PRECOMPUTE_SUBQUERY hint

I’ve just found out that we can specify query block for PRECOMPUTE_SUBQUERY: /*+ precompute_subquery(@sel$2) */
So we can use it now with SQL profiles, SPM baselines and patches.

SQL> select/*+ precompute_subquery(@sel$2) */ * from dual where dummy in (select chr(level) from dual connect by level<=100);

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SQL> @last

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID  c437vsqj7c4jy, child number 0
-------------------------------------
select/*+ precompute_subquery(@sel$2) */ * from dual where dummy in
(select chr(level) from dual connect by level<=100)

Plan  (more...)

How to find out session info about session that comes from remote database through dblink

It is well known thing and you can even find it on MOS, but I have a little more simple script for it, so I want to show little example.

First of all we need to start script on local database:

SQL>                                                                                                                                                                   
SQL> @transactions/global.sql
Enter filters(empty for any)...
Sid           :
Globalid mask :
Remote_db mask:

 INST_ID  SID    SERIAL# USERNAME REMOTE_DB REMOTE_DBID TRANS_ID         DIRECTION   GLOBALID                                           EVENT                      
-------- ----  (more...)

Standalone sqlplus script for plans comparing

I have a couple scripts for plans comparing:

1. https://github.com/xtender/xt_scripts/blob/master/diff_plans.sql
2. http://github.com/xtender/xt_scripts/blob/master/plans/diff_plans_active.sql

But they have dependencies on other scripts, so I decided to create a standalone script for more convenient use without the need to download other scripts and to set up the sql*plus environment.
I’ve tested it already with firefox, so you can try it now: http://github.com/xtender/xt_scripts/blob/master/plans/diff_plans_active_standalone.sql

Some screenshots:
diff_plans.sql:
diff_plans

plans_active.sql:
plans_active

Usage:
1. plans_active:

SQL> @plans_active  (more...)

Bug with xmltable, xmlnamespaces and xquery_string specified using bind variable

Today I was asked about strange problem: xmltable does not return data, if xquery specified by bind variable and xml data has xmlnamespaces:

SQL> var x_path varchar2(100);
SQL> var x_xml  varchar2(4000);
SQL> col x format a100;
SQL> begin
  2      :x_path:='/table/tr/td';
  3      :x_xml :=q'[
  4                  <table xmlns="http://www.w3.org/tr/html4/">
  5                    <tr>
  6                      <td>apples</td>
  7                      <td>bananas</td>
  8                    </tr>
  9                  </table>
 10                  ]';
 11  end;
 12  /

PL/SQL procedure successfully completed.

SQL> select
  2        i, x
  3   from  (more...)

REGEXP_LIKE: strange unspecified value in parameter “modifier”

Today I noticed strange thing in predicate section of execution plan for simple query with regexp_like, where was not specified 3rd parameter “MODIFIER”:

SQL> select * from dual where regexp_like(dummy,'.');

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SQL> select * from table(dbms_xplan.display_cursor);

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------
SQL_ID  97xuqf9cmjsta, child number 0
-------------------------------------
select * from dual where regexp_like(dummy,'.')

Plan hash value: 272002086

--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
 (more...)

Strange moving filter predicates from index to table

It seems strange to me:
When all needed columns are in the index, filter predicates are expectedly applied to the index

select a,b from xt_test where a=1 and
(:b is null or b = :b)

-----------------------------------------------------------------------------------------
| Id  | Operation        | Name       | Starts | E-Rows | A-Rows |   A-Time   | Buffers |
-----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT |            |      1 |        |      1 |00:00:00.01 |       2 |
|*  1 |  INDEX RANGE  (more...)

To_char, Infinity and NaN

Funny that oracle can easily cast ‘nan’,'inf’,'infinity’,'-inf’,'-infinity’ to corresponding binary_float_infinity,binary_double_nan, but there is no any format models for to_char(binary_float_infinity,format) or to_binary_***(text_expr,format) that can output the same as to_char(binary_float_infinity)/to_binary_float(‘inf’) without format parameter:

If a BINARY_FLOAT or BINARY_DOUBLE value is converted to CHAR or NCHAR, and the input is either infinity or NaN (not a number), then Oracle always returns the pound signs to replace the value.

Little example:

SQL> select to_binary_float('inf') from dual;

TO_BINARY_FLOAT('INF')
----------------------
                   Inf

 (more...)

Deterministic functions, result_cache and operators

In previous posts about caching mechanism of determinstic functions I wrote that cached results are kept only between fetch calls, but there is one exception from this rule: if all function parameters are literals, cached result will not be flushed every fetch call.
Little example with difference:

SQL> create or replace function f_deterministic(p varchar2)
  2     return varchar2
  3     deterministic
  4  as
  5  begin
  6     dbms_output.put_line(p);
  7     return p;
  8  end;
  9  /
SQL> set  (more...)

When v$sesstat statistics are updated

Craig Shallahamer wrote excellent article “When is v$sesstat really updated?”.
And my today post just a little addition and correction about the difference of updating ‘Db time’ and ‘CPU used by this session’ statistics.

Test #1

In this test I want to show that the statistics will be updated after every fetch call.
I have set arraysize=2, so sql*plus will fetch by 2 rows:
(full script)

-- Result will be fetched by  (more...)

Single SQL vs SQL+PL/SQL

Everyone knows Tom Kyte’s mantra:

You should do it in a single SQL statement if at all possible.

But we all know that “Every rule has an exception
There are many different cases when pl/sql with sql can be more efficient than only sql, and i dont want to catalog them. I just want to show a couple examples of such exceptions:

1. Running totals by several dimensions

Simple example from forum:

 (more...)