description: Script for running on Windows (replace port and path + database file with your env.):

set names utf8;
set list on;
shell del C:\MIX\firebird\QA\fbt-repo\tmp\c4881.fdb 2>nul;
create database 'localhost/3333:C:\MIX\firebird\QA\fbt-repo\tmp\c4881.fdb' default character set utf8;
commit;

set list on;
out nul;
select rpad('', 10921,'∏Ω∑') as multi_byte_10922 from rdb$database;
select rpad('', 10922,'∏Ω∑') as multi_byte_10922 from rdb$database;
out;
quit;

Similar script for running on Linux:

set names utf8;
set list on;
shell rm -f /var/db/fb30/c4881.fdb 2>nul;
create database 'localhost/3333:/var/db/fb30/c4881.fdb' default character set utf8;
commit;

set list on;
out /dev/null;
select rpad('', 10921,'∑∏Ω') as multi_byte_10921 from rdb$database;
select rpad('', 10922,'∑∏Ω') as multi_byte_10922 from rdb$database;
out;
quit;

Result in any case will be:

Statement failed, SQLSTATE = 22001
arithmetic exception, numeric overflow, or string truncation
-string right truncation
-expected length 8191, actual 10921

Statement failed, SQLSTATE = 22000
Malformed string

IMHO, second expression (which tries to do RPAD up to 10922 should raise the same exception as first one, no matter of its argument.

PS.

All three characters in the padded string '∑', '∏' and 'Ω' -- requires three byte for encoding. Symbol '∏' should not be confused with cyrillic upper 'Pe', it is mathematical sign of number producing.

=>

Script for running on Windows (replace port and path + database file with your env.):

set names utf8;
set list on;
shell del C:\MIX\firebird\QA\fbt-repo\tmp\c4881.fdb 2>nul;
create database 'localhost/3333:C:\MIX\firebird\QA\fbt-repo\tmp\c4881.fdb' default character set utf8;
commit;

set list on;
out nul;
select rpad('', 10921,'∏Ω∑') as multi_byte_10921 from rdb$database;
select rpad('', 10922,'∏Ω∑') as multi_byte_10922 from rdb$database;
out;
quit;

Similar script for running on Linux:

set names utf8;
set list on;
shell rm -f /var/db/fb30/c4881.fdb 2>nul;
create database 'localhost/3333:/var/db/fb30/c4881.fdb' default character set utf8;
commit;

set list on;
out /dev/null;
select rpad('', 10921,'∑∏Ω') as multi_byte_10921 from rdb$database;
select rpad('', 10922,'∑∏Ω') as multi_byte_10922 from rdb$database;
out;
quit;

Result in any case will be:

Statement failed, SQLSTATE = 22001
arithmetic exception, numeric overflow, or string truncation
-string right truncation
-expected length 8191, actual 10921

Statement failed, SQLSTATE = 22000
Malformed string

IMHO, second expression (which tries to do RPAD up to 10922 should raise the same exception as first one, no matter of its argument.

PS.

All three characters in the padded string '∑', '∏' and 'Ω' -- requires three byte for encoding. Symbol '∏' should not be confused with cyrillic upper 'Pe', it is mathematical sign of number producing.