How to search particular string in long varchar column?

REGEXP_LIKE works fine on LONG data types like LONG VARCHAR.

Example:

dbadmin=> CREATE TABLE t (c LONG VARCHAR);
CREATE TABLE

dbadmin=> INSERT INTO t SELECT 'Some junk';
 OUTPUT
--------
      1
(1 row)

dbadmin=> SELECT * FROM t WHERE REGEXP_LIKE(c, 'junk');
     c
-----------
 Some junk
(1 row)

The error you are getting is because you've stored some non UTF-8 data in the column in some record(s).

Take a look here:
https://forum.vertica.com/discussion/240991/remove-non-utf-8-characters-from-varchar-column

Like this:

dbadmin=> INSERT INTO t SELECT E'\xa0' || 'Test!' || E'\xa0';
 OUTPUT
--------
      1
(1 row)

dbadmin=> SELECT * FROM t;
     c
-----------
 Some junk
 ▒Test!▒
(2 rows)

dbadmin=> SELECT * FROM t WHERE REGEXP_LIKE(c, '[^\t\r\n\x20-\x7E]+', 'b');
    c
---------
 ▒Test!▒
(1 row)

But, you can probably just run your original query with the B option of the REGEXP_LIKE function...

SELECT query, user_name FROM analytics.query_tracker where REGEXP_LIKE(query, 'junk', 'b');

@Jim_Knicely you are awesome !