That all makes sense.
And recreating the index in the hopes that the error was an unlikely event, but
which should make the problem go away.
To the scenario possibilities I will add a delete operation that was
interrupted in an unfriendly way before the commit. Hmm, I wonder if an aborted
insert could burn space it shouldn’t permanently burn, leaving the index leaf
marked with plenty of space available, but the free space in the block somehow
trashed.
All the “fixes” that come to mind would destroy this as a research case, and if
something in their operational schedule triggers an otherwise unusual event,
the usual thing that happens when problems that go away unexplained will
probably happen.
Thanks for saving me from drilling into the actual details.
It certainly would be tedious to dump the blocks of the index looking for
perversity, and then tying anything odd to a past event is likely to be an
anecdotal search. “What unusual thing did all y’all do after the last ‘good’
run (that might have cleaned some things up) before the next ‘bad’ run?”
Happy Holidays.
From: oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l-bounce@xxxxxxxxxxxxx] On ;
Behalf Of Jonathan Lewis
Sent: Wednesday, November 29, 2023 4:45 AM
To: Oracle L
Subject: Re: Same query with no plan change or volume but runs for hours vs
minutes
Mark,
I'm still trying to work out a consistent story of what the names of the
statistics are trying to tell us, but it seems likely that part (possibly a
major part) of the problem relates to Oracle trying to find usable blocks in
the index segment. One very clear indicator is that the fast run doesn't show a
value for "ASSM gsp:Optimized index block rejects" whlie the slow run reports
438M
Other observations from the slow stats:
ASSM gsp:reject db = 622315351
ASSM cbk:blocks examined = 184M which is very close to: "reject db - optimizer
index blocks reject"
I think this tells us that "optimized" means I checked a "bit" so I didn't need
to check the block it references.
So when we see
ASSM gsp:L2 bitmaps examined = 151,000
ASSM gsp:Optimized reject l1 = 1,150
ASSM gsp:L1 bitmaps examined = 42M
I think that's telling us that there are lots of L1 bitmaps that should have
been flagged in the L2 as "no space in my data blocks", but haven't.
Somewhere in the code path we have something is failing to set bits in the L1.
The trouble is, I don't think it's the insert that's doing it wrong. Check
these three stats:
leaf node splits 4,602
branch node splits 8
ASSM cbk:blocks marked full 4,610
We know that Oracle had to allocate (or re-use) 4,610 index leaf blocks because
of the block splits - and it has immediately marked those blocks as "full"
(index blocks are only ever "unformatted" "newly formatted", "reusable (FS2)"
or "in use (FULL)" as far as the bitmaps are concerned).
The only ideas I have at present are:
a) an index coalesce or a shrink space has taken place, freed up no (or very
few) blocks, but erroneously marked the leaf blocks as reusable.
b) something happened to cause two copies of the code to run, and one
eventually failed or stuck; or the PL/SQL block controlling the insert rolled
back and restarted, and the internal code left lots of blocks in the wrong
state for some reason.
Regards
Jonathan Lewis
On Tue, 28 Nov 2023 at 23:20, Mark W. Farnham <mwf@xxxxxxxx> wrote:
“So ideally it should be a brand new segment without any fragmentation etc.”
? Does that apply to both the table partition and the index?
(A quick skim of what JL wrote makes me thing that index space available for
new inserts is the problem, but I really just glanced.)
? Do you insert rows with key values and lots of attributes null and then
immediately update the null attributes to the real values?
(If you’re doing that, then do it someplace else (with a huge pctfree) and copy
the fully formed rows into the real destination. This interim location does not
need to be indexed, let the secondary insert into the “real” destination handle
any relational integrity issues, and if your initial insert is really noisy
with violations possibly test index it for uniqueness after the initial insert.)
? What is the size of the initial extent of the new partition compared to the
amount of a batch being stuffed in?
IF the answer to Dominic’s question is that your function is not direct insert
friendly, then think about putting the fully formed rows into an interim
location unindexed and then use those fully formed rows as a source for direct
inserts into the “real” destination.
Good luck.
mwf
From: oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l-bounce@xxxxxxxxxxxxx] On ;
Behalf Of Dominic Brooks
Sent: Tuesday, November 28, 2023 4:24 PM
To: loknath.73@xxxxxxxxx
Cc: jlewisoracle@xxxxxxxxx; Oracle L
Subject: Re: Same query with no plan change or volume but runs for hours vs
minutes
Is there a good reason for not using direct path inserts?
Sent from my iPhone
On 28 Nov 2023, at 21:03, Lok P <loknath.73@xxxxxxxxx> wrote:
Thank you Jonathan.
So, It means the time spent is really on doing INSERT only but not the
"reading/select" part of the query. While loading , it spends a lot of time
scanning blocks that are full and thus unable to insert rows in them. However,
onething want to share here, that the target partition key/date is hard coded
in this query, which means the data inserted into a fresh new blank partition
each day. So ideally it should be a brand new segment without any fragmentation
etc.
"When I asked about a "catastrophic" event, I wanted you to think whether there
was any large-scale activity you might have done in the recent past. "
In the recent past this database moved from the older version of exadata to the
latest X9 and also the database was moved to multitenant PDB. How can that be
related? But then why do we see the slowness on certain days only but not daily
when this data load happens?
Also we see some high "block" count for almost the same amount num_rows,. I.e
for the same ~30million rows , some partitions showing the blocks as ~1million
and some are showing ~1.5million. Is this expected?.
