Re: Index Contention / Sequence Caching

The first example cited by Jonathan Lewis was for Oracle 8i; running the code on 11.2.0.3 I don't see that same behavior:

SQL> select
  2      /*+
  3          leading(t2) use_nl(t1)
  4      */
  5      t2.object_name, t1.object_name
  6  from
  7      t2      t2,
  8      t1_at_poojooba    t1
  9  where
 10      t2.object_name = 'DUAL'
 11  and t1.object_id = t2.object_id
 12  ;

OBJECT_NAME                    OBJECT_NAME
------------------------------ ------------------------------
DUAL                           DUAL
DUAL                           DUAL


Execution Plan
----------------------------------------------------------
Plan hash value: 3485226535

-------------------------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     | Inst   |IN-OUT|
-------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |     1 |    54 |   221   (0)| 00:00:03 |        |      |
|   1 |  NESTED LOOPS      |      |     1 |    54 |   221   (0)| 00:00:03 |        |      |
|*  2 |   TABLE ACCESS FULL| T2   |     1 |    24 |   134   (0)| 00:00:02 |        |      |
|   3 |   REMOTE           | T1   |     1 |    30 |    87   (0)| 00:00:02 | POOJO~ | R->S |
-------------------------------------------------------------------------------------------

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

   2 - filter("T2"."OBJECT_NAME"='DUAL')

Remote SQL Information (identified by operation id):
----------------------------------------------------

   3 - SELECT /*+ USE_NL ("T1") */ "OBJECT_NAME","OBJECT_ID" FROM "T1" "T1" WHERE
       "OBJECT_ID"=:1 (accessing 'POOJOOBA' )



Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
        478  consistent gets
          0  physical reads
          0  redo size
        670  bytes sent via SQL*Net to client
        519  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          2  rows processed

SQL>
SQL> alter index t1_i1 rebuild reverse;

Index altered.

SQL>
SQL> select
  2      /*+
  3          leading(t2) use_nl(t1)
  4      */
  5      t2.object_name, t1.object_name
  6  from
  7      t2      t2,
  8      t1_at_poojooba    t1
  9  where
 10      t2.object_name = 'DUAL'
 11  and t1.object_id = t2.object_id
 12  ;

OBJECT_NAME                    OBJECT_NAME
------------------------------ ------------------------------
DUAL                           DUAL
DUAL                           DUAL


Execution Plan
----------------------------------------------------------
Plan hash value: 3485226535

-------------------------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     | Inst   |IN-OUT|
-------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |     1 |    54 |   221   (0)| 00:00:03 |        |      |
|   1 |  NESTED LOOPS      |      |     1 |    54 |   221   (0)| 00:00:03 |        |      |
|*  2 |   TABLE ACCESS FULL| T2   |     1 |    24 |   134   (0)| 00:00:02 |        |      |
|   3 |   REMOTE           | T1   |     1 |    30 |    87   (0)| 00:00:02 | POOJO~ | R->S |
-------------------------------------------------------------------------------------------

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

   2 - filter("T2"."OBJECT_NAME"='DUAL')

Remote SQL Information (identified by operation id):
----------------------------------------------------

   3 - SELECT /*+ USE_NL ("T1") */ "OBJECT_NAME","OBJECT_ID" FROM "T1" "T1" WHERE
       "OBJECT_ID"=:1 (accessing 'POOJOOBA' )



Statistics
----------------------------------------------------------
          0  recursive calls
          1  db block gets
        478  consistent gets
          0  physical reads
        256  redo size
        670  bytes sent via SQL*Net to client
        519  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          2  rows processed

SQL>

I do not disagree with the points raised in Jonathan's second post; those are valid concerns and should be considered carefully before implementing a reverse-key index.


 
David Fitzjarrell
Primary author, "Oracle Exadata Survival Guide"




On Monday, March 17, 2014 10:28 AM, Riyaj Shamsudeen <riyaj.shamsudeen_at_gmail.com> wrote:
 
Hello Stephen
  In addition to Houri (and JL) has pointed out already, effect of a reverse key index is, to spread the values among ALL the leaf blocks of the index. So, the buffer cache(s) can be polluted with all these leaf blocks potentially. This problem is magnified if the number of leaf block in the index is huge. 




Cheers

Riyaj Shamsudeen
Principal DBA,
Ora!nternals -  http://www.orainternals.com - Specialists in Performance, RAC and EBS
Blog: http://orainternals.wordpress.com/
Oracle ACE Director and OakTable member

Co-author of the books: Expert Oracle Practices, Pro Oracle SQL, Expert RAC Practices 12c. Expert PL/SQL practices

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