Hi Jomy,
to execute a join the database (simply formulated, I know the precise logic will be more sophisticated) loops over the first table and for each entrry of the first table searches for entries in the second table that matches the join condition. In a classical database I would make shure that there is a primary or secondary index for the second table that can be used (if the second table is big).
In HANA index are often not used, because HANA can loop over big tables very fast. But in this case HANA has to do this loop over the second table quite often, as also the first table is big. Therefore an index on the second table should improve the performance. SAP proposes to use index only if the performance is not sufficient without, but in this case it may be the case, that an index is needed.
With kind regards
Rainer