Difference between the design of clusters PCLx and others like RFBLG etc .

Question

There are a few nagging questions which I was not able to find in the forum hence i have to post a new question.
I am a little confused about the difference between the different clusters .
If i start with RFBLG i.e. the cluster for BSEG BSEC etc ,I can see that the tables which are part of this cluster
can be viewed through different methods like
1) whr usd list for RFBLG
2) dd02l table and give the required parameters there
now when I compare this with another so-called cluster PCL1 if find that PCL1 is not recognized as  a cluster
and also I am not able to see the same in dd02l table when i give PCL1 and the tabtype as cluster which I was able to see
for the RFBLG ,there are other tables similar to RFBLG .
1) SO what is the difference between the RFBLG type of clusters and the PCLx type of cluster
2) are pclx and rfblg..type of clusters same ?
3) why does PCL1 shows that it is a transparent table ? where as rfblg shows in a diff way in se11
4) i know we access data from PCL1 using import and export stmts ,DO OR CAN WE DO THE SAME FOR RFBLG
5) I found that each and evry cluster table had diff fields ,this was kinda surprising for me as I had been thinking
that all cluster tables need to follow a certain rule ,SO WHO DECIDES THE FIELDS OF A TABLE CLUSTER ?
6) PCL1 has the index button enabled ,which again I think is not according to the cluster table rules?how?
7) I understand that we can save data in form of internal tables in the PCL1 cluster ,can we do the same in RFBLG ?
8) Can I think on lines that PCL1 and RFBLG type of cluster are two totally different types of data dictionary objects
and the usage and implementation of both of them is different and that the design and the BASE of both of such objects
is different .
I know this is a long list but I am sure that answers to these questions would really require some one who has really really work hard and invested a lot of time in understanding the dictionary system.I am awaiting a few answers ,few hints and a healthy discussion till we get them .
Thanks ...
a

Answer 1

Hello,
1/
BSEG is a typical Cluster Table.
This means that the physical table BSEG does NOT exist in the database, physical data for BSEG is stored in the database (table) cluster RFBLG.
In ABAP however you can perform selects on BSEG (with all fields from the SAP repository structure, see SE11 on BSEG), during execution the SAP database layer will translate these statements to physical selects in the RFBLG database table, so in ABAP this is transparant.
More info :
[http://help.sap.com/saphelp_nw04/helpdata/en/cf/21f083446011d189700000e8322d00/content.htm|http://help.sap.com/saphelp_nw04/helpdata/en/cf/21f083446011d189700000e8322d00/content.htm]
2/
PCL1, PCL2, ... are normal SAP transparent tables, however in HR they are often called HR cluster table.
Transparent tables are SAP objects where there is also a database table with the same name that contains the physical data.
However the PCL tables are somewhat different from normal transparent tables (data is compressed, external programs can not interpret the data, ...).
This means that in ABAP you can not use simple SQL statements to access data in PCL tables (because of compressed format).
In stead statements like EXPORT TO DATABASE and IMPORT FROM DATABASE need to be used.
More info :
[http://fuller.mit.edu/hr/cluster_tables.html|http://fuller.mit.edu/hr/cluster_tables.html]
Wim