drop table if exists customer;
create table customer(my_id serial, my_txt char(20));
insert into customer values (0, "first");
insert into customer values (0, "second");
insert into customer values (0, "third");
SELECT idsxmlparse(genxmlschemaclob(customer, 'customer')) FROM customer;

 select idsxmlparse(
replace(replace(col, "ru_RU.915", "ISO8859-5"), '<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">')
)
from table(genxmlqueryhdr('customer','select * from customer')) as tmptb(col)

drop table if exists customer;
create table customer(my_id serial, my_txt char(20));
insert into customer values (0, "first");
insert into customer values (0, "second");
insert into customer values (0, "third");
insert into customer values (0, "первый");
insert into customer values (0, "второй");
insert into customer values (0, "третий");
select xsltransform(
idsxmlparse(replace(replace(col, "ru_RU.915", "ISO-8859-5"), '<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">'))
,'<?xml version="1.0" encoding="ISO-8859-5"?><xsl:stylesheet version="2.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform"><xsl:output method="string" encoding="ISO-8859-5"/><xsl:template match="/"><xsl:for-each select="*"><xsl:value-of select="."/></xsl:for-each></xsl:template></xsl:stylesheet>'
)
from table(genxmlqueryhdr('customer','select * from customer')) as tmptb(col);

<?xml version="1.0" encoding="ISO-8859-5"?>
1
first               

2
second              

3
third               

4
первый              

5
второй              

6
третий