pgstat: store statistics in shared memory.

Previously the statistics collector received statistics updates via UDP and
shared statistics data by writing them out to temporary files regularly. These
files can reach tens of megabytes and are written out up to twice a
second. This has repeatedly prevented us from adding additional useful
statistics.

Now statistics are stored in shared memory. Statistics for variable-numbered
objects are stored in a dshash hashtable (backed by dynamic shared
memory). Fixed-numbered stats are stored in plain shared memory.

The header for pgstat.c contains an overview of the architecture.

The stats collector is not needed anymore, remove it.

By utilizing the transactional statistics drop infrastructure introduced in a
prior commit statistics entries cannot "leak" anymore. Previously leaked
statistics were dropped by pgstat_vacuum_stat(), called from [auto-]vacuum. On
systems with many small relations pgstat_vacuum_stat() could be quite
expensive.

Now that replicas drop statistics entries for dropped objects, it is not
necessary anymore to reset stats when starting from a cleanly shut down
replica.

Subsequent commits will perform some further code cleanup, adapt docs and add
tests.

Bumps PGSTAT_FILE_FORMAT_ID.

Author: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Author: Andres Freund <andres@anarazel.de>
Author: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-By: Andres Freund <andres@anarazel.de>
Reviewed-By: Thomas Munro <thomas.munro@gmail.com>
Reviewed-By: Justin Pryzby <pryzby@telsasoft.com>
Reviewed-By: "David G. Johnston" <david.g.johnston@gmail.com>
Reviewed-By: Tomas Vondra <tomas.vondra@2ndquadrant.com> (in a much earlier version)
Reviewed-By: Arthur Zakirov <a.zakirov@postgrespro.ru> (in a much earlier version)
Reviewed-By: Antonin Houska <ah@cybertec.at> (in a much earlier version)
Discussion: https://postgr.es/m/20220303021600.hs34ghqcw6zcokdh@alap3.anarazel.de
Discussion: https://postgr.es/m/20220308205351.2xcn6k4x5yivcxyd@alap3.anarazel.de
Discussion: https://postgr.es/m/20210319235115.y3wz7hpnnrshdyv6@alap3.anarazel.de

3 files changed, 19 insertions, 1 deletions

diff --git a/src/test/modules/worker_spi/worker_spi.c b/src/test/modules/worker_spi/worker_spi.c
index 48829df29c3..5b541ec47f1 100644
--- a/src/test/modules/worker_spi/worker_spi.c
+++ b/src/test/modules/worker_spi/worker_spi.c
@@ -265,7 +265,7 @@ worker_spi_main(Datum main_arg)
 		PopActiveSnapshot();
 		CommitTransactionCommand();
 		debug_query_string = NULL;
-		pgstat_report_stat(false);
+		pgstat_report_stat(true);
 		pgstat_report_activity(STATE_IDLE, NULL);
 	}
 
diff --git a/src/test/regress/expected/stats.out b/src/test/regress/expected/stats.out
index 494fb262375..64e2ff6b296 100644
--- a/src/test/regress/expected/stats.out
+++ b/src/test/regress/expected/stats.out
@@ -17,6 +17,8 @@ SET enable_indexscan TO on;
 -- for the moment, we don't want index-only scans here
 SET enable_indexonlyscan TO off;
 -- save counters
+BEGIN;
+SET LOCAL stats_fetch_consistency = snapshot;
 CREATE TABLE prevstats AS
 SELECT t.seq_scan, t.seq_tup_read, t.idx_scan, t.idx_tup_fetch,
        (b.heap_blks_read + b.heap_blks_hit) AS heap_blks,
@@ -25,6 +27,7 @@ SELECT t.seq_scan, t.seq_tup_read, t.idx_scan, t.idx_tup_fetch,
   FROM pg_catalog.pg_stat_user_tables AS t,
        pg_catalog.pg_statio_user_tables AS b
  WHERE t.relname='tenk2' AND b.relname='tenk2';
+COMMIT;
 -- function to wait for counters to advance
 create function wait_for_stats() returns void as $$
 declare
@@ -34,6 +37,8 @@ declare
   updated3 bool;
   updated4 bool;
 begin
+  SET LOCAL stats_fetch_consistency = snapshot;
+
   -- We don't want to wait forever.  No timeout suffices if the OS drops our
   -- stats traffic because an earlier test file left a full UDP buffer.
   -- Hence, don't use PG_TEST_TIMEOUT_DEFAULT, which may be large for
@@ -163,6 +168,8 @@ SELECT wait_for_stats();
 (1 row)
 
