Date: Wed, 13 Oct 2021 17:42:37 -0300
From: Alvaro Herrera <alvherre@alvh.no-ip.org>
To: Andres Freund <andres@anarazel.de>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Andrew Dunstan <andrew@dunslane.net>,
	"Bossart, Nathan" <bossartn@amazon.com>,
	Kyotaro Horiguchi <horikyota.ntt@gmail.com>,
	"masao.fujii@oss.nttdata.com" <masao.fujii@oss.nttdata.com>,
	"pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>,
	"mengjuan.cmj@alibaba-inc.com" <mengjuan.cmj@alibaba-inc.com>,
	"Jakub.Wartak@tomtom.com" <Jakub.Wartak@tomtom.com>,
	Ryo Matsumura <matsumura.ryo@fujitsu.com>
Subject: Re: prevent immature WAL streaming
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This works nicely with the TAP test:


-- 
Álvaro Herrera         PostgreSQL Developer  —  https://www.EnterpriseDB.com/
"Postgres is bloatware by design: it was built to house
 PhD theses." (Joey Hellerstein, SIGMOD annual conference 2002)

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	filename="0001-do-away-with-small-tuple-writing.patch"

From e9bcdb3b44f36ffa1a2377f252d73ff3d7210fd9 Mon Sep 17 00:00:00 2001
From: Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: Wed, 13 Oct 2021 17:41:31 -0300
Subject: [PATCH] do away with small tuple writing

---
 .../recovery/t/026_overwrite_contrecord.pl    | 39 +++++++++++--------
 1 file changed, 22 insertions(+), 17 deletions(-)

diff --git a/src/test/recovery/t/026_overwrite_contrecord.pl b/src/test/recovery/t/026_overwrite_contrecord.pl
index 1725a97531..b7d3b6e03d 100644
--- a/src/test/recovery/t/026_overwrite_contrecord.pl
+++ b/src/test/recovery/t/026_overwrite_contrecord.pl
@@ -22,27 +22,32 @@ $node->init(allows_streaming => 1);
 $node->append_conf('postgresql.conf', 'wal_keep_size=1GB');
 $node->start;
 
-$node->safe_psql('postgres', 'create table filler (a int)');
-# First, measure how many bytes does the insertion of 1000 rows produce
-my $start_lsn =
-  $node->safe_psql('postgres', q{select pg_current_wal_insert_lsn() - '0/0'});
-$node->safe_psql('postgres',
-	'insert into filler select * from generate_series(1, 1000)');
-my $end_lsn =
-  $node->safe_psql('postgres', q{select pg_current_wal_insert_lsn() - '0/0'});
-my $rows_walsize = $end_lsn - $start_lsn;
+$node->safe_psql('postgres', 'create table filler (a int, b text)');
 
 # Now consume all remaining room in the current WAL segment, leaving
 # space enough only for the start of a largish record.
 $node->safe_psql(
-	'postgres', qq{
-WITH setting AS (
-  SELECT setting::int AS wal_segsize
-    FROM pg_settings WHERE name = 'wal_segment_size'
-)
-INSERT INTO filler
-SELECT g FROM setting,
-  generate_series(1, 1000 * (wal_segsize - ((pg_current_wal_insert_lsn() - '0/0') % wal_segsize)) / $rows_walsize) g
+	'postgres', q{
+DO $$
+DECLARE
+    wal_segsize int := setting::int FROM pg_settings WHERE name = 'wal_segment_size';
+    remain int;
+    iters  int := 0;
+BEGIN
+    LOOP
+        INSERT into filler
+        select g, repeat(md5(g::text), (random() * 60 + 1)::int)
+        from generate_series(1, 10) g;
+
+        remain := wal_segsize - (pg_current_wal_insert_lsn() - '0/0') % wal_segsize;
+        IF remain < 2 * setting::int from pg_settings where name = 'block_size' THEN
+            RAISE log 'exiting after % iterations, % bytes to end of WAL segment', iters, remain;
+            EXIT;
+        END IF;
+        iters := iters + 1;
+    END LOOP;
+END
+$$;
 });
 
 my $initfile = $node->safe_psql('postgres',
-- 
2.30.2


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