To me, ACS is an indication that you're using binds instead of literals.
Binds exist partly so you can have SQL and execution plans shared for all
values.
With a massive sweeping generalisation, a whole bunch of additional complicated
features exist to combat the misuse of with binds or literals.
In plsql you have the flexibility to rewrite the code to either always use
literals or to code different sql statements for different sets of values.
Cheers
Dominic
Sent from my Windows Phone
________________________________
From: Patrick Jolliffe<mailto:jolliffe@xxxxxxxxx>
Sent: 29/03/2017 09:36
To: oracle-l<mailto:oracle-l@xxxxxxxxxxxxx>
Subject: Bind Sensitivity and PL/SQL cursor caching
Hi List,
Been investigating a PL/SQL process which was failing because a non-optimal
plan was being used due to a combination of Bind Variable Peeking and data skew.
A /*+bind_aware*/ hint resolved my simple test when the statement it was
executed from SQLPlus, however retesting via PL/SQL showed that the plan being
used was still that of the first bind variables encountered.
I guessed problem was due to PL/SQL cursor caching, and this seems to have been
proved by disabling this (session_cached_cursors=0).
I have vastly simplified testcase and workaround and provide it below.
I understand what is going on, and have this workaround, but just wanted to
throw this out there to see if any better solutions.
(Yes I know I could lock in an acceptable plan using SPM or manipulating stats,
but I think that is not really optimal, we really do need different plans for
different bind variables).
What I really want is somehow to get this bind_aware/sensitivity stuff working
with PL/SQL, but without having set/reset session_cached_cursors.
A /*+NO_SESSION_CACHE*/ hint would be ideal I think (any downsides or
complexities for implementation?)
Any other ideas? (btw Oracle 12.1.0.2)
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', METHOD_OPT=>'FOR ALL
COLUMNS SIZE SKEWONLY');
create or replace procedure get_skew(p in number)
is
dummy number;
begin
select /*+ bind_aware sktest */ count(*) INTO dummy FROM skew_table where
c10000 = p;
end;
/
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$sql where sql_id = '1rg2w46daksr4';
CHILD_NUMBER EXECUTIONS PARSE_CALLS I I
------------ ---------- ----------- - -
0 2 1 Y Y
declare
dummy number;
begin
execute immediate 'ALTER SESSION SET session_cached_cursors = 0';
get_skew(0);
get_skew(1);
execute immediate 'ALTER SESSION RESET session_cached_cursors';
end;
/
SQL> select child_number, executions, parse_calls, is_bind_sensitive,
is_bind_aware from v$sql where sql_id = '1rg2w46daksr4';
CHILD_NUMBER EXECUTIONS PARSE_CALLS I I
------------ ---------- ----------- - -
0 3 3 Y Y
1 1 0 Y Y