 -- check effects
+BEGIN;
+SET LOCAL stats_fetch_consistency = snapshot;
 SELECT relname, n_tup_ins, n_tup_upd, n_tup_del, n_live_tup, n_dead_tup
   FROM pg_stat_user_tables
  WHERE relname like 'trunc_stats_test%' order by relname;
@@ -202,6 +209,7 @@ FROM prevstats AS pr;
  t
 (1 row)
 
+COMMIT;
 DROP TABLE trunc_stats_test, trunc_stats_test1, trunc_stats_test2, trunc_stats_test3, trunc_stats_test4;
 DROP TABLE prevstats;
 -- test BRIN index doesn't block HOT update - we include this test here, as it
diff --git a/src/test/regress/sql/stats.sql b/src/test/regress/sql/stats.sql
index d0ba1f6d7ba..85a253bcd43 100644
--- a/src/test/regress/sql/stats.sql
+++ b/src/test/regress/sql/stats.sql
@@ -15,6 +15,8 @@ SET enable_indexscan TO on;
 SET enable_indexonlyscan TO off;
 
 -- save counters
+BEGIN;
+SET LOCAL stats_fetch_consistency = snapshot;
 CREATE TABLE prevstats AS
 SELECT t.seq_scan, t.seq_tup_read, t.idx_scan, t.idx_tup_fetch,
        (b.heap_blks_read + b.heap_blks_hit) AS heap_blks,
@@ -23,6 +25,7 @@ SELECT t.seq_scan, t.seq_tup_read, t.idx_scan, t.idx_tup_fetch,
   FROM pg_catalog.pg_stat_user_tables AS t,
        pg_catalog.pg_statio_user_tables AS b
  WHERE t.relname='tenk2' AND b.relname='tenk2';
+COMMIT;
 
 -- function to wait for counters to advance
 create function wait_for_stats() returns void as $$
@@ -33,6 +36,8 @@ declare
   updated3 bool;
   updated4 bool;
 begin
+  SET LOCAL stats_fetch_consistency = snapshot;
+
   -- We don't want to wait forever.  No timeout suffices if the OS drops our
   -- stats traffic because an earlier test file left a full UDP buffer.
   -- Hence, don't use PG_TEST_TIMEOUT_DEFAULT, which may be large for
@@ -158,6 +163,9 @@ RESET enable_bitmapscan;
 SELECT wait_for_stats();
 
 -- check effects
+BEGIN;
+SET LOCAL stats_fetch_consistency = snapshot;
+
 SELECT relname, n_tup_ins, n_tup_upd, n_tup_del, n_live_tup, n_dead_tup
   FROM pg_stat_user_tables
  WHERE relname like 'trunc_stats_test%' order by relname;
@@ -177,6 +185,8 @@ SELECT st.heap_blks_read + st.heap_blks_hit >= pr.heap_blks + cl.relpages,
 SELECT pr.snap_ts < pg_stat_get_snapshot_timestamp() as snapshot_newer
 FROM prevstats AS pr;
 
+COMMIT;
+
 DROP TABLE trunc_stats_test, trunc_stats_test1, trunc_stats_test2, trunc_stats_test3, trunc_stats_test4;
 DROP TABLE prevstats;