Fix issues in pg_stat_wal.

1) Previously there were both pgstat_send_wal() and pgstat_report_wal()
   in order to send WAL activity to the stats collector. With the former being
   used by wal writer, the latter by most other processes. They were a bit
   redundant and so this commit merges them into pgstat_send_wal() to
   simplify the code.

2) Previously WAL global statistics counters were calculated and then
   compared with zero-filled buffer in order to determine whether any WAL
   activity has happened since the last submission. These calculation and
   comparison were not cheap. This was regularly exercised even in read-only
   workloads. This commit fixes the issue by making some WAL activity
   counters directly be checked to determine if there's WAL activity stats
   to send.

3) Previously pgstat_report_stat() did not check if there's WAL activity
   stats to send as part of the "Don't expend a clock check if nothing to do"
   check at the top. It's probably rare to have pending WAL stats without
   also passing one of the other conditions, but for safely this commit
   changes pgstat_report_stats() so that it checks also some WAL activity
   counters at the top.

This commit also adds the comments about the design of WAL stats.

Reported-by: Andres Freund
Author: Masahiro Ikeda
Reviewed-by: Kyotaro Horiguchi, Atsushi Torikoshi, Andres Freund, Fujii Masao
Discussion: https://postgr.es/m/[email protected]

1 files changed, 12 insertions, 0 deletions

diff --git a/src/include/executor/instrument.h b/src/include/executor/instrument.h
index fc87eed4fb2..2f9905b7c8e 100644
--- a/src/include/executor/instrument.h
+++ b/src/include/executor/instrument.h
@@ -16,6 +16,11 @@
 #include "portability/instr_time.h"
 
 
+/*
+ * BufferUsage and WalUsage counters keep being incremented infinitely,
+ * i.e., must never be reset to zero, so that we can calculate how much
+ * the counters are incremented in an arbitrary period.
+ */
 typedef struct BufferUsage
 {
 	int64		shared_blks_hit;	/* # of shared buffer hits */
@@ -32,6 +37,13 @@ typedef struct BufferUsage
 	instr_time	blk_write_time; /* time spent writing */
 } BufferUsage;
 
+/*
+ * WalUsage tracks only WAL activity like WAL records generation that
+ * can be measured per query and is displayed by EXPLAIN command,
+ * pg_stat_statements extension, etc. It does not track other WAL activity
+ * like WAL writes that it's not worth measuring per query. That's tracked
+ * by WAL global statistics counters in WalStats, instead.
+ */
 typedef struct WalUsage
 {
 	int64		wal_records;	/* # of WAL records produced */