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@xxxxxxxxx>
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@xxxxxxxxx>:
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@xxxxxxxxx> 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(@"SEL$1")
FULL(@"SEL$1" "P"@"SEL$1")
END_OUTLINE_DATA
*/
Note
-----
- dynamic statistics used: statistics for conventional DML