PARTITION_NAME NUM_ROWS BLOCKS AVG_ROW_LEN COMPRESSION
PBDRA_DTL_11272023_P 33734616 1487844 197 DISABLED
PBDRA_DTL_11262023_P 33469912 980070 198 DISABLED
PBDRA_DTL_11252023_P 33527112 994892 198 DISABLED
PBDRA_DTL_11242023_P 33948522 991827 198 DISABLED
PBDRA_DTL_11232023_P 32667974 993492 198 DISABLED
PBDRA_DTL_11222023_P 34653004 1529017 198 DISABLED
PBDRA_DTL_11212023_P 33233838 968435 198 DISABLED
PBDRA_DTL_11202023_P 32870086 1472631 198 DISABLED
PBDRA_DTL_11192023_P 32485084 947193 198 DISABLED
Yet to find the space usage percentage from the new segments/partitions. Hope i
can use the scripts in the blog below for the same.
https://jonathanlewis.wordpress.com/2016/09/11/space-usage/
On Tue, Nov 28, 2023 at 11:38 PM Jonathan Lewis <jlewisoracle@xxxxxxxxx> wrote:
an index on (large sequence value, date_value) should store about 310 to 320
index entries per leaf block when full.
So the leaf node splits suggest you've inserted roughly 4602 * 310 = 1.4M rows
in the bad case and 40928 * 310 = 12.7M rows in the good case.
These figures are in line with the execute count (etc.) of 1424230 and
12752800. This suggests that virtually every row that gets through the other
join predicates executes the select statement in fun1 and survives the
predicate check.
The "index range scans" statistic match the "execution count" statistic in both
cases, and the various figures for consistent gets suggest that the access path
is the same in both cases (and is index only-only). So you can discount
something odd happening with the function.
Take the execute count as the number of rows inserted
Take "ASSM gsp: get free block" - "leaf node splits" - "branch node splits" as
the number of new blocks requested for the table.
Divide "rows inserted" by "new table blocks" and the answer is about 34.5:
multiply that by 203 (avg_row_len + 5 for the row overhead) and the answer is
about 7,000 - so the figures about data size etc. hang together quite well, and
make it reasonable to look for the problem in "other activity">
So in the bad case the session requests some ASSM space for inserting table
rows 41,500 times (46,150 - 4,600). To find that space is examines
To find that space Oracle examines 42.6M Level 1 bitmap blocks: (ASSM gsp:L1
bitmaps examined 42601388)
and 184M data blocks: (ASSM cbk:blocks examined 184273393)
Of these(!), Oracle reports 633M rejected (ASSM gsp:reject db
622315351)
I think the inconsistency in the examined/rejected tells us that there's an
error somewhere in how Oracle is totalling up different subsets, and there are
lots of ways you can kick around all the examine/reject figures to try and
guess what they mean and how they've been summed incorrectly.
What I do think, though, is that the 184M blocks examined, and the 42M L1
bitmap blocks example are where your code is burning its excess CPU in the slow
case -- checking blocks which aren't flagged as FULL, but can't accept the data
that needs to be inserted. In passing 184M + 42M = 226M, which is most of the
session logical I/O.
When I asked about a "catastrophic" event, I wanted you to think whether there
was any large-scale activity you might have done in the recent past. If I had
wanted to know if you had done a big delete I would have asked if you had done
a big delete. We need to find a reason why there are lots of blocks that seem
to be marked as "space available" in the L1 bitmap, but aren't usable once
Oracle gets to the block. (At some point in the past Oracle Corp. added code
for a session to record a list of blocks that it had visited unsuccessfully "a
few" times so that it stopped visiting them, but that may have changed since
the initial implementation.)
You could use the dbms_space.space_usage procedure to check the state of the
blocks in one of the target partitions to see if there are lots of blocks that
between 75 and 10% full. This might be a bit of information to pass to Oracle.
Regards
Jonathan Lewis
There exists only one composite (TXN_ID, WOD_DT) primary key index on this
table-PBDRA_DTL. This table is daily range partitioned on a date column. And
also the sequence -PBDRAD_SEQ does fed into the leading column(TXN_ID) of the
PK index.
The AVG_ROW_LEN i see in dba_tab_partition for last few of the partitions are
showing ~198 and similar for others too.
PARTITION_NAME NUM_ROWS BLOCKS AVG_ROW_LEN COMPRESSION
PBDRA_DTL_11272023_P 33734616 1487844 197 DISABLED
PBDRA_DTL_11262023_P 33469912 980070 198 DISABLED
PBDRA_DTL_11252023_P 33527112 994892 198 DISABLED
PBDRA_DTL_11242023_P 33948522 991827 198 DISABLED
PBDRA_DTL_11232023_P 32667974 993492 198 DISABLED
PBDRA_DTL_11222023_P 34653004 1529017 198 DISABLED
PBDRA_DTL_11212023_P 33233838 968435 198 DISABLED
PBDRA_DTL_11202023_P 32870086 1472631 198 DISABLED
PBDRA_DTL_11192023_P 32485084 947193 198 DISABLED
Select list doesnt have any plsql function.
Today I have captured the sql monitor and related stats from v$sesstat for a
~5minutes delta period for a quick/good run in which the complete query
finished in ~15minutes. I have posted the sql monitor and stats in same github
link, The session stats i have posted as comments to the same github linc.
https://gist.github.com/oraclelearner/722a77ef5ebce23396d82e2835958303
When you mentioned "some catastrophic activity on the base table might cause
the ASSM related stats", are you pointing to delete or anything? Because we
normally dont performa DELETE but just use partition DROP for data purge on
these tables. Below is the stats specific to ASSM i do observe , and those i
have not noticed in past frequently. And also as we encounter this slow run
intermittently, so not sure if we are encountering any bugs related to ASSM
here?
