Re: Advantage off parallel access at external tables
Date: Fri, 4 Apr 2008 13:36:12 +0200
"SePp" <C_o_z_m_o_at_gmx.de> schreef in bericht
> Hi all,
> I did some testing to find an advantage of parallel access on external
> tables. I did an Select count(*) statement
> on an external table with around 450.000 records. The table reads the
> data from one file.
> But the results are quit different. It takes around 14 to 26 seconds
> and sometimes the parallel access is faster and sometimes they have
> the same speed. Is there no real advantage or is the way I'm testing
> it wrong?
> Do I understand the hole parallel access wrong and I have only an
> advantage at the moment I'm access two or more external files.
> I read somewhere (can't find the source again) the parallel access is
> only recommended on huge external files. Can somebody confirm that?
> Thank you very much in advance for you information and help!
> Kind regards,
From the docs:
Parallel Access to External Tables
After the metadata for an external table is created, you can query the external data directly and in parallel, using SQL. As a result, the external table acts as a view, which lets you run any SQL query against external data without loading the external data into the database.
The degree of parallel access to an external table is specified using standard parallel hints and with the PARALLEL clause. Using parallelism on an external table allows for concurrent access to the datafiles that comprise an external table. Whether a single file is accessed concurrently is dependent upon the access driver implementation, and attributes of the datafile(s) being accessed (for example, record formats).
So it seems possible to have an external table in multiple datafiles, in which case parallelism might help.
I don't know whether the standard SQL loader access driver (which is used in most cases) supports parallallism on single files ; maybe some guru can tell us...
Shakespeare Received on Fri Apr 04 2008 - 06:36:12 CDT