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 cfaa625356e66f8024a5a3b9804c63a2a604344f..cb43962d010a3e710ca0701a638b241af10fcea2 100644 (file)
--- a/src/backend/replication/logical/applyparallelworker.c
+++ b/src/backend/replication/logical/applyparallelworker.c
@@ -811,6 +811,15 @@ 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