Re: Unique index access path seems very slow

The reason you're not likely to have seen anything about tuning the Bloom filter is because you're one of a small set of people doing a certain type of processing with very large volumes of data on Exadata. The tuning we're trying to handle has come up because we're trying to get the largest possible Bloom filter that will be sent down to the cell and applied there. It's a little unlikely that anyone would notice the possible effect of the combination. On top of that the initial allocation of memory depends on what Oracle thinks it will need - and the effectiveness of the hash table will be affected by the size of the initial estimates.

I thought I'd written a reply explaining the effects you were seeing from the different cardinality hints you were trying, but if I have it's not going into the mail. I'll try to find time to rewrite over the weekend.

You might like to try a series of test to find a sweet spot. After see the effect of 50K I think I'd try 100K, 150K, 200K, 250K. as the cardinality goes up I think the offload will increase until it suddenly drops off because the Bloom filter has got too big to send to the cell. (To be honest I'd like to get my hands on your database for a few hours to experiment with this detail.)

Regards
Jonathan Lewis

On Fri, 10 Feb 2023 at 18:16, Pap <oracle.developer35_at_gmail.com> wrote:

> Thank You Jonathan and Yudhi.
>
> I think one key learning I have got here which is not mentioned in any
> blog or book, I. E, I have never thought of there exists a way to control
> the size of bloom filters. i.e. the technique by which if we want to
> influence oracle to do more work in terms of how big bloom filters it
> applies there to filter out maximum rows , it can be done using simple
> cardinality hints. And this will be beneficial if we want to send a minimum
> amount of rows to the subsequent steps of the execution, also this can be
> helpful to minimize tempspill in case it's happening in subsequent rows
> passed to the hash join. This is awesome. Thanks to Jonathan.
>
> With regards to the number of hash partitions I am not sure if there is a
> best practice to follow for deciding the number of hash partitions here?
> From your response it seems <=10GB is the optimal size per partition.
> Correct me if wrong.
>
>
> On Thu, Feb 9, 2023 at 1:43 PM yudhi s <learnerdatabase99_at_gmail.com>
> wrote:
>
>> Regarding your question on number of HASH partitions for this ~900GB
>> table. So it depends up on multiple factors like , say you should not make
>> it very large number of partition so that can be overhead on parsing time
>> for any quick queries and it will also flood your data dictionary (say
>> histogram for each column across all partition etc). Also if you create a
>> local index there will be those number of index partitions/segments to be
>> scanned by the query which will use that index and that would not be a good
>> thing. But looking your current use case, you want to perform partition
>> wise join with the other table as efficiently as possible, so it seems your
>> 256 hash partition will keep each partition size <4GB(Considering ~900GB
>> table size) and also if in future your data becomes double then too each of
>> your partition size will stays <10GB.
>>
>> Regarding your HASH JOIN trace file i cant comment much. Jonathan may put
>> his thought if any clue there which may improve things. BTW I was thinking
>> if you pass the hash_area_size higher(~2GB) for that session using
>> workarea_size_policy as manual will that help anyway?
>>
>>
>>
>> On Thu, 9 Feb, 2023, 12:42 am Pap, <oracle.developer35_at_gmail.com> wrote:
>>
>>> Thank you so much Jonathan and Yudhi.
>>>
>>> Jonathan, To your point "*Side note: Unless things have changed in
>>> recent versions a 10104 trace will tell you about what's going on with the
>>> hash join, including the number of buckets in the hash table and the
>>> distribution of rows per bucket - that might be quite interesting and
>>> corroborate some of my comments about collisions/false positives etc"*
>>>
>>> I was struggling to generate the '10104' trace initially as I was trying
>>> to do it as level 10. But then I just removed the level from there and saw
>>> the HASH join information populated in the trace file, below is the git
>>> link. Not able to interpret much of it , however I do see a few
>>> things...like say, In one case 'number of partitions fit in memory' is 32
>>> VS 8 in another. The Total number of rows in in-memory partitions is
>>> 2548554 in both cases.
>>>
>>> Not sure if it gives any clue which would help here making existing
