Re: Insert Query issue

It's good to see the session activity stats for a timed period.

In this case your figures are showing us that the two runs are basically doing the same work in the same way, but the slower session is suffering from slow (real) disk I/O, and getting a slighlty smaller fraction of its "disk" reads from cell flash cache. (Note that physical reads = physical read I/O requests, so it's all single block, which is consistent with seeking and updating the index; note also that the "cell flash cache read hits" is only a small fraction of the physical read I/O requests. so the drop from 2.07% to 1.69% is not particularly significant.

There is one interesting pair of numbers - though it's not telling us anything about performance. You've inserted (in the good run) roughly 38M rows in this interval; but the "Heap Segment Array Inserts" is 1.2M, which means (internally) Oracle is inserting about 30 rows per array (compared to your (external) array size of 352). But your figures for the size of the table (215GB, 1.7B rows) suggest a block should hold about 65 rows, and your delete "where cr_date < sysdate - 3" with no supporting index suggests a tablescan path which we might expect to empty blocks completely. This combination suggests there's a pattern created by past activity that could be producing interesting side-effects. (NOTE, though that whatever is happening it's happening in both the good and bad here, so this oddity isn't an explanation for the difference).

Although the session activity stats haven't provided an answer, they have eliminated some possibilities, so it is a step forward. The thing to do now is to extend the information you are collecting. Most of the time is I/O wait time, but there is some other time to account, and the exact breakdown of I/O time would be interesting; so next steps would be:

1. repeat this capture, but gather the session's event wait time (v$session_event) at the same time, and the SYSTEM stats and event times so that we can see if there's anything happening outside this session that is causing this session's I/O time. If possible, add to that capture the capture of the event histogram - v$event_histogram which will give you some idea of how even (or not) the slowness is by giving you a numeric summary histogram of the event times. There's an example at this URL: https://jonathanlewis.wordpress.com/2008/06/15/event-snapshots/ showing how I've set up some tests in the past, but it's easier in the short term to just take two snapshots and find the difference.
2. If all else fails to supply any clues enable sql_trace remotely for the session for a limited period with wait state tracing enabled to see if you can spot any pattern in the disk reads and times taken.

Regards
Jonathan Lewis

On Fri, 30 Jul 2021 at 12:13, Pap <oracle.developer35_at_gmail.com> wrote:

