Implement rate-limiting logic on how often backends will attempt to send

messages to the stats collector.  This avoids the problem that enabling
stats_row_level for autovacuum has a significant overhead for short
read-only transactions, as noted by Arjen van der Meijden.  We can avoid
an extra gettimeofday call by piggybacking on the one done for WAL-logging
xact commit or abort (although that doesn't help read-only transactions,
since they don't WAL-log anything).

In my proposal for this, I noted that we could change the WAL log entries
for commit/abort to record full TimestampTz precision, instead of only
time_t as at present.  That's not done in this patch, but will be committed
separately.

2 files changed, 12 insertions, 0 deletions

diff --git a/src/test/regress/expected/stats.out b/src/test/regress/expected/stats.out
index fc76d14225..af0f630a8d 100644
--- a/src/test/regress/expected/stats.out
+++ b/src/test/regress/expected/stats.out
@@ -73,6 +73,14 @@ SELECT count(*) FROM tenk2 WHERE unique1 = 1;
      1
 (1 row)
 
+-- force the rate-limiting logic in pgstat_report_tabstat() to time out
+-- and send a message
+SELECT pg_sleep(1.0);
+ pg_sleep 
+----------
+ 
+(1 row)
+
 -- wait for stats collector to update
 SELECT wait_for_stats();
  wait_for_stats 
diff --git a/src/test/regress/sql/stats.sql b/src/test/regress/sql/stats.sql
index cde38b3a37..9900b9114a 100644
--- a/src/test/regress/sql/stats.sql
+++ b/src/test/regress/sql/stats.sql
@@ -60,6 +60,10 @@ SELECT count(*) FROM tenk2;
 -- do an indexscan
 SELECT count(*) FROM tenk2 WHERE unique1 = 1;
 
+-- force the rate-limiting logic in pgstat_report_tabstat() to time out
+-- and send a message
+SELECT pg_sleep(1.0);
+
 -- wait for stats collector to update
 SELECT wait_for_stats();