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
