Stabilize pg_stat_io writes test

Counting writes only for io_context = 'normal' is unreliable, as backends
using a buffer access strategy could flush all of the dirty buffers out from
under the other backends and checkpointer. Change the test to count writes in
any context. This achieves roughly the same coverage anyway.

Reported-by: Justin Pryzby <pryzby@telsasoft.com>
Author: Melanie Plageman <melanieplageman@gmail.com>
Discussion: https://www.postgresql.org/message-id/ZAnWU8WbXEDjrfUE%40telsasoft.com

src/test/regress/expected/stats.out

@@ -1137,6 +1137,9 @@ SELECT pg_stat_get_subscription_stats(NULL);
 -- extends.
 SELECT sum(extends) AS io_sum_shared_before_extends
   FROM pg_stat_io WHERE io_context = 'normal' AND io_object = 'relation' \gset
+SELECT sum(writes) AS writes, sum(fsyncs) AS fsyncs
+  FROM pg_stat_io
+  WHERE io_object = 'relation' \gset io_sum_shared_before_
 CREATE TABLE test_io_shared(a int);
 INSERT INTO test_io_shared SELECT i FROM generate_series(1,100)i;
 SELECT pg_stat_force_next_flush();
@@ -1155,15 +1158,12 @@ SELECT :io_sum_shared_after_extends > :io_sum_shared_before_extends;
 
 -- After a checkpoint, there should be some additional IOCONTEXT_NORMAL writes
 -- and fsyncs.
-SELECT sum(writes) AS writes, sum(fsyncs) AS fsyncs
-  FROM pg_stat_io
-  WHERE io_context = 'normal' AND io_object = 'relation' \gset io_sum_shared_before_
 -- See comment above for rationale for two explicit CHECKPOINTs.
 CHECKPOINT;
 CHECKPOINT;
 SELECT sum(writes) AS writes, sum(fsyncs) AS fsyncs
   FROM pg_stat_io
-  WHERE io_context = 'normal' AND io_object = 'relation' \gset io_sum_shared_after_
+  WHERE io_object = 'relation' \gset io_sum_shared_after_
 SELECT :io_sum_shared_after_writes > :io_sum_shared_before_writes;
  ?column? 
 ----------

src/test/regress/sql/stats.sql

@@ -549,6 +549,9 @@ SELECT pg_stat_get_subscription_stats(NULL);
 -- extends.
 SELECT sum(extends) AS io_sum_shared_before_extends
   FROM pg_stat_io WHERE io_context = 'normal' AND io_object = 'relation' \gset
+SELECT sum(writes) AS writes, sum(fsyncs) AS fsyncs
+  FROM pg_stat_io
+  WHERE io_object = 'relation' \gset io_sum_shared_before_
 CREATE TABLE test_io_shared(a int);
 INSERT INTO test_io_shared SELECT i FROM generate_series(1,100)i;
 SELECT pg_stat_force_next_flush();
@@ -558,16 +561,12 @@ SELECT :io_sum_shared_after_extends > :io_sum_shared_before_extends;
 
 -- After a checkpoint, there should be some additional IOCONTEXT_NORMAL writes
 -- and fsyncs.
-SELECT sum(writes) AS writes, sum(fsyncs) AS fsyncs
-  FROM pg_stat_io
-  WHERE io_context = 'normal' AND io_object = 'relation' \gset io_sum_shared_before_
 -- See comment above for rationale for two explicit CHECKPOINTs.
 CHECKPOINT;
 CHECKPOINT;
 SELECT sum(writes) AS writes, sum(fsyncs) AS fsyncs
   FROM pg_stat_io
-  WHERE io_context = 'normal' AND io_object = 'relation' \gset io_sum_shared_after_
+  WHERE io_object = 'relation' \gset io_sum_shared_after_
-
 SELECT :io_sum_shared_after_writes > :io_sum_shared_before_writes;
 SELECT current_setting('fsync') = 'off'
   OR :io_sum_shared_after_fsyncs > :io_sum_shared_before_fsyncs;