Author Archives: Sayan Malakshinov

Little quiz: Ordering/Grouping – Guess the output

Posted on by Posted in curious, oracle, SQL Leave a comment

How many times have you guessed the right answer? ๐Ÿ™‚

1 

select * from dual order by -1;
select * from dual order by 0;

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2 

select *                   from dual                                     order by -(0.1+0/1) desc;
select 1 n,0 n,2 n,0 n,1 n from dual group by grouping sets(1,2,3,2,1,0) order by -(0.1+0/1) desc;

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3 

select 1 n,0 n,2 n,0 n,1 n from dual group by grouping sets(1,2,3,2,1,0) order by 0;
select 1 n,0 n,2 n,0 n,1 n from dual group by grouping sets(1,2,3,2,1,0) order by 0+0;
select 1 n,0 n,2 n,0 n,1 n from dual group by grouping sets(1,2,3,2,1,0) order by 3+7 desc;
select 1 n,0 n,2 n,0 n,1 n from dual group by grouping sets(1,2,3,2,1,0) order by -(3.1+0f) desc;

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4 

select column_value x,10-column_value y from table(ku$_objnumset(5,4,3,1,2,3,4)) order by 1.9;
select column_value x,10-column_value y from table(ku$_objnumset(5,4,3,1,2,3,4)) order by 2.5;
select column_value x,10-column_value y from table(ku$_objnumset(5,4,3,1,2,3,4)) order by 2.7 desc;
select column_value x,10-column_value y from table(ku$_objnumset(5,4,3,1,2,3,4)) order by -2.7 desc;

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12c: Little test of “TABLE ACCESS INMEMORY FULL” with count stopkey

Posted on by Posted in inmemory 5 Comments

The table has 9M rows:

SQL> with function f return int is
  2       begin
  3          for r in (select value from v$mystat natural join v$statname where name like 'IM scan rows') loop
  4             dbms_output.put_line(r.value);
  5             return r.value;
  6          end loop;
  7       end;
  8  select f() from t_inmemory where rownum<=1
  9  ;
 10  /

       F()
----------
         0

1 row selected.

SQL> /

       F()
----------
    491436

1 row selected.

SQL> /

       F()
----------
    982872

1 row selected.
DDL and Plan 

create table t_inmemory inmemory
as 
with gen as (select 0 id from dual connect by level<=3e3)
select 0 n from gen,gen;

SQL_ID  cpgrrfv9h6m52, child number 0
-------------------------------------
with function f return int is      begin         for r in (select value
from v$mystat natural join v$statname where name like 'IM scan rows')
loop            dbms_output.put_line(r.value);            return
r.value;         end loop;      end; select f() from t_inmemory where
rownum<=1

Plan hash value: 3697881339

----------------------------------------------------------------------------------
| Id  | Operation                   | Name       | Rows  | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |            |       |     3 (100)|          |
|*  1 |  COUNT STOPKEY              |            |       |            |          |
|   2 |   TABLE ACCESS INMEMORY FULL| T_INMEMORY |     1 |     3   (0)| 00:00:01 |
----------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter(ROWNUM<=1)

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select * from table where rownum=1

Posted on by Posted in adaptive serial direct path reads, CBO, oracle, troubleshooting 1 Comment

I never thought I would have to optimize so simple query as 

select col1, col2, col4, col7 from table where rownum=1

(even though I read recently “SELECT * FROM TABLE” Runs Out Of TEMP Space)
But a few days ago frequent executions of this query caused big problems on the one of our databases(11.2.0.3) because of adaptive serial direct path reads.

I don’t know why, but I felt intuitively that full table scan with “First K rows” optimization (“_optimizer_rownum_pred_based_fkr“=true) should turn off adaptive serial direct path reads. It seems quite logical to me.

PS. Unfortunately I had a little time, so I didn’t investigate what process and why it was doing that, I just created profile with “index full scan” access, and it completely solved the problem.

INDEX FULL SCAN (MIN/MAX) with two identical MIN()

Posted on by Posted in bug, CBO, oracle 1 Comment

I’ve just noticed an interesting thing:

Assume, that we have a simple query with “MIN(ID)” that works through “Index full scan(MIN/MAX)”:

SQL> explain plan for
  2  select
  3     min(ID)      as x
  4  from tab1
  5  where ID is not null;

Explained.

