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If I use many CLOB variables in the PL/SQL stored procedure to store many long length string , are there any performance issues? Is the length of the CLOB is also variable? Are there any known limitations/disadvantages for CLOB instead using varchar2 and long?
CLOBs are variable in length, yes. The upper limit varies according to which version of Oracle you are on and your database block size. For 11G the limit is “4G * value of DB_BLOCK_SIZE parameter” (from 11G PL/SQL Language Reference). VARCHAR2 values are limited to 32767 bytes in PL/SQL.
I don’t have any definitive information on the relative perforance of CLOB vs. VARCHAR2 in PL/SQL, and a brief Google didn’t return anything useful either. However I would strongly suspect that VARCHAR2 performs better than CLOB in general (for data that can be stored in either), since it is simpler. You could easily set up a test to prove this right or wrong by writing the same simple program once with VARCHAR2 and once with CLOB, and running each 1000s of times and comparing the total elapsed time. Tom Kyte’s Runstats utility is a great help here, and shows other differences between 2 approaches in terms of Oracle resources.
My advice would be: if your data will exceed 32K then you must use CLOB; if not then don’t use CLOB, use VARCHAR2.
NB: if CLOB were better than VARCHAR2 for all sizes of character data, Oracle would probably deprecate VARCHAR2, as they did with the LONG datatype – but they haven’t.