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Updated: 15 hours 50 min ago

REGEXP_LIKE: strange unspecified value in parameter “modifier”

Tue, 2014-07-22 15:05

Today I noticed strange thing in predicate section of execution plan for simple query with regexp_like, where 3rd parameter “MODIFIER” was not specified:

SQL> select * from dual where regexp_like(dummy,'.');

D
-
X

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

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------
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     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |       |       |     2 (100)|          |
|*  1 |  TABLE ACCESS FULL| DUAL |     1 |     2 |     2   (0)| 00:00:01 |
--------------------------------------------------------------------------

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

   1 - filter( REGEXP_LIKE ("DUMMY",'.',HEXTORAW('F07FD85CFF0700006A1116
              45010000000000000000000000FC12164501000000000000000000000000000000000000
              0010000000000000001880D85CFF07000002000000000000000000000081000000') ))


20 rows selected.

It is particularly interesting that the values in HEXTORAW() are always different for different first parameters:

SQL> select * from dual where regexp_like(dummy,'x');
...
   1 - filter( REGEXP_LIKE ("DUMMY",'x',HEXTORAW('3895D330FF0700006A1116
              45010000000000000000000000FC12164501000000000000000000000000000000000000
              0011000000000000006895D330FF07000002000000000000000000000081000000') ))
SQL> select * from dual where regexp_like(dummy,'y');
...
   1 - filter( REGEXP_LIKE ("DUMMY",'y',HEXTORAW('00DA3C3FFF0700006A1116
              45010000000000000000000000FC12164501000000000000000000000000000000000000
              00110000000000000030DA3C3FFF07000002000000000000000000000081000000') ))
SQL> select * from dual where regexp_like(dummy||'','x')
...
   1 - filter( REGEXP_LIKE ("DUMMY"||'','x',HEXTORAW('70964F2FFF0700006A
              111645010000000000000000000000FC1216450100000000000000000000000000000000
              0000001100000000000000A0964F2FFF07000002000000000000000000000081000000')
               ))

I don’t know, what does it mean, but it looks like garbage from memory.
When I noticed this, I decided to check how regexp_like will work in function-based indexes:

SQL> create table xtest as
  2    select dummy||level as str
  3    from dual
  4    connect by level<=30;

Table created.

SQL> select * from xtest where case when regexp_like(str,'1') then 1 end = 1;
...
12 rows selected.

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

PLAN_TABLE_OUTPUT
-----------------------------------------------------------------------------------------
SQL_ID  7ztp0k8c1zn2h, child number 0
-------------------------------------
select * from xtest where case when regexp_like(str,'1') then 1 end = 1

Plan hash value: 4207139086

---------------------------------------------------------------------------
| Id  | Operation         | Name  | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |       |       |       |     3 (100)|          |
|*  1 |  TABLE ACCESS FULL| XTEST |    12 |   264 |     3   (0)| 00:00:01 |
---------------------------------------------------------------------------

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

   1 - filter(CASE  WHEN  REGEXP_LIKE
              ("STR",'1',HEXTORAW('68F9CB32FF0700006A111645010000000000000000000000FC1
              216450100000000000000000000000000000000000000110000000000000098F9CB32FF0
              7000002000000000000000000000081000000') ) THEN 1 END =1)

SQL> create index xtest_fbi on xtest(case when regexp_like(str,'1') then 1 end);

Index created.

SQL> select * from xtest where case when regexp_like(str,'1') then 1 end = 1;
...
12 rows selected.

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

PLAN_TABLE_OUTPUT
-----------------------------------------------------------------------------------------
SQL_ID  7ztp0k8c1zn2h, child number 0
-------------------------------------
select * from xtest where case when regexp_like(str,'1') then 1 end = 1

Plan hash value: 1479471124

-----------------------------------------------------------------------------------------
| Id  | Operation                   | Name      | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |           |       |       |     2 (100)|          |
|   1 |  TABLE ACCESS BY INDEX ROWID| XTEST     |    12 |   300 |     2   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN          | XTEST_FBI |    12 |       |     1   (0)| 00:00:01 |
-----------------------------------------------------------------------------------------

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

   2 - access("XTEST"."SYS_NC00002$"=1)

SQL> select column_expression from user_ind_expressions e where e.index_name='XTEST_FBI';

COLUMN_EXPRESSION
-----------------------------------------------------------------------------------------
CASE  WHEN  REGEXP_LIKE ("STR",'1') THEN 1 END

As you can see it works fine, although the predicate from first execution plan differs from the FBI expression.
Then I dumped 10053 trace and noticed that the HEXTORAW(…) function appeared in “Explain Plan Dump” only, so it looks just like plan output bug.

Categories: Development

Strange moving filter predicates from index to table

Wed, 2014-04-23 15:41

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))

Spoiler:: Full test case SelectShow

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'));


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))