RE: Bind Sensitivity and PL/SQL cursor caching

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

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

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