If you have defined a column set as a composite index, it makes no difference whether you collect statistics on the composite using the COLUMN keyword or the INDEX keyword. The result is identical.
For example, suppose you have created an index on the column set (x1,y1) of table t1. The following COLLECT STATISTICS requests gather the identical statistics for the named QCD into the identical QCD table columns:
COLLECT STATISTICS FOR SAMPLE 30 PERCENT ON t1 INTO myqcd COLUMN (x1,y1); COLLECT STATISTICS FOR SAMPLE 30 PERCENT ON t1 INTO myqcd INDEX (x1,y1);