DECLARE GLOBAL TEMPORARY TABLE Vs CREATE GLOBAL TEMPORARY TABLE in DB2
There is a good article from Craig S. Mullins that covers the major differences between the two. For most purposes, they work the same.
Created Temp Tables are created in DSNDB07, which is the working file database (the same storage area used during SQL statements that need working storage). Declared Temp Tables are stored in temporary tablespace that you have to create.
There are some cons for CTT's:
Because they are not persistent, some typical database operations including locking, logging, and recovery do not apply to created temporary tables.
Indexes can not be created on created temporary tables so all access is by a complete table scan.
Constraints can not be created on created temporary tables.
A null is the only default value permitted for columns of a created temporary table.
Created temporary tables can not be referenced by DB2 utilities.
Created temporary tables can not be specified as the object of an UPDATE statement.
When deleting from a created temporary table, all rows must be deleted.
Although views can be created on created temporary tables, the WITH CHECK OPTION can not be specified.
DTT are usually much more flexible:
Declared temporary tables can have indexes and CHECK constraints defined on them.
You can issue UPDATE statements and positioned DELETE statements against a declared temporary table.
You can implicitly define the columns of a declared temporary table and use the result table from a SELECT.
Now for your numbered questions:
1. & 2. There isn't one. I believe (and I'm not 100% sure if this is accurate, our shop pretty much uses DTTs in all cases) that a CTT is declared once (by a DBA), and then the application programmers can use it in any session. Each connection will have their own copy, and once the application disconnects, the data stored in that CTT in that session will go away.
3. SESSION is just the schema identifier for DTTs. It shows that it is a temporary
table that does not persist.
4. I think both will have about the same performance. They will be faster than normal tables, because locking, logging, recovery, etc will not apply.
5. In general, I would say that DTTs are the way to go, but CTTs are useful (as Craig says in his article):
(CTTs) should be considered primarily when no updating of temporary data is needed and access to the temporary data is purely sequential.
Craig S. Mullins article applies to DB2 for OS/390 platform. In DB2 9.7 you actually can create indexes for both DGTT and CGTT. Also you can enable logging in CGTT. For 9.7 read here: https://www.ibm.com/support/knowledgecenter/en/SSEPGG_9.7.0/com.ibm.db2.luw.admin.dbobj.doc/doc/r0054491.html