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

PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------------------
Plan hash value: 4170136576

---------------------------------------------------------------------------------------
| Id  | Operation                   | Name    | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |         |     1 |     4 |     3   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE             |         |     1 |     4 |            |          |
|   2 |   FIRST ROW                 |         |     1 |     4 |     3   (0)| 00:00:01 |
|*  3 |    INDEX FULL SCAN (MIN/MAX)| IX_TAB1 |     1 |     4 |     3   (0)| 00:00:01 |
---------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   3 - filter("ID" IS NOT NULL)
Test tables 
create table tab1(id, x, padding)
as 
   with gen as (select level n from dual connect by level<=1000)
   select g1.n, g2.n, rpad(rownum,10,'x')
   from gen g1,gen g2;
create index ix_tab1 on tab1(id, x);
exec dbms_stats.gather_table_stats('','TAB1');

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But look what will happen if we add one more “MIN(ID)”:

SQL> explain plan for
  2  select
  3     min(ID)      as x
  4   , min(ID)+1000 as x1000
  5  from tab1
  6  where ID is not null;

Explained.

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

PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------------
Plan hash value: 3397888171

---------------------------------------------------------------------------------
| Id  | Operation             | Name    | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------
|   0 | SELECT STATEMENT      |         |     1 |     4 |  3075  (17)| 00:00:02 |
|   1 |  SORT AGGREGATE       |         |     1 |     4 |            |          |
|*  2 |   INDEX FAST FULL SCAN| IX_TAB1 |   999K|  3906K|  3075  (17)| 00:00:02 |
---------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - filter("ID" IS NOT NULL)

Continue reading→

Just a couple of screenshots of sqlplus+rlwrap+cygwin+console

Posted on by Posted in oracle, SQL*Plus 2 Comments

I previously wrote that I peeped the idea about showing the session information in terminal title from Timur Akhmadeev’s screenshots, and Timur wrote:

I’m using (a bit modified) Tanel Poder’s login.sql available in his TPT scripts library: http://tech.e2sn.com/oracle-scripts-and-tools

Scripts:
Tanel’s i.sql
My title.sql and on_login.sql
View screenshots

Little script for finding tables for which dynamic sampling was used

Posted on by Posted in CBO, oracle, statistics 3 Comments

You can always download latest version here: http://github.com/xtender/xt_scripts/blob/master/dynamic_sampling_used_for.sql
Current source code:

col owner         for a30;
col tab_name      for a30;
col top_sql_id    for a13;
col temporary     for a9;
col last_analyzed for a30;
col partitioned   for a11;
col nested        for a6;
col IOT_TYPE      for a15;
with tabs as (
      select 
         to_char(regexp_substr(sql_fulltext,'FROM "([^"]+)"."([^"]+)"',1,1,null,1))  owner
        ,to_char(regexp_substr(sql_fulltext,'FROM "([^"]+)"."([^"]+)"',1,1,null,2))  tab_name
        ,count(*)                                                                    cnt
        ,sum(executions)                                                             execs
        ,round(sum(elapsed_time/1e6),3)                                              elapsed
        ,max(sql_id) keep(dense_rank first order by elapsed_time desc)               top_sql_id
      from v$sqlarea a
      where a.sql_text like 'SELECT /* OPT_DYN_SAMP */%'
      group by
         to_char(regexp_substr(sql_fulltext,'FROM "([^"]+)"."([^"]+)"',1,1,null,1))
        ,to_char(regexp_substr(sql_fulltext,'FROM "([^"]+)"."([^"]+)"',1,1,null,2))
)
select tabs.* 
      ,t.temporary
      ,t.last_analyzed
      ,t.partitioned
      ,t.nested
      ,t.IOT_TYPE
from tabs
    ,dba_tables t
where 
     tabs.owner    = t.owner(+)
 and tabs.tab_name = t.table_name(+)
order by elapsed desc
/
col owner         clear;
col tab_name      clear;
col top_sql_id    clear;
col temporary     clear;
col last_analyzed clear;
col partitioned   clear;
col nested        clear;
col IOT_TYPE      clear;

ps. Or if you want to find queries that used dynamic sampling, you can use query like that:

select s.*
from v$sql s
where 
  s.sql_id in (select p.sql_id 
               from v$sql_plan p
               where p.id=1
                 and p.other_xml like '%dynamic_sampling%'
              )

PRECOMPUTE_SUBQUERY hint

Posted on by Posted in CBO, hints, undocumented 3 Comments

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);

D
-
X

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 hash value: 272002086

---------------------------------------------------------------------------
| Id  | Operation         | Name | E-Rows |E-Bytes| Cost (%CPU)| E-Time   |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |        |       |     2 (100)|          |
|*  1 |  TABLE ACCESS FULL| DUAL |      1 |     2 |     2   (0)| 00:00:01 |
---------------------------------------------------------------------------

Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------

   1 - SEL$1 / DUAL@SEL$1

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter(("DUMMY"='' OR "DUMMY"='' OR "DUMMY"='โ™ฅ' OR "DUMMY"='โ™ฆ'
              OR "DUMMY"='โ™ฃ' OR "DUMMY"='โ™ ' OR "DUMMY"='' OR "DUMMY"=' OR
              "DUMMY"=' ' OR "DUMMY"=' ' OR "DUMMY"='' OR "DUMMY"='' OR "DUMMY"=' '
              OR "DUMMY"='' OR "DUMMY"='' OR "DUMMY"='โ–บ' OR "DUMMY"='โ—„' OR
              "DUMMY"='' OR "DUMMY"='' OR "DUMMY"='' OR "DUMMY"='' OR "DUMMY"=''
              OR "DUMMY"='' OR "DUMMY"='โ†‘' OR "DUMMY"='โ†“' OR "DUMMY"='' OR
              "DUMMY"=' OR "DUMMY"='' OR "DUMMY"='' OR "DUMMY"='' OR "DUMMY"=''
              OR "DUMMY"=' ' OR "DUMMY"='!' OR "DUMMY"='"' OR "DUMMY"='#' OR
              "DUMMY"='$' OR "DUMMY"='%' OR "DUMMY"='&' OR "DUMMY"='''' OR
              "DUMMY"='(' OR "DUMMY"=')' OR "DUMMY"='*' OR "DUMMY"='+' OR "DUMMY"=','
              OR "DUMMY"='-' OR "DUMMY"='.' OR "DUMMY"='/' OR "DUMMY"='0' OR
              "DUMMY"='1' OR "DUMMY"='2' OR "DUMMY"='3' OR "DUMMY"='4' OR "DUMMY"='5'
              OR "DUMMY"='6' OR "DUMMY"='7' OR "DUMMY"='8' OR "DUMMY"='9' OR
              "DUMMY"=':' OR "DUMMY"=';' OR "DUMMY"='<' OR "DUMMY"='=' OR "DUMMY"='>'
              OR "DUMMY"='?' OR "DUMMY"='@' OR "DUMMY"='A' OR "DUMMY"='B' OR
              "DUMMY"='C' OR "DUMMY"='D' OR "DUMMY"='E' OR "DUMMY"='F' OR "DUMMY"='G'
              OR "DUMMY"='H' OR "DUMMY"='I' OR "DUMMY"='J' OR "DUMMY"='K' OR
              "DUMMY"='L' OR "DUMMY"='M' OR "DUMMY"='N' OR "DUMMY"='O' OR "DUMMY"='P'
              OR "DUMMY"='Q' OR "DUMMY"='R' OR "DUMMY"='S' OR "DUMMY"='T' OR
              "DUMMY"='U' OR "DUMMY"='V' OR "DUMMY"='W' OR "DUMMY"='X' OR "DUMMY"='Y'
              OR "DUMMY"='Z' OR "DUMMY"='[' OR "DUMMY"='\' OR "DUMMY"=']' OR
              "DUMMY"='^' OR "DUMMY"='_' OR "DUMMY"='`' OR "DUMMY"='a' OR "DUMMY"='b'
              OR "DUMMY"='c' OR "DUMMY"='d'))

PS. I’m not sure, but as far as i remember, when I tested it on 10.2, it didn’t work with specifying a query block.
And I have never seen such usage.

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

Posted on by Posted in oracle, remote statements, SQL*Plus 6 Comments

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                      
-------- ---- ---------- -------- --------- ----------- ---------------- ----------- -------------------------------------------------- ---------------------------
       1  275       4469 XTENDER  BAIKAL     1742630060 8.20.7119        FROM REMOTE 4241494B414C2E63616336656437362E382E32302E37313139 SQL*Net message from client

Then we need to copy GLOBALID of interested session and run script on database that shown in column REMOTE_DBID, but with specifieng GLOBALID:

SQL>                                                                                                                                                                                                 
SQL> conn sys/syspass@baikal as sysdba
Connected.

======================================================================
=======  Connected to  SYS@BAIKAL(baikal)(BAIKAL)
=======  SID           203
=======  SERIAL#       38399
=======  SPID          6536
=======  DB_VERSION    11.2.0.4.0
======================================================================

SQL> @transactions/global.sql
Enter filters(empty for any)...
Sid           :
Globalid mask : 4241494B414C2E63616336656437362E382E32302E37313139
Remote_db mask:

INST_ID   SID    SERIAL# USERNAME  REMOTE_DB  REMOTE_DBID TRANS_ID   DIRECTION   GLOBALID                                            STATE                     
------- ----- ---------- --------- ---------- ----------- ---------- ----------- --------------------------------------------------  --------------------------
      1     9      39637 XTENDER   BAIKAL      1742630060 8.20.7119  TO REMOTE   4241494B414C2E63616336656437362E382E32302E37313139  [ORACLE COORDINATED]ACTIVE

It’s quite simple and fast.