Поменять тип полей, с очисткой

DO $$
DECLARE
  _rec record;
  _tag_name varchar(32);
begin
_tag_name := 'tag_678';
 for _rec in
 -----------------------------
SELECT attrelid::regclass as tablename,
       attname,
       (select typname from pg_type where oid=atttypid) as col_type
FROM   pg_attribute
WHERE  --attrelid = 'er.editor_comment_tmp'::regclass AND  
       attnum > 0
and    attname = _tag_name
AND    NOT attisdropped
and    (select typname from pg_type where oid=atttypid)='varchar'
 -----------------------------
 loop
   begin  
 execute 'update '||_rec.tablename||' set '||_tag_name||'=null';
 execute 'alter table '||_rec.tablename||' alter column '||_tag_name||' type numeric using '||_tag_name||'::numeric';
    raise notice 'OK: table - %',_rec.tablename;
   exception
    when others then raise notice 'ERROR: table - %',_rec.tablename;
   end;
 end loop;
END
$$