Thanks foe the details Stefan.
Your mention of Bryn makes me realize my idea of a NO_SESSION_CACHE hint
wouldn't really be workable, as this change would have to come from the
PL/SQL layer, whereas SQL hints are only handled (I think) during SQL
optimization.
On 29 March 2017 at 16:58, Stefan Koehler <contact@xxxxxxxx> wrote:
Hey Patrick,
what you see here is an effect of the (old) soft-parse avoidance scheme
implementation. We already had discussions with Bryn about these issues as
it
also affects statistics / cardinality feedback.
There are some open bugs / enhancement requests (e.g. #8357294 or #
25158799) for these issues but Oracle has not adjusted the implementation
until yet
and if you look at the dates of these bugs - i guess we can't expect some
enhancements in near future.
Best Regards
Stefan Koehler
Independent Oracle performance consultant and researcher
Website: http://www.soocs.de
Twitter: @OracleSK
Upcoming online seminar: http://tinyurl.com/17-06-13-Shared-Pool-Internals
Patrick Jolliffe <jolliffe@xxxxxxxxx> hat am 29. März 2017 um 10:34geschrieben:
non-optimal plan was being used due to a combination of Bind Variable
Hi List,
Been investigating a PL/SQL process which was failing because a
Peeking and
data skew.was executed from SQLPlus, however retesting via PL/SQL showed that the plan
A /*+bind_aware*/ hint resolved my simple test when the statement it
being used was still that of the first bind variables encountered.have been proved by disabling this (session_cached_cursors=0).
I guessed problem was due to PL/SQL cursor caching, and this seems to
I have vastly simplified testcase and workaround and provide it below.wanted to throw this out there to see if any better solutions.
I understand what is going on, and have this workaround, but just
(Yes I know I could lock in an acceptable plan using SPM ormanipulating stats, but I think that is not really optimal, we really do
need different
plans for different bind variables).working with PL/SQL, but without having set/reset session_cached_cursors.
What I really want is somehow to get this bind_aware/sensitivity stuff
A /*+NO_SESSION_CACHE*/ hint would be ideal I think (any downsides orcomplexities for implementation?)
Any other ideas? (btw Oracle 12.1.0.2)METHOD_OPT=>'FOR ALL COLUMNS SIZE SKEWONLY');
Thanks in advance
Patrick
drop table skew_table;
create table skew_table nologging as
select rownum id,
case mod(rownum, 10000) when 0 then 0 else 1 end c10000,
rpad('X', 255, 'X') padding
from dual
connect by level <= 1e6;
create index skew_index on skew_table(c10000);
exec dbms_stats.gather_table_stats(NULL, 'SKEW_TABLE',
where c10000 = p;
create or replace procedure get_skew(p in number)
is
dummy number;
begin
select /*+ bind_aware sktest */ count(*) INTO dummy FROM skew_table
end;is_bind_aware from v$sql where sql_id = '1rg2w46daksr4';
/
declare
dummy number;
begin
get_skew(0);
get_skew(1);
end;
/
select child_number, executions, parse_calls, is_bind_sensitive,
is_bind_aware from v$sql where sql_id = '1rg2w46daksr4';
CHILD_NUMBER EXECUTIONS PARSE_CALLS I I
------------ ---------- ----------- - -
0 2 1 Y Y
declare
dummy number;
begin
execute immediate 'ALTER SESSION SET session_cached_cursors = 0';
get_skew(0);
get_skew(1);
execute immediate 'ALTER SESSION RESET session_cached_cursors';
end;
/
SQL> select child_number, executions, parse_calls, is_bind_sensitive,
CHILD_NUMBER EXECUTIONS PARSE_CALLS I I
------------ ---------- ----------- - -
0 3 3 Y Y
1 1 0 Y Y