Oracle Tips  by Burleson Consulting

The Data Warehouse Development Life Cycle

Parallelism And Oracle Data Warehousing
Freelists And Oracle Parallel Server Contention

Freelists are especially important for Oracle data warehouses that experience a high volume of localized update activity. A freelist is the parameter used when more than one concurrent process is expected to access a table. Oracle keeps one freelist for each table in memory, and uses the freelist in order to determine what database block to use when an SQL INSERT occurs. When a row is added, the freelist is locked. If more than one concurrent process is attempting to insert into your table, one of the processes may need to wait until the freelist has been released by the previous task. To see if adding a freelist to a table will improve performance, you will need to evaluate how often Oracle has to wait for a freelist. Fortunately, Oracle keeps a V$ table called V$WAITSTAT for this purpose. The following query example tells you how many times Oracle has waited for a freelist to become available. As you can see from the following query, Oracle does not tell you which freelists are experiencing the contention problems:

SELECT CLASS, COUNT
FROM V$WAITSTAT
SEE CODE DEPOT FOR FULL SCRIPT

CLASS COUNT
--------------- ------------
free list 83

Here, we see that Oracle had to wait 83 times for a table freelist to become available. This could represent a wait of 83 times on the same table or perhaps a single wait for 83 separate tables. We have no idea. While 83 may seem to be a large number, remember that Oracle may perform hundreds of I/Os each second, so 83 may be quite insignificant to the overall system. In any case, if you suspect that you know which table’s freelist is having the contention, the table can be exported, dropped, and redefined to have more freelist. While an extra freelist consumes more of Oracle’s memory, additional freelists can help the throughput on tables that have lots of inserts. Generally, you should define extra freelists only on those tables that have many concurrent update operations. Now, let’s take a look at some table definitions and see if we can infer the type of activity that will be taking place against the tables. Listings 7.1 and 7.2 each present a table definition.

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