SYSIQJOINIXCOLUMN system table

CREATE TABLE SYS.SYSIQJOINIXCOLUMN (
	joinindex_id UNSIGNED INT NOT NULL,
	left_table_id UNSIGNED INT NOT NULL,
	left_column_id UNSIGNED INT NOT NULL,
	join_type CHAR(4) NOT NULL,
	right_table_id UNSIGNED INT NOT NULL,
	right_column_id UNSIGNED INT NOT NULL,
	order_num				UNSIGNED INT NOT NULL,
	left_order_num					UNSIGNED INT NOT NULL,
	right_order_num					UNSIGNED INT NOT NULL,
	key_type			CHAR(8) NOT NULL,
	coalesce			CHAR(1)			NOT NULL,
	PRIMARY KEY ( joinindex_id, left_table_id, 
left_column_id, right_table_id, right_column_id )
)

The rows of SYSIQJOINIXCOLUMN describes the columns which explicitly participate in a join index.