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?
