                else