NLSSORT function not used with CHAR and VARCHAR2 column index

Hello!

Create a test bench:
CREATE TABLE scott.nls_demo
      (
      col_varchar            varchar2(4),
      col_char               char(4),
      col_varchar_NLS_GERMAN varchar2(4),
      col_char_NLS_GERMAN    char(4)
      );
 
INSERT INTO scott.nls_demo (
      col_varchar,
      col_char,
      col_varchar_NLS_GERMAN,
      col_char_NLS_GERMAN  )
  SELECT 
         substr(object_name,1,4),
         substr(object_name,1,4),
         substr(object_name,1,4),
         substr(object_name,1,4)
  FROM all_objects where rownum<5000;

COMMIT; 

create index scott.i_varchar on scott.nls_demo (col_varchar);  
create index scott.i_char    on scott.nls_demo (col_char);

create index scott.i_varchar_NLS_GERMAN on scott.nls_demo ( NLSSORT(col_varchar_NLS_GERMAN,'nls_sort=''GERMAN_CI'''));  
create index scott.i_char_NLS_GERMAN    on scott.nls_demo ( NLSSORT(col_char_NLS_GERMAN,   'nls_sort=''GERMAN_CI'''));
Now "explain plan" these 8 select statements in SQL * more:
variable c char(4);
variable v varchar2(4);

exec :c:= 'abc';
exec :v:= 'abc';

explain plan for SELECT /* 1*/ * FROM scott.nls_demo where col_varchar=:v;
SELECT * FROM TABLE(dbms_xplan.display);

explain plan for SELECT /* 2*/ * FROM scott.nls_demo where col_char=:c;
SELECT * FROM TABLE(dbms_xplan.display);

explain plan for SELECT /* 3*/ * FROM scott.nls_demo where col_varchar_NLS_GERMAN=:v;
SELECT * FROM TABLE(dbms_xplan.display);

explain plan for SELECT /* 4*/ * FROM scott.nls_demo where col_char_NLS_GERMAN=:c;
SELECT * FROM TABLE(dbms_xplan.display);

ALTER SESSION SET NLS_COMP = linguistic;
ALTER SESSION SET NLS_SORT = german_ci;

explain plan for SELECT /* 5*/ * FROM scott.nls_demo where col_varchar=:v;
SELECT * FROM TABLE(dbms_xplan.display);

explain plan for SELECT /* 6*/ * FROM scott.nls_demo where col_char=:c;
SELECT * FROM TABLE(dbms_xplan.display);

explain plan for SELECT /* 7*/ * FROM scott.nls_demo where col_varchar_NLS_GERMAN=:v;
SELECT * FROM TABLE(dbms_xplan.display);

explain plan for SELECT /* 8*/ * FROM scott.nls_demo where col_char_NLS_GERMAN=:c;
SELECT * FROM TABLE(dbms_xplan.display);
What I see on 11.2.0.2 is:

1.) statement 1 would use the I_VARCHAR index, that is what I expected.
2.) statement 2 would use the I_CHAR index, that is what I expected.
3.) no clue used, because none are available right here for you. Understood.
4.) no clue used, because none are available right here for you. Understood.

And when define us NLS_SORT = german_ci:

5.) no clue used, because none are available right here for you. Understood.
6.) no clue used, because none are available right here for you. Understood.
7.) I_VARCHAR_NLS_GERMAN we used. Large.
8.) no index used, although I think that "i_char_NLS_GERMAN" would do the job...

Why the index 'i_char_NLS_GERMAN' is not used with the 8 statement? Jonathan? Someone else?

Thanks for your help!
Marcus

Not really a reason to not use TANK - although there are many of them.

More than one reason to not always rely to "explain the plan for?

All the variables passed in EXPLAINING the PLAN for are treated as VARCHAR2.
In addition, there is no point setting the values of the variable because they will not cast a look either with MAP to EXPLAIN.

SQL> CREATE TABLE nls_demo
  2        (
  3        col_varchar            varchar2(4),
  4        col_char               char(4),
  5        col_varchar_NLS_GERMAN varchar2(4),
  6        col_char_NLS_GERMAN    char(4)
  7        );

Table created.

SQL>
SQL> INSERT INTO nls_demo (
  2        col_varchar,
  3        col_char,
  4        col_varchar_NLS_GERMAN,
  5        col_char_NLS_GERMAN  )
  6    SELECT
  7           substr(object_name,1,4),
  8           substr(object_name,1,4),
  9           substr(object_name,1,4),
 10           substr(object_name,1,4)
 11    FROM all_objects where rownum<5000;

4999 rows created.

SQL>
SQL> commit; 

Commit complete.

SQL>
SQL> create index i_varchar on nls_demo (col_varchar);  

Index created.

SQL> create index i_char    on nls_demo (col_char);

Index created.

SQL>
SQL> create index i_varchar_NLS_GERMAN on nls_demo ( NLSSORT(col_varchar_NLS_GERMAN,'nls_sort=''GERM
AN_CI'''));  

Index created.

SQL> create index i_char_NLS_GERMAN    on nls_demo ( NLSSORT(col_char_NLS_GERMAN,   'nls_sort=''GERM
AN_CI'''));

Index created.

SQL>
SQL> variable c char(4);
SQL> variable v varchar2(4);
SQL>
SQL> exec :c:= 'abc';

PL/SQL procedure successfully completed.

SQL> exec :v:= 'abc';

PL/SQL procedure successfully completed.

SQL>
SQL> ALTER SESSION SET NLS_COMP = linguistic;

Session altered.

SQL> ALTER SESSION SET NLS_SORT = german_ci;

Session altered.

SQL>
SQL> SELECT /* 8*/ * FROM nls_demo where col_char_NLS_GERMAN=:c;

no rows selected

SQL> SELECT * FROM TABLE(dbms_xplan.display_cursor);

PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------
SQL_ID  9su0j5vzuwzyj, child number 0
-------------------------------------
SELECT /* 8*/ * FROM nls_demo where col_char_NLS_GERMAN=:c

Plan hash value: 2830339923

-------------------------------------------------------------------------------------------------
| Id  | Operation                   | Name              | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |                   |       |       |     3 (100)|          |
|   1 |  TABLE ACCESS BY INDEX ROWID| NLS_DEMO          |    50 |  2150 |     3   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN          | I_CHAR_NLS_GERMAN |    20 |       |     1   (0)| 00:00:01 |
-------------------------------------------------------------------------------------------------

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

   2 - access("NLS_DEMO"."SYS_NC00006$"=NLSSORT(:C,'nls_sort=''GERMAN_CI'''))

Note
-----
   - dynamic sampling used for this statement (level=2)

23 rows selected.

SQL>

Published by: Dom Brooks on February 21, 2011 15:39

Tags: Database

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