Tim,
"Load as select" means it's doing direct path load. The additional step of
"OPTIMIZER STATISTICS GATHERING" also only appears for direct path load.
We can also tell that the insertion into the table is done in parallel as
we are still using PCWP in the IN-OUT column, it only serializes about the
load.
Thanks,
Andy
On Tue, 7 Jul 2020 at 23:25, Tim Gorman <tim.evdbt@xxxxxxxxx> wrote:
Ram,
Perhaps it is serializing on the INSERT phase of the CTAS, keeping in mind
that the CREATE clause both creates the table and does the insert?
Perhaps...
ALTER SESSION ENABLE PARALLEL DDL;
CREATE TABLE ... PARALLEL 8 SELECT /*+ full(ce) parallel(ce,8) */
CE.* FROM ... ;
Unless you are required to perform CTAS in one statement, could you
perhaps split it into two steps, the first to create the table and the
second to populate it? So as an alternative...
CREATE TABLE .... SELECT * FROM IX.S_CLN_EVNT WHERE 1 = 0;
ALTER SESSION ENABLE PARALLEL DML;
INSERT /*+ APPEND PARALLEL(x,8) */ INTO REQ_27333_CLN_EVENT SELECT
/*+ full(ce) parallel(ce,8) */ CE.* FROM ... ;
This way you can use the APPEND hint to force direct-path INSERTs which
are far more efficient than conventional INSERTs; I'm not sure if LOAD AS
SELECT (HYBRID TSM/HWMB)" is direct-path or conventional? At any rate,
with an APPEND hint you can be certain. You'll also get to specify the
PARALLEL hint on the INSERT statement as well as the SELECT statement, to
make sure of PX pipelining. Also, not sure if your session is enabled for
parallel DDL or parallel DML, but if you break it into two operations, you
will only need parallel DML.
Hope this helps?
-Tim
On 7/7/2020 2:48 PM, Ram Raman wrote:
Hi,
We are trying to create a table by pulling information from a big table,
the create statement is taking too long. However when I run select count(*)
with the same query it comes back in seconds. While the CREATE statement is
running I see waits on parallel read almost all the time, however, I do not
see the same waits during select count(*).
Tried a few variations with parallelism degree in the Create statement.
Also tried inserting after truncating the table, used INSERT with APPEND
hint, same slow results. v12c. FORCE_LOGGING is not on at DB level,
inserted table in nologging mode. During the run, the IO on the data disks
is about 50 to 70% util. 2 cores are consistently at 99-100% (most of them
are %iowait at ~95%) with the other cores idle during this run the last
time I checked - I only checked the individual core usage this last time it
was run. I was checking average before. The wait events are the same in all
cases.
The attached link
<https://drive.google.com/file/d/1N8re2eAWLl0FHYdGs0c9I93e5ruoSNN6/view?usp=sharing>
has all the commands and the information. Any help is appreciated.
Thanks,
Ram.
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