select name,colnames from sysibm.sysindexes
where tbname='my_table_name'
See also:
How to get index column names of a table from SQL Server
How to get index column names of a table from MySQL
How to get index column names of a table from PostgreSQL
How to get index column names of a table from SQLite
How to get index column names of a table from Oracle
Some DB2 tools you can try: https://www.withdata.com/db2/