This post demonstrates influence of characterset on use of indexes. See also Language and Use of Indexes in Oracle

/*
DROP TABLE t;
ALTER SESSION SET NLS_LANGUAGE='AMERICAN';
*/

SELECT PARAMETER, VALUE FROM V$NLS_PARAMETERS;

CREATE TABLE t (x VARCHAR2(255));

INSERT INTO t VALUES ('a');
INSERT INTO t VALUES ('b');
INSERT INTO t VALUES ('c');
INSERT INTO t VALUES ('d');

CREATE INDEX t_idx ON t (x);

-- the index is used
UPDATE t SET x='a' WHERE x='a';

ALTER SESSION SET NLS_LANGUAGE='CZECH';

-- the index is still used if NLS_CHARACTERSET=WE8MSWIN1252
-- the index is not used if NLS_CHARACTERSET=WE8ISO8859P1
UPDATE t SET x='a' WHERE x='a';