Tag Archives: cbo

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');

[collapse]

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→

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"='♥' 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"='' 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.

Strange moving filter predicates from index to table

Posted on by Posted in CBO, curious, oracle 9 Comments

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 SCAN| PK_XT_TEST |      1 |      1 |      1 |00:00:00.01 |       2 |
-----------------------------------------------------------------------------------------

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

   1 - access("A"=1)
       filter((:B IS NULL OR "B"=:B))

But if I add another column “PAD”, the filter moves to the table filters:

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

---------------------------------------------------------------------------------------
| Id  | Operation                   | Name       | Starts | E-Rows | A-Rows | Buffers |
---------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |            |      1 |        |      1 |       4 |
|*  1 |  TABLE ACCESS BY INDEX ROWID| XT_TEST    |      1 |      1 |      1 |       4 |
|*  2 |   INDEX RANGE SCAN          | PK_XT_TEST |      1 |     10 |     10 |       2 |
---------------------------------------------------------------------------------------

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

   1 - filter((:B IS NULL OR "B"=:B))
   2 - access("A"=1)

As workaround we can use something like that:

select--+ NO_ELIMINATE_JOIN(t) NO_ELIMINATE_JOIN(t2@sel$2) gather_plan_statistics
   a,b,pad
from xt_test t
where t.rowid in ( select t2.rowid
                   from xt_test t2
                   where a=1
                     and (:b is null or b = :b)
                 );

Plan hash value: 1464320522

--------------------------------------------------------------------------------------
| Id  | Operation                   | Name       | Starts | E-Rows | A-Rows |Buffers |
--------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |            |      1 |        |      1 |      3 |
|   1 |  NESTED LOOPS               |            |      1 |      1 |      1 |      3 |
|*  2 |   INDEX RANGE SCAN          | PK_XT_TEST |      1 |      1 |      1 |      2 |
|   3 |   TABLE ACCESS BY USER ROWID| XT_TEST    |      1 |      1 |      1 |      1 |
--------------------------------------------------------------------------------------

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

   2 - access("A"=1)
       filter((:B IS NULL OR "B"=:B))
Full test case 
create table xt_test(a,b,pad,constraint pk_xt_test primary key(a,b))
as select 
      mod(rownum,10) a
     ,rownum         b
     ,rpad(rownum,10) pad
   from dual
   connect by level<=100;
call dbms_stats.gather_table_stats('','XT_TEST');
var b number;
exec :b:=1;
select/*+ gather_plan_statistics */
  a,b,pad
from xt_test 
where a=1 and (:b is null or b = :b);
select * from table(dbms_xplan.display_cursor('','','allstats last'));

select/*+ gather_plan_statistics */
  a,b
from xt_test 
where a=1 and (:b is null or b = :b);
select * from table(dbms_xplan.display_cursor('','','allstats last'));

select--+ NO_ELIMINATE_JOIN(t) NO_ELIMINATE_JOIN(t2@sel$2) gather_plan_statistics
   a,b,pad 
from xt_test t
where t.rowid in ( select t2.rowid 
                   from xt_test t2 
                   where a=1 
                     and (:b is null or b = :b)
                 );
select * from table(dbms_xplan.display_cursor('','','allstats last'));

[collapse]

Update:
I just forgot to mention that there is another workaround – to force concatenation:

SQL> select--+ use_concat(or_predicates(2))
  2     a,b,pad
  3  from xt_test where a=1 and (:b is null or b = :b);

         A          B PAD
---------- ---------- ----------------------------------------
         1          1 1

Plan hash value: 3582916188

----------------------------------------------------------------------------------------------------------------------------
| Id  | Operation                     | Name       | Starts | E-Rows | Cost (%CPU)| A-Rows |   A-Time   | Buffers | Reads  |
----------------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT              |            |      1 |        |     3 (100)|      1 |00:00:00.01 |       2 |      2 |
|   1 |  CONCATENATION                |            |      1 |        |            |      1 |00:00:00.01 |       2 |      2 |
|   2 |   TABLE ACCESS BY INDEX ROWID | XT_TEST    |      1 |      1 |     1   (0)|      1 |00:00:00.01 |       2 |      2 |
|*  3 |    INDEX UNIQUE SCAN          | PK_XT_TEST |      1 |      1 |     0   (0)|      1 |00:00:00.01 |       1 |      1 |
|*  4 |   FILTER                      |            |      1 |        |            |      0 |00:00:00.01 |       0 |      0 |
|   5 |    TABLE ACCESS BY INDEX ROWID| XT_TEST    |      0 |     10 |     2   (0)|      0 |00:00:00.01 |       0 |      0 |
|*  6 |     INDEX RANGE SCAN          | PK_XT_TEST |      0 |     10 |     1   (0)|      0 |00:00:00.01 |       0 |      0 |
----------------------------------------------------------------------------------------------------------------------------

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

   3 - access("A"=1 AND "B"=:B)
   4 - filter(:B IS NULL)
   6 - access("A"=1)
       filter(LNNVL("B"=:B))

Patch for “Bug 16516751 : Suboptimal execution plan for query with join and in-list using composite index” is available now

Posted on by Posted in 12c, bug, CBO, oracle, query optimizing 2 Comments

Bug about which i wrote previously is fixed now in 12.2, and patch 16516751 is available now for 11.2.0.3 Solaris64.
Changes:
1. CBO can consider filters in such cases now
2. Hint NUM_INDEX_KEYS fixed and works fine

UPD: Very interesting solution by Igor Usoltsev(in russian):
Ignored hint USE_CONCAT(OR_PREDICATES(N)) allows to avoid inlist iterator.
Example:

select--+ USE_CONCAT(OR_PREDICATES(32767))
 * from xt1,xt2
where
     xt1.b=10
 and xt1.a=xt2.a
 and xt2.b in (1,2)
/

Plan hash value: 2884586137          -- good plan:
 
----------------------------------------------------------------------------------------
| Id  | Operation                     | Name   | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT              |        |       |       |   401 (100)|          |
|   1 |  NESTED LOOPS                 |        |       |       |            |          |
|   2 |   NESTED LOOPS                |        |   100 | 36900 |   401   (0)| 00:00:01 |
|   3 |    TABLE ACCESS BY INDEX ROWID| XT1    |   100 | 31000 |   101   (0)| 00:00:01 |
|*  4 |     INDEX RANGE SCAN          | IX_XT1 |   100 |       |     1   (0)| 00:00:01 |
|*  5 |    INDEX RANGE SCAN           | IX_XT2 |     1 |       |     2   (0)| 00:00:01 |
|   6 |   TABLE ACCESS BY INDEX ROWID | XT2    |     1 |    59 |     3   (0)| 00:00:01 |
----------------------------------------------------------------------------------------
 
Predicate Information (identified by operation id):
---------------------------------------------------
 
   4 - access("XT1"."B"=10)
   5 - access("XT1"."A"="XT2"."A")
       filter(("XT2"."B"=1 OR "XT2"."B"=2))

From 10053 trace on nonpatched 11.2.0.3:
inlist_concat_diff_10053