Submitted by: @pavel-zotov

Scenario and its output for illustrating the problem:

create database 'localhost/3333:C:\FBTESTING\qa\fbt-repo\tmp\db_cset_utf8.fdb' default character set utf8;

set list on;

select rdb$character_set_name from rdb$database;

/************************************************
::: STDOUT :::

RDB$CHARACTER_SET_NAME UTF8
************************************************/

commit;

create or alter view v_table_field_cset as
select
rf.rdb$relation_name tab_name
,rf.rdb$field_name fld_name
,cs.rdb$character_set_name fld_cset
,rb.RDB$CHARACTER_SET_NAME db_default_cset
from rdb$relation_fields rf
join rdb$fields ff on rf.rdb$field_source = ff.rdb$field_name
join rdb$character_sets cs on ff.rdb$character_set_id = cs.rdb$character_set_id
cross join rdb$database rb
;
commit;

-------- CHANGE DATABASE DEFAULT CHARSET, COMMAND #⁠ 1 --------

alter database set default character set dos850;
commit;

create table tab_850( txt_850 varchar(10)); commit;
select * from v_table_field_cset where tab_name=upper('tab_850');

/************************************************
::: STDOUT :::

TAB_NAME TAB_850
FLD_NAME TXT_850
FLD_CSET DOS850 ---- yes, changed. But this is last time for current attachment.
DB_DEFAULT_CSET DOS850
************************************************/

-------- CHANGE DATABASE DEFAULT CHARSET, COMMAND #⁠ 2 --------

alter database set default character set dos866;
commit;

create table tab_866( txt_866 varchar(10)); commit;
select * from v_table_field_cset where tab_name=upper('tab_866');

/************************************************
::: STDOUT :::

TAB_NAME TAB_866
FLD_NAME TXT_866
FLD_CSET DOS850 ---- field charset remains OLD!
DB_DEFAULT_CSET DOS866
************************************************/

-------- CHANGE DATABASE DEFAULT CHARSET, COMMAND #⁠ 3 --------

alter database set default character set win1252;
commit;

create table tab_1252( txt_1252 varchar(10)); commit;
select * from v_table_field_cset where tab_name=upper('tab_1252');

/************************************************
::: STDOUT :::

TAB_NAME TAB_1252
FLD_NAME TXT_1252
FLD_CSET DOS850 ---- field charset remains OLD!
DB_DEFAULT_CSET WIN1252

************************************************/

-------- CHANGE DATABASE DEFAULT CHARSET TO NON-EXISTED VALUE --------

alter database set default character set FOO_BAR_8859_4;
commit; -- PASSED!

-- Now make reconnect:
connect 'localhost/3333:C:\FBTESTING\qa\fbt-repo\tmp\db_cset_utf8.fdb' user 'sysdba' password 'masterke';

create table tab_foo( txt_8859_4 varchar(10) );

/************************************************
::: STDERR :::

Statement failed, SQLSTATE = HY004
unsuccessful metadata update
-CREATE TABLE TAB_FOO failed
-Dynamic SQL Error
-SQL error code = -204
-Data type unknown
-CHARACTER SET FOO_BAR_8859_4 is not defined
************************************************/

Commits: 193b892 FirebirdSQL/fbt-repository@79b5b00