Re: Explain Plan and Security

From: Jonathan Lewis <jonathan_at_jlcomp.demon.co.uk>
Date: Thu, 14 Jun 2018 17:01:27 +0000
Message-ID: <MM1P123MB084233161C3EDB4720AC90E2A57D0_at_MM1P123MB0842.GBRP123.PROD.OUTLOOK.COM>


Andy,

I don't think I'd even be that generous. I can't think of any detail where dbms_sql/dbms_xplan.display_cursor() gives you safer information than explain plan / dbms_xplan.display(). I suppose the extra complexity of using dbms_sql might make you a little more careful as you set up a test, and that could be a benefit.

Regards
Jonathan Lewis



From: andyklock_at_gmail.com <andyklock_at_gmail.com> on behalf of Andy Klock <andy_at_oracledepot.com> Sent: 14 June 2018 17:15
To: Jonathan Lewis
Cc: oracle-l_at_freelists.org
Subject: Re: Explain Plan and Security

Ah! Indeed. Thanks for that Jonathan. So, the takeaway is that DBMS_SQL is slightly better than EXPLAIN PLAN. But, only slightly?

Thanks!

Andy K

On Thu, Jun 14, 2018 at 12:08 PM, Jonathan Lewis <jonathan_at_jlcomp.demon.co.uk<mailto:jonathan_at_jlcomp.demon.co.uk>> wrote:

Andy,

If you add a "describe columns" to your SQL PARSE TEST BIND you find that you do get a plan before the execute.

DECLARE
  cursor_name INTEGER;
  rows_processed INTEGER;
  m_desc_table dbms_sql.desc_tab;
  m_colcount number;

BEGIN
  cursor_name := dbms_sql.open_cursor;

  dbms_sql.parse(cursor_name, 'select /* SQL_PARSE TEST BIND */ * from parse_test where n = :n', dbms_sql.NATIVE);
  dbms_sql.bind_variable(cursor_name, ':n', 2);
  dbms_sql.describe_columns( cursor_name, m_colcount, m_desc_table );
END;
/

SQL> select sql_id, plan_hash_value, sql_text from V$sql where sql_text like '%SQL_PARSE TEST BINDwq%'   2 /

SQL_ID PLAN_HASH_VALUE
------------- ---------------
SQL_TEXT



911jt1m3dxrba 903671040
select sql_id, plan_hash_value, sql_text from V$sql where sql_text like '%SQL_PARSE TEST BINDwq%'

Regards
Jonathan Lewis



From: andyklock_at_gmail.com<mailto:andyklock_at_gmail.com> <andyklock_at_gmail.com<mailto:andyklock_at_gmail.com>> on behalf of Andy Klock <andy_at_oracledepot.com<mailto:andy_at_oracledepot.com>> Sent: 14 June 2018 16:56
To: mauro.pagano_at_gmail.com<mailto:mauro.pagano_at_gmail.com> Cc: Jonathan Lewis; oracle-l_at_freelists.org<mailto:oracle-l_at_freelists.org>; l.flatz_at_bluewin.ch<mailto:l.flatz_at_bluewin.ch> Subject: Re: Explain Plan and Security

Agreed. It's kind of a cool idea, however, BINDs are checked after the DBMS_SQL.PARSE call and is only evaluated after the call to DBMS_SQL.EXECUTE

https://docs.oracle.com/database/121/ARPLS/d_sql.htm#i996870

I ran a quick test to see what shows up in the cursor cache after setting bind_variable, but not calling EXECUTE and as expected you don't get a plan at all.

SQL> DECLARE
 cursor_name INTEGER;
 rows_processed INTEGER;
BEGIN
  cursor_name := dbms_sql.open_cursor;
  dbms_sql.parse(cursor_name, 'select /* SQL_PARSE TEST */ * from parse_test where n = 1', dbms_sql.NATIVE); END;
/

PL/SQL procedure successfully completed.

SQL> DECLARE
 cursor_name INTEGER;
 rows_processed INTEGER;
BEGIN
  cursor_name := dbms_sql.open_cursor;
  dbms_sql.parse(cursor_name, 'select /* SQL_PARSE TEST BIND */ * from parse_test where n = :n', dbms_sql.NATIVE);   dbms_sql.bind_variable(cursor_name, ':n', 2); END;
/

PL/SQL procedure successfully completed.

