CLUSTER TABLE

There seems to be some kind of misunderstanding here. You cannot create a secondary index on a cluster table. A cluster table does not exist as a separate physical table in the database; it is part of a "physical cluster". In the case of BSEG for instance, the physical cluster is RFBLG. The only fields of the cluster table that also exist as fields of the physical cluster are the leading fields of the primary key. Taking again BSEG as the example, the primary key includes the fields MANDT, BUKRS, BELNR, GJAHR, BUZEI. If you look at the structure of the RFBLG table, you will see that it has primary key fields MANDT, BUKRS, BELNR, GJAHR, PAGENO. The first four fields are those that all cluster tables inside BSEG have in common. The fifth field, PAGENO, is a "technical" field giving the sequence number of the current record in the series of cluster records sharing the same primary key.

All the "functional" fields of the cluster table (for BSEG this is field BUZEI and everything beyond that) exist only inside a raw binary object. The database does not know about these fields, it only sees the raw object (the field VARDATA of the physical cluster). Since the field does not exist in the database, it is impossible to create a secondary index on it. If you try to create a secondary index on a cluster table in transaction SE11, you will therefore rightly get the error "Index maintenance only possible for transparent tables".

Theoretically you could get around this by converting the cluster table to a transparent table. You can do this in the SAP dictionary. However, in practice this is almost never a good solution. The table becomes much larger (clusters are compressed) and you lose the advantage that related records are stored close to each other (the main reason for having cluster tables in the first place). Apart from the performance and disk space hit, converting a big cluster table like BSEG to transparent would take extremely long.

In cases where "indexing" of fields of a cluster table is worthwhile, SAP has constructed "indexing tables" around the cluster. For example, around BSEG there are transparent tables like BSIS, BSAS, etc. Other clusters normally do not have this, but that simply means there is no reason for having it. I have worked with the SAP dictionary for over 12 years and I have never met a single case where it was necessary to convert a cluster to transparent.

If you try to select on specific values of a non-transparent field in a cluster without also specifying selections for the primary key, then the database will have to do a serial read of the whole physical cluster (and the ABAP DB interface will have to decompress every single record to extract the fields). The performance of that is monstrous -- maybe that was the reason of your question. However, the solution then is (in the case of BSEG) to query via one of the index tables (where you are free to create secondary indexes since those tables are transparent).