Re: Locking issue

Thank you.
Do you mean to say even the application query is SELECT query but because it's using dynamic statistics so its updating some internal dictionary cache objects and thus needs a lock. And we know that stats gather does updates data dictionary objects , so eventually they are entering in this conflicting situation.
 And in this case if we would have ensured accurate stats on the table and then it wouldn't have gone for dynamic sampling. Correct me if wrong.

It's version 19.9.0.0.0 of oracle.

On Thu, 20 May 2021, 2:23 am Nenad Noveljic, <nenad.noveljic_at_gmail.com> wrote:

> There is a buggy (in my opinion) code path in statistics gathering which
> acquires a row cache lock in the shared mode and then waits on a library
> cache lock held by another session. The other session holds a library cache
> lock and wants to acquire the row cache lock being held by the statistics
> job.
>
> I reproduced a such case with the debugger:
> https://nenadnoveljic.com/blog/deadlock-row-cache-lock-library-cache-lock/
>
> A boundary condition leading to the problem in your case - I’m guessing-
> might be dynamic sampling, which might require row cache lock to update
> sampled statistics.
>
> Best regards,
> Nenad
>
> Von meinem iPhone gesendet
>
> Am 19.05.2021 um 22:21 schrieb Lok P <loknath.73_at_gmail.com>:
>
> 
> Also initially these application queries which were experiencing "row
> cache lock" and were not in parsing stage but later on some other session
> spawn and were stuck in "cursor: pin S wait on X" and were in parse
> stage only. So I'm wondering why it's behaving in such a way ?
>
> Also as the cache id for the blocked session were referring to dc_realtime_tabst
> and dc_realtime_colst , is it correct to assume that the stats gathering
> session must be having some kind of read lock on these dictionary cache
> objects? But ideally it should not block read queries.
>
> Regards
> Lok
>
> On Thu, May 20, 2021 at 1:39 AM Lok P <loknath.73_at_gmail.com> wrote:
>
>> We are seeing some strange locking situations because of statistics
>> gathering. While the stats gathering was running on an object and at same
>> time a SELECT on that object getting 'row cache lock', We want to
>> understand why it's happening so? Has it anything to do with the 'dynamic
>> statistics usage' which we are seeing in the note section of the plan?
>>
>> Below is the occurrence of the issue..
>>
>> Stats gather started non partition table -TAB1 as part of auto gather job
>> (i.e. gather_database_stats_job_proc) and it started around 10:08PM. And
>> multiple internal sqls were running as part of that stats gather and around
>> 11:06PM we last saw the below statement executed in that session.
>>
>> select /*+ opt_param('_optimizer_use_auto_indexes'
>> 'on') no_parallel_index(t, "XXX") dbms_stats cursor_sharing_exact
>> use_weak_name_resl dynamic_sampling(0) no_monitoring xmlindex_sel_idx_tbl
>> opt_param('optimizer_inmemory_aware' 'false') no_substrb_pad no_expand
>> index_ffs(t,"XXXX") */ count(*) as nrw,count(distinct
>> sys_op_lbid(228647,'L',t.rowid)) as nlb,count(distinct
>> hextoraw(sys_op_descend("XXX")||sys_op_descend("XXX")||sys_op_descend("XXX")))
>> as ndk,sys_op_countchg(substrb(t.rowid,1,15),1) as clf from "USER1"."TAB1"
>> sample block ( .0413656293,1) t where "XXX" is not null or "XXX" is not
>> null or "XXXX" is not null
>>
>> Then at around 11:07:00.687 PM, we have below application query started
>> running from multiple sessions and started experiencing "row cache lock"
>> with P1(cache id=62) as "dc_realtime_colst" , P2(mode) as 3 i.e. 'share
>> mode' and P3(request) as 3 i.e. 'share mode'. Nothing found in the blocking
>> session. Subsequently few other session of same query went into "row cache
>> lock" with P1(cache id=63) as "dc_realtime_tabst" , P2(mode) as 0 i.e.
>> 'null mode - not locked' and P3(request) as 3 i.e. 'share mode'.
>>
>> Then we noticed at around 11:07:00.695 PM , the same stats gather
>> session itself started experiencing "row cache lock" with P1(cache id=62)
>> as "dc_realtime_colst" , P2(mode) as 0 i.e. 'null mode - not locked' and
>> P3(request) as 5 i.e. 'exclusive mode'.
>>
>> SELECT /*+ FULL(P) +*/ * FROM "TAB1" P
>>
>>
>> ------------------------------------------------------------------------------------------------------------------
>>
>> | Id | Operation | Name | Rows | Bytes | Cost
>> (%CPU)| Time |
>>
>>
>> ------------------------------------------------------------------------------------------------------------------
>>
>> | 0 | SELECT
>> STATEMENT | | | | 3777K(100)| |
>>
>> | 1 | TABLE ACCESS STORAGE FULL FIRST ROWS|
>> TAB1 | 733M| 169G| 3777K (1)| 00:02:28 |
>>
>>
>> ------------------------------------------------------------------------------------------------------------------
>>
>> Outline Data
>>
>> -------------
>>
>> /*+
>>
>> BEGIN_OUTLINE_DATA
>>
>> IGNORE_OPTIM_EMBEDDED_HINTS
>>
>> OPTIMIZER_FEATURES_ENABLE('19.1.0')
>>
>> DB_VERSION('19.1.0')
>>
>> OPT_PARAM('_fix_control' '27268249:0')
>>
>> FIRST_ROWS
>>
>> OUTLINE_LEAF(_at_"SEL$1")
>>
>> FULL(_at_"SEL$1" "P"_at_"SEL$1")
>>
>> END_OUTLINE_DATA
>>
>> */
>>
>>
>>
>> Note
>>
>> -----
>>
>> - dynamic statistics used: statistics for conventional DML
>>
>>
>>
>

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