SQL> DECLARE
 cursor_name INTEGER;
 rows_processed INTEGER;
BEGIN

  cursor_name := dbms_sql.open_cursor;

  dbms_sql.parse(cursor_name, 'select /* SQL_PARSE TEST BIND 2 */ * from parse_test where n = :n', dbms_sql.NATIVE);
  dbms_sql.bind_variable(cursor_name, ':n', 2);
  rows_processed := dbms_sql.execute(cursor_name);
  dbms_sql.close_cursor(cursor_name);

END;
/

PL/SQL procedure successfully completed.

SQL> _at_findsq SQL_PARSE

SQL_ID PLAN_HASH_VALUE LENGTH(SQL_FULLTEXT) SUB_TEXT EXECUTIONS AVG_ELAPSED

------------- --------------- -------------------- -------------------------------------------------- ---------- -----------
4wbhzrjq0k7fd     464636435 57 select /* SQL_PARSE TEST */ * from parse_test wher        0     .002671
8xbgz1hbjk0vz     0 63 select /* SQL_PARSE TEST BIND */ * from parse_test        0     .000467
0bavj3vaszvw2     464636435 65 select /* SQL_PARSE TEST BIND 2 */ * from parse_te        1      .00503

Final note, if you don't actually execute the SQL then you don't get all that other Oracle runtime stuff like cardinality feedback or dynamic sampling, etc which adds to even more headaches.

Andy K

On Thu, Jun 14, 2018 at 11:45 AM, Mauro Pagano <mauro.pagano_at_gmail.com<mailto:mauro.pagano_at_gmail.com><mailto:mauro.pagano_at_gmail.com<mailto:mauro.pagano_at_gmail.com>>> wrote: Lothar,

To add on Jonathan's "odd note", because of 9630032 (disabled by default) you might see an even odder behavior (difference between describe vs exec). Just saying that DBMS_SQL might translate in some headaches :-(

Cheers,
Mauro

On Thu, Jun 14, 2018 at 7:13 AM, Jonathan Lewis <jonathan_at_jlcomp.demon.co.uk<mailto:jonathan_at_jlcomp.demon.co.uk><mailto:jonathan_at_jlcomp.demon.co.uk<mailto:jonathan_at_jlcomp.demon.co.uk>>> wrote:

As far as I know explain plan will produce a misleading plan only if:

  1. the query uses bind variables - which can't be peeked and are assumed to be character or
  2. the literals used in the explain plan are a bad choice compared to what happens in production (which includes wrong type, wrong character set, wrong implicit date format etc.)

Using dbms_sql won't (necessarily) be any better. If you supply a statement with a bind variable in the text the call to dbms_parse will assume that it's an unknown varchar - just as explain plan will. This is why you sometimes see systems with lots of statements parsed twice per execute - the first time was a parse call the that used guesses for bind types, the second was with information about the actual bind types.

(I have an odd note from 16 years ago that you don't get the plan on the call to dbms_parse, but have to call dbms_describe_colums as well).

Regards
Jonathan Lewis



From: oracle-l-bounce_at_freelists.org<mailto:oracle-l-bounce_at_freelists.org><mailto:oracle-l-bounce_at_freelists.org<mailto:oracle-l-bounce_at_freelists.org>> <oracle-l-bounce_at_freelists.org<mailto:oracle-l-bounce_at_freelists.org><mailto:oracle-l-bounce_at_freelists.org<mailto:oracle-l-bounce_at_freelists.org>>> on behalf of l.flatz_at_bluewin.ch<mailto:l.flatz_at_bluewin.ch><mailto:l.flatz_at_bluewin.ch<mailto:l.flatz_at_bluewin.ch>> <l.flatz_at_bluewin.ch<mailto:l.flatz_at_bluewin.ch><mailto:l.flatz_at_bluewin.ch<mailto:l.flatz_at_bluewin.ch>>> Sent: 14 June 2018 13:36:46
To: oracle-l_at_freelists.org<mailto:oracle-l_at_freelists.org><mailto:oracle-l_at_freelists.org<mailto:oracle-l_at_freelists.org>> Subject: Explain Plan and Security

Hi,

you might know Kerryīs classic blog: http://kerryosborne.oracle-guy.com/2008/10/explain-plan-lies/. Normally my work around for explain plan issues is to run the query and use dbms_xplan.display_cursor. Now I am working in an environment where I must not run a query, but I can do explain plan. But still I think I can not tolerate explain plan weaknesses. I think it should be possble to use DBMS_SQL to parse a statement and receive a proper plan without actually running the statement. Then use dbms_xplan.display_cursor.
Before I spent time, has anybody done it already?

Regards

Lothar

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Received on Thu Jun 14 2018 - 19:01:27 CEST

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