+my $ddd = compress('');
+my $msg = sub { { ts => 0, ds => 0, num => $_[0] } };
+foreach my $s ('', undef) {
+ $over->add_over($msg->(98), [ 'a' ], [], $s, $ddd);
+ $over->add_over($msg->(99), [ 'b' ], [], $s, $ddd);
+ my $msgs = [ map { $_->{num} } @{$over->get_thread('a')} ];
+ is_deeply([98], $msgs,
+ 'messages not linked by empty subject');
+}
+
+$over->add_over($msg->(98), [ 'a' ], [], 's', $ddd);
+$over->add_over($msg->(99), [ 'b' ], [], 's', $ddd);
+foreach my $mid (qw(a b)) {
+ my $msgs = [ map { $_->{num} } @{$over->get_thread('a')} ];
+ is_deeply([98, 99], $msgs, 'linked messages by subject');
+}
+$over->add_over($msg->(98), [ 'a' ], [], 's', $ddd);
+$over->add_over($msg->(99), [ 'b' ], ['a'], 'diff', $ddd);
+foreach my $mid (qw(a b)) {
+ my $msgs = [ map { $_->{num} } @{$over->get_thread($mid)} ];
+ is_deeply([98, 99], $msgs, "linked messages by Message-ID: <$mid>");
+}
+isnt($over->max, 0, 'max is non-zero');
+
+$over->rollback_lazy;
+
+# L<perldata/"Version Strings">
+my $v = eval 'v'.$over->{dbh}->{sqlite_version};
+SKIP: {
+ skip("no WAL in SQLite version $v < 3.7.0", 1) if $v lt v3.7.0;
+ $over->{dbh}->do('PRAGMA journal_mode = WAL');
+ $over = PublicInbox::OverIdx->new("$tmpdir/over.sqlite3");
+ is($over->dbh->selectrow_array('PRAGMA journal_mode'), 'wal',
+ 'WAL journal_mode not clobbered if manually set');
+}
+
+# ext index additions
+$over->eidx_prep;
+{
+ my @arg = qw(1349 2019 adeadba7cafe example.key);
+ ok($over->add_xref3(@arg), 'first add');
+ ok($over->add_xref3(@arg), 'add idempotent');
+ my $xref3 = $over->get_xref3(1349);
+ is_deeply($xref3, [ 'example.key:2019:adeadba7cafe' ], 'xref3 works');
+
+ @arg = qw(1349 2018 deadbeefcafe example.kee);
+ ok($over->add_xref3(@arg), 'add another xref3');
+ $xref3 = $over->get_xref3(1349);
+ is_deeply($xref3, [ 'example.key:2019:adeadba7cafe',
+ 'example.kee:2018:deadbeefcafe' ],
+ 'xref3 works forw two');
+
+ is($over->dbh->do(<<''), 1, 'remove first');
+DELETE FROM xref3 WHERE xnum = 2019 AND docid = 1349
+
+ $xref3 = $over->get_xref3(1349);
+ is_deeply($xref3, [ 'example.kee:2018:deadbeefcafe' ],
+ 'confirm removal successful');
+ $over->rollback_lazy;
+}
+