Refactor some code related to backend statistics

This commit changes the way pending backend statistics are tracked by
moving them into a new structure called PgStat_BackendPending, removing
PgStat_BackendPendingIO.  PgStat_BackendPending currently only includes
PgStat_PendingIO for the pending I/O stats.

pgstat_flush_backend() is extended with a "flags" argument to control
which parts of the stats of a backend should be flushed.

With this refactoring, it becomes easier to plug into backend statistics
more data.  A patch to add information related to WAL in this stats kind
is under discussion.

Author: Bertrand Drouvot
Discussion: https://postgr.es/m/Z3zqc4o09dM/Ezyz@ip-10-97-1-34.eu-west-3.compute.internal

1 files changed, 13 insertions, 4 deletions

diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index 0d8427f27d1..f1dfe2b5e91 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -375,15 +375,24 @@ typedef struct PgStat_IO
 	PgStat_BktypeIO stats[BACKEND_NUM_TYPES];
 } PgStat_IO;
 
-/* Backend statistics store the same amount of IO data as PGSTAT_KIND_IO */
-typedef PgStat_PendingIO PgStat_BackendPendingIO;
-
 typedef struct PgStat_Backend
 {
 	TimestampTz stat_reset_timestamp;
-	PgStat_BktypeIO stats;
+	PgStat_BktypeIO io_stats;
 } PgStat_Backend;
 
+/* ---------
+ * PgStat_BackendPending	Non-flushed backend stats.
+ * ---------
+ */
+typedef struct PgStat_BackendPending
+{
+	/*
+	 * Backend statistics store the same amount of IO data as PGSTAT_KIND_IO.
+	 */
+	PgStat_PendingIO pending_io;
+} PgStat_BackendPending;
+
 typedef struct PgStat_StatDBEntry
 {
 	PgStat_Counter xact_commit;