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> @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:


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:

Some screenshots:


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="">
  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,'.');


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

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     |

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;



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:


Friday prank: select from join join join

Valid funny queries :)

select the join from join join join using(the,some) 
select some join from left join join right using(some,the) 
select 1 join from join join join join join using(the) on 1=1
select the some from join 
where the=some( the(select some from join) 
               ,the(select the  from join)

Spoiler:: tables SelectShow