Parallel unfriendly

Take a look at the following Parallel section of a SQL Monitor report:

Any query which produces such a report won't care about how much parallel you're running because virtually all the time is spent by the query coordinator (which is a serial process) being busy.

In this case the query in question is quite simple:
select /*+ parallel(t,8) */ median(basket_amount) from whs.fact_sale t
The reason it behaves the way it does has everything to do with how Oracle executes it:
Execution Plan
Plan hash value: 712547042

| Id  | Operation                     | Name       | Rows  | Bytes  (more...)

In-memory PQ and physical reads

In my previous post I've demonstrated how in-memory PQ can access the table directly from the buffer cache even when you're using manual DOP.

One interesting question, however, is what happens when PQ slave needs to read some blocks from disk given that object has been qualified for in-memory (cached) access? Would the slave do it using direct or buffered I/O?

The answer becomes somewhat clear once you realize that in-memory PQ is enabled by simply utilizing buffered reads. Since direct path reads can not take advantage of any blocks stored in the buffer cache (local or remote), trying to (more...)

Does in-memory PQ work with PARALLEL_DEGREE_POLICY=MANUAL?

In-memory parallel execution seems to be gaining popularity especially among people running x2-8 and x3-8 Exadata systems or any other system that have large amounts of memory capable of caching lots of data.

Oracle documentation suggests that in order to utilize in-memory PQ, parallel_degree_policy needs to be set to auto.


One of the parameters influenced by parallel_degree_policy is _parallel_cluster_cache_policy. When using Auto DOP _parallel_cluster_cache_policy will be set to cached. The question then becomes what happens if we set _parallel_cluster_cache_policy=cached while still keeping Manual DOP? Will the system use in-memory PQ?

Test table

Below is a test table setup:
SQL>  (more...)

GoldenGate and transient PK updates

The problem of transient PK updates is well known and comes from the fact that pretty much every replication solution on the market applies changes using row-by-row approach.

To quickly recap the problem, if you have a table like:
SQL> create table tpk (n number primary key);
Table created
SQL> insert into tpk values (1);
1 row inserted
SQL> insert into tpk values (2);
1 row inserted
SQL> commit;
Commit complete
Then executing the following statement...
update tpk set n=n+1
...will result in a transient PK problem since the replication solution will have to decompose it into the following two (more...)

Oracle GoldenGate Sequence Replication

When using Oracle GoldenGate sequence replication there is a number of issues you need to be aware of especially if you replicate quite a lot of busy sequences.

The first issue is that GoldenGate sequence replication does not use bind variables. Let's execute the following statements on the source system:
SQL> create sequence rep1.s1 nocache;
Sequence created
SQL> select rep1.s1.nextval from dual;
SQL> select rep1.s1.nextval from dual;
GoldenGate uses PL/SQL procedure called replicateSequence each time it needs to sync sequence values. The following calls will be made on the (more...)

ZFS Home Storage Network at 10GbE

About a year ago I've decided to put all my data on a home built ZFS storage server. The growing amount of devices around my household prompted for an easier and much faster way to share the data. Since then the box was happily serving both CIFS as well as iSCSI over 1GbE network without any issues.

I was keen on upgrading to 10GbE for quite some time as both my server as well as clients could easily saturate 1GbE link when ZFS had all the required data in ARC. 32GB RAM in my storage server usually left me with (more...)

Exporting DBFS via NFS

Anybody who was thinking about exporting DBFS via NFS have probably stumbled upon the fact the Oracle says it can not be done:
DBFS does not support exporting NFS or SAMBA exports
What's wrong with DBFS?

There is nothing wrong with DBFS itself. The problem originated form the fact that FUSE did not have proper interfaces implemented to support exporting by the kernel. Newer versions of the Linux kernel fully support exporting. I know that OEL 6.x works for sure as I did the DBFS exports myself through both NFS as well as Samba. The common minimum kernel version (more...)