> Thank you Mark.
>
> I was wondering if it's a good idea to partition this table just to help
> it purge in a cleaner fashion. As because, if we just partition it to make
> our purging better we also have to see the impact on the read queries. To
> have the table partitioned with the delete key(i.e. create_timestamp), the
> primary key also has to include the delete key in it or else it will not
> allow it to make it a local index. And in case of global index we will have
> to go through index invalidation/rebuild stuff both in case of drop or
> partition exchange etc. And also the read queries now have to go through
> multiple local index segments scan vs the current one segment scan.
>
> I was trying to see/prove if it's really delayed block cleanout impacting
> INSERT, So i tried to collect the stats from v$sesstat for two different
> executions/data loads for close to ~10minutes duration though it's not
> exact(i think i monitored the bad execution for a bit longer duration). And
> as v$sesstat accumulates the statistics so I collected the delta value for
> each run. As per application, the first execution was running slow and the
> next one was running with better response. Attached are the stats for the
> same. But surprisingly I see the delta value of statistics 'transaction
> table consistent read - Undo records applied' is zero in both the cases. So
> I'm wondering if there are some other phenomena(other than delayed
> block cleanout) which are happening here and how can we fix? In both the
> cases I see from ASH the wait event (cell single block physical read) was
> highest logged against the table followed by the PK index.
>
>
>
>
>
>
>
>
>
> On Fri, Jul 30, 2021 at 3:10 AM Mark W. Farnham <mwf_at_rsiz.com> wrote:
>
>> That’s right.
>>
>>
>>
>> But what you probably **should** do is partition the table by your
>> delete key. And if you can change the insert into append, then you just
>> partition exchange out and drop the unwanted rows with nary a delete seen.
>> That also gives you a cheap ability to archive the swapped out partition to
>> a table to back up as a stand alone period of time.
>>
>>
>>
>> Good luck. IF Lothar’s suggestion of delayed block cleanout is correct,
>> either of these things should fix it. IF the deleted space being used
>> dramatically increasing the number of blocks you need to read via an
>> indexed access path by fubaring the actual cluster factor, the partitioning
>> fixes that.
>>
>>
>>
>> *From:* Pap [mailto:oracle.developer35_at_gmail.com]
>> *Sent:* Thursday, July 29, 2021 3:49 PM
>> *To:* Mark W. Farnham
>> *Cc:* Lothar Flatz; Oracle L
>> *Subject:* Re: Insert Query issue
>>
>>
>>
>> Thanks Mark. If I get it correct , you are suggesting a method to scan
>> the blocks in the buffer cache which are getting deleted. But considering
>> we are going to hold ~3days worth of data at any point in time, I was
>> thinking if it's easy one to just scan or gather stats with auto sample
>> size after delete to get the blocks cleaned. but before that is it the same
>> effect i.e. delayed block cleanout which we are suffering from here? I
>> thought Lothar might be pointing to some other similar effect but not
>> exactly delayed block cleanout. Please correct me if I'm wrong.
>>
>>
>>
>> On Fri, Jul 30, 2021 at 12:41 AM Mark W. Farnham <mwf_at_rsiz.com> wrote:
>>
>> If you stash (probably insert append, then after the subsequent read
>> truncate) into a table the rowid of all the rows you are about to delete
>> (using the same predicate), then do the delete, then commit, that will give
>> you the list of the candidate blocks. If you read from the table you
>> deleted from where rowid in (select rowid from the new table) from a
>> different node on your Exadata, you’re probably going to get that in sga.
>> IF not, you can pl/sql loop them one block at a time.
>>
>>
>>
>> *From:* oracle-l-bounce_at_freelists.org [mailto:
>> oracle-l-bounce_at_freelists.org] *On Behalf Of *Pap
>> *Sent:* Thursday, July 29, 2021 2:34 PM
>> *To:* Lothar Flatz
>> *Cc:* Oracle L
>> *Subject:* Re: Insert Query issue
>>
>>
>>
>> Thank you Lothar.
>>
>>
>>
>> It appears to be something matching to delayed block cleanout effect in
>> which post DML sometimes we have to perform a segment scan so that the
>> commit scn can be updated for all the blocks and for that sometimes we have
>> to do it using manual stats gathering or forced segment full scan to pass
>> all the data blocks through buffer cache. But for that we used to see one
>> stats i.e. 'transaction table consistent read undo records applied' on the
>> SELECT query execution.
>>
>>
>>