>>> query better, but in coming days, yes we are planning to make the
>>> tab_encrypt table(which is ~900GB in size) as ~256 HASH partitions so that
>>> each partition would be small enough i.e. ~3.5GB in size. And the same
>>> number of HASH subpartitions for the transaction table, I hope that will
>>> work in this scenario.
>>>
>>> Below is the trace with cardinality hint 10M(where the bloom filter was
>>> more effective) vs cardinality hint 50K(where the bloom filter was small).
>>>
>>> ALTER SESSION SET EVENTS '10104 trace name context forever';
>>>
>>> **** with cardiality hint of 10Million ********
>>> https://gist.github.com/oracle9999/fb0598d04b0c0938bb6de695c20131fb
>>>
>>> **** with cardinality hint of 50K ********
>>> https://gist.github.com/oracle9999/a9145310a6158f151ac9e04f2bf3ba31
>>>
>>>
>>>
>>> On Thu, Feb 2, 2023 at 8:51 PM Jonathan Lewis <jlewisoracle_at_gmail.com>
>>> wrote:
>>>
>>>>
>>>> Yuhdi,
>>>>
>>>> As I pointed out, I don't think that there's likely to be *much*
>>>> change in performance by hacking in a different cardinality estimates; but
>>>> we have seen that the two different figures produce significant changes in
>>>> WHERE the time is spent and some change in the effectiveness of
>>>> off-loading. Given that clue (and assuming that there isn't a more
>>>> important task to address) I would have spent an hour or two re-running the
>>>> query with a few different cardinality hints between 49K and 2M to see if
>>>> there was a sweet spot that reduced the CPU required to apply the filter,
>>>> maximised the effectiveness of offloading, and minimised the number of rows
>>>> passed up the plan.
>>>>
>>>> IIRC none of the plans showed any writes on the hash join, so I wasn't
>>>> thinking about overheads of hash joins spilling to disk.
>>>>
>>>> The suggestion for re-engineering the data so that Oracle could iterate
>>>> through a partition-wise join was also about offload and CPU efficiency. On
>>>> smaller data volumes a hash table could have both a smaller number of
>>>> buckets and be more accurate in its distribution, so a Bloom filter could
>>>> be more effective and cheaper to use on the offload.
>>>>
>>>> The switch to RAW, of course, is mostly about reducing I/O: the very
>>>> slow runs are probably about resource use by other users on the Cell
>>>> Servers so a smaler data size means less I/O which means less impact when
>>>> the hardware gets busy; it did occur to me to wonder if the CPU cost of
>>>> hashing a 64 byte raw would be less than the cost of hashing a 128 byte
>>>> varchar (answer: probably) which would also reduce run time and the load on
>>>> the cell server (and that last one woudl reduce the risk of large volumes
>>>> of data being sent unprocessed to the database server).
>>>>
>>>>
>>>>
>>>> Regards
>>>> Jonathan Lewis
>>>>
>>>>
>>>>
>>>>
>>>> On Tue, 31 Jan 2023 at 19:54, yudhi s <learnerdatabase99_at_gmail.com>
>>>> wrote:
>>>>
>>>>> So Jonathan, OP has supplied plans which shows both cases, I. E, with
>>>>> large estimation the bigger bloom filter is consuming additional CPU cycle
>>>>> and with smaller estimation the hash join is consuming higher CPU cycle.
>>>>> But both the cases the total query execution time is closely equal, also op
>>>>> mentioned both the plans running for ~30minutes+ many times of the day
>>>>> so...
>>>>>
>>>>> when you said below I. E favoring large bloom filter option, so I am
>>>>> wondering if it's because it might help in less temp spill? Or say, do you
>>>>> mean its better option of hinting the inline view or tran_tab estimation
>>>>> very high so that a bigger bloom filter will be applied and the lesser
>>>>> amounts of rows will be passed to the hash join which may also benefit in
>>>>> case of large data volume as temp spill will be minimal?
>>>>>
>>>>> *Note that the Offload Returned Bytes was 300GB for the 49K estimate
>>>>> with the small Bloom filter, and 500GB for the 2M estimate with the large
>>>>> Bloom filter.*
>>>>> *It looks like we need to "fake" the system so that the Bloom filter
>>>>> (estimate) is large enough to eliminate a lot of data while being small
>>>>> enough to be sent to the cell server so that the 14 concurrently active
>>>>> cells can do the row elimination. Beyond that I don't think there's a way
>>>>> to make the query go faster than the (roughly) 650 seconds you've seen so
>>>>> far*.
>>>>>
>>>>>
>>>>> On Tue, 31 Jan, 2023, 3:51 am Jonathan Lewis, <jlewisoracle_at_gmail.com>
>>>>> wrote:
>>>>>
>>>>>> Comparing the 5 hash join plans you've posted:
>>>>>>
>>>>>>

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