JL: Nice blog post: http://oakweb02.oaktable.net/node/8065
Brings up the question of whether we need to track for and snipe idle
entries in v$pq_slave to prevent anomalous choices or failures of Oracle to
use the expected parallel degree (let's hope not, or hope some bug is
detected and fixed before we have to resort to that; we'd at least have to
pay the restart cost of the idle slave processes that should be used.
Perhaps they are pinned unusable)
Charles: JL's post nicely explains why adding a column spreads the work out
enough to keep the slaves more equally busy since then many entries become
not all null. As indicated by JL's analysis the title "cannot" is a bit off;
minimally usefully would be more on target. As for having many indexes
needing to be created on each table for many tables and indexes, have you
experimented with kicking off sets of index creation in parallel (each
serial) up to the load you want to consume? My "all else equal" bet would be
that a healthy number of creates being run in parallel would finish all the
work to be completed in less elapsed time than using parallel degree and
running the creates serially unless just a few index creations are so large
that they prolongs the work window. (In which case get all the little stuff
done in sets and then kick the big ones off maximally parallel each.)
mwf
From: oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l-bounce@xxxxxxxxxxxxx]
On Behalf Of Jonathan Lewis
Sent: Wednesday, July 15, 2015 12:48 AM
To: sacrophyte@xxxxxxxxx
Cc: ORACLE-L
Subject: RE: Curiosity: single-column index on sparse data cannot be built
in parallel
Charles,
Apologies for the delay - I had to shut down for the day before your reply
came through.
I've written a blog note about the anomaly - planned to re-read it and
polish it a little this morning.
Your non-null data is 3% of your index, and a parallel build would mean
RANGE distribution of the data, so if your build ran anything less than
parallel 33 (approx) the point would come where only one slave (in the
second set) would be doing anything because it would be the one that got all
the real data and had to sort and build. That's why you (I believe) you
might see a point where only one process seemed to be doing any work. There
is still benefit in running parallel, of course, since you have to scan and
discard the 97% of the data that is NULL and can't go into the index.
The anomaly is that when I examined v$pq_tqstat after creating my 310,000
row index from my 10,000,000 row table (running parallel 4) the first set of
slaves had passed 10 million rows to the second set of slaves whiich means
they must have been passing all the (null, rowid) entries rather than
discarding them.
Are you licensed to dig back into the ASH history ? Can you find out if the
one running process was a Pnnn process or a normal shadow process ? If it
was a normal shadow the problem might simply be that at the moment the
create index started all the PX slaves were involved in building other
indexes. If you can run an AWR report for the time around the start of
build you might get a clue from the statistics about "Parallel operations
downgraded to ..."
Regards
Jonathan Lewis
http://jonathanlewis.wordpress.com
@jloracle
_____
From: Charles Schultz [sacrophyte@xxxxxxxxx]
Sent: 15 July 2015 01:05
To: Jonathan Lewis
Cc: ORACLE-L
Subject: Re: Curiosity: single-column index on sparse data cannot be built
in parallel
I didn't check $pq_tqstat, but I show results when I next do the test. I say
it was not running in parallel because only one session was active via
Enterprise Manager (yeah, I was being lazy, I know) and the creation took
over 1 hour the first test, 51 minutes the next test, whereas all my other
indexes (mix of local and non-partitioned) on the same table were taking
less than 20 minutes (some less than 10).
Jonathan, can you elaborate a little more on the "interesting anomaly"? :)
It is entirely possible I am hitting that, but would like to learn more.
As far as parallel 100, I was just trying to open the doors to see where my
bottlenecks are. The extent sizes are included in the index ddl (5m); again,
I don't have issues with the other indexes, but there is something about
this being on a single column, so you may be on the right track with large
extents and small volumes of data per slave. You are making me curious now.
:) I love the anthromorphic angle on "refusing to cooperate". *grin*
PS - in case it is not obvious, this is a vendor supplied table and index.
PS2 - tests currently executing on Sun T5440, for those that care about such
things.
PS3 - as to "why"; I got a lot of indexes to build in a short time, so I am
looking for the bounds. :)
On Tue, Jul 14, 2015 at 4:12 PM, Jonathan Lewis
<jonathan@xxxxxxxxxxxxxxxxxx> wrote:
I'm not going to try for 500M rows, but I just built a table with 10M rows
with your pattern of data and ran a parallel create index.
There is an interesting anomaly to the create index (11.2.0.4) that might
make the build look like a serial build - how are you showing that the build
isn't parallel ?
I queried v$pq_tqstat after the build.
Parallel 100 seems a little optimistic for such a "small" index - 17M rows
at about 13 bytes per row gives about 220MB or 2MB per slave to build.
What's your default extent size - perhaps there's something about large
extents and small volumes of data per slave that makes Oracle refuse to
co-operate.
Regards
Jonathan Lewis
http://jonathanlewis.wordpress.com
@jloracle
_____
From: oracle-l-bounce@xxxxxxxxxxxxx [oracle-l-bounce@xxxxxxxxxxxxx] on
behalf of Charles Schultz [sacrophyte@xxxxxxxxx]
Sent: 14 July 2015 21:59
To: ORACLE-L
Subject: Curiosity: single-column index on sparse data cannot be built in
parallel
Good day,
I am trying to find the technical reason for why Oracle cannot use parallel
slaves to build a single-column index on a sparse column with few distinct
values:
F COUNT(*)
- ----------
538937561
Y 51464
N 17486819
Just by playing around, I discovered that if I put this column as the
leading edge on an index with many columns, it can be built in parallel.
According to the relevant documentation
<http://docs.oracle.com/cd/E11882_01/server.112/e25523/parallel007.htm#i1009
131> , I would expect that the base table is sampled, but perhaps the random
sampling returns 0 keys due to the sparse nature of the column?
Facts:
Oracle Enterprise Edition 11.2.0.4
table is partitioned, but not on this key (lol)
parallel_max_servers=3600
ddl extracted via datapump:
CREATE INDEX "FIMSMGR"."FGBTRND_ENCD_INDEX" ON "FIMSMGR"."FGBTRND"
("FGBTRND_DEFER_GRANT_IND")
PCTFREE 10 INITRANS 2 MAXTRANS 255
STORAGE(INITIAL 5242880 NEXT 5242880 MINEXTENTS 1 MAXEXTENTS 2147483645
PCTINCREASE 0 FREELISTS 1 FREELIST GROUPS 1
BUFFER_POOL DEFAULT FLASH_CACHE DEFAULT CELL_FLASH_CACHE DEFAULT)
TABLESPACE "FIN_LARGE_INDX" PARALLEL 100 ;
--
Charles Schultz
--
Charles Schultz