>> So is this tidying up of block post delete something similar as above and
>> we can get that verified and confirmed from some session statistics from
>> v$sesstat while the insert is running at run time? Also I am thinking what
>> can be done so as to tidying up the blocks after delete, if a force segment
>> full scan or stats gather post deletion can anyway help, so as to not
>> impact the insert performance?
>>
>>
>>
>>
>>
>>
>>
>>
>>
>> *Betreff: *
>>
>> Re: Insert Query issue
>>
>> *Datum: *
>>
>> Thu, 29 Jul 2021 19:59:25 +0200
>>
>> *Von: *
>>
>> Lothar Flatz <l.flatz_at_bluewin.ch> <l.flatz_at_bluewin.ch>
>>
>> *An: *
>>
>> oracle-l_at_freelists.org
>>
>>
>>
>> Hi,
>>
>> there reason should be that after a delete rows are marked as technical
>> deleted, but the block is not yet tidied up. (I suppose that would mean
>> e.g. that other rows are not yet shifted for contiguous space etc.)
>> The tidying is done on the next insert, which therefore will be
>> substantially slower for once.
>> I heard this explanation long ago , but suppose it still holds.
>> My whole understanding of this is a bit vague.
>> I guess Jonathan could fill in some details here.
>>
>> Regards
>>
>> Lothar
>>
>> Am 29.07.2021 um 18:10 schrieb Pap:
>>
>> This legacy database is on version 11.2.0.4 of oracle Exadata.
>>
>> We are having below conventional insert query introduced and its a bulk
>> insert happening with an arraysize of ~352 from informatica. And the
>> response time was ~3millisec/execution i.e. for one chunk or ~352 rows. But
>> we noticed suddenly the response time becomes
>> tripled(~8-9millisec/execution) for the same set of rows/array size insert.
>> No changes done to the underlying table structure.
>>
>> During digging into the AWR views we found that the increase in response
>> time for this insert query started after we introduced the delete script to
>> purge data from this table. The delete is also happening in bulk fashion
>> with an arraysize of 100K through plsql procedure and its deleting data
>> with CRE_TS<sysdate-3. And it's clear from dba_hist_sqlstat that it's the
>> cpu_time_delta and IOWAIT_DELTA which increased thus making the
>> ELAPSED_TIME_delta go up by three times for the INSERT query. No such
>> increase in BUFFER_GETS_delta observed. But one thing we notice is that the
>> DELETE is not running at exactly the same time as the insert/data load
>> script runs. So howcome this delete is impacting the INSERT query? Is this
>> that as delete is making some blocks empty so that now the INSERT has to
>> now see through all to find the empty blocks before insert and thus
>> spending more time?
>>
>> The table is ~215GB in size and is not partitioned holding ~1.7billion
>> rows. There exists only a primary key composite index on two number data
>> type columns. With Blevel-3 , and having index size ~65GB with ~7.3million
>> leaf blocks.
>>
>>
>> Below is the AWR plan for the INSERT and DELETE queries.
>>
>> INSERT INTO TAB1(TAID,TID,AID,APP_NM,CRE_TS) VALUES ( :1, :2, :3,:4, :5)
>>
>>
>> -------------------------------------------------
>> | Id | Operation | Name | Cost |
>> -------------------------------------------------
>> | 0 | INSERT STATEMENT | | 1 |
>> | 1 | LOAD TABLE CONVENTIONAL | | |
>> -------------------------------------------------
>>
>> Query Block Name / Object Alias (identified by operation id):
>> -------------------------------------------------------------
>>
>> 1 - INS$1
>>
>> Note
>> -----
>> - cpu costing is off (consider enabling it)
>>
>>
>> DELETE FROM USER1.TAB1 WHERE ROWID = :1
>>
>> Plan hash value: 2488984540
>>
>>
>> ----------------------------------------------------------------------------------------------------
>> | Id | Operation | Name | Rows |
>> Bytes | Cost (%CPU)| Time |
>>
>> ----------------------------------------------------------------------------------------------------
>> | 0 | DELETE STATEMENT | | |
>> | 1 (100)| |
>> | 1 | DELETE | TAB1 | |
>> | | |
>> | 2 | TABLE ACCESS BY USER ROWID| TAB1 | 1 | 25
>> | 1 (0)| 00:00:01 |
>>
>> ----------------------------------------------------------------------------------------------------
>>
>> Query Block Name / Object Alias (identified by operation id):
>> -------------------------------------------------------------
>>
>> 1 - DEL$1
>> 2 - DEL$1 / TAB1_at_DEL$1
>>
>> Outline Data
>> -------------
>>
>> /*+
>> BEGIN_OUTLINE_DATA
>> IGNORE_OPTIM_EMBEDDED_HINTS
>> OPTIMIZER_FEATURES_ENABLE('11.2.0.4')
>> DB_VERSION('11.2.0.4')
>> ALL_ROWS
>> OUTLINE_LEAF(_at_"DEL$1")
>> ROWID(_at_"DEL$1" "TAB1"_at_"DEL$1")
>> END_OUTLINE_DATA
>> */
>>
>>
>>
>>

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