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INTERNAL_FUNCTION() Impact April 21, 2010

Posted by mwidlake in internals, performance.
Tags: , ,
13 comments

You may occasionally see something like the following in the filter predicates of an Explain Plan:

INTERNAL_FUNCTION(“COL_1″)>:P0

What is INTERNAL_FUNCTION, what is the use of INTERNAL_FUNCTION() in Oracle explain plans and why am I putting things like ORACLE INTERNAL_FUNCTION partition exclusion and meaning of internal_function performance almost as random strings in this blog?

Well, I want to get the hit rate up for this post as when I did a quick search on INTERNAL_FUNCTION the current top hit is This unfortunately misleading posting that tells you “It is not a significant overhead in SQL execution performance”. This internal function IS potentially a significant overhead in SQL Execution performance. You can’t add comments to the above site {just book a course or order a book}, so I’m having to create my own page…

INTERNAL_FUNCTION is, as the above posting says, where oracle does an implicit data conversion. In the shown case, as well as in my example in this blog and in most cases I have seen, from timestamp to date.

Why is it an issue? Because it is a function on a column and as such it can have a disasterous impact on sql execution performance.

This code below ( which has most select columns removed for simplicity and to protect the innocent) was performing very poorly, taking over 30 minutes to complete. This is all on 10.2.0.3 Enterprise edition of Oracle.

SELECT trans0_.ID as ID3_1_
...
, round(transfers0_.AMOUNT,2) as formula10_0_
FROM VW_PMTT trans0_
WHERE  (trans0_.MODIFIED_DT between :p0 AND :p1
        AND trans0_.PERS_ID = :p2)
and transfers0_.ACCOUNT_ID=:p3
ORDER BY transfers0_.MODIFIED_DT

This is the plan and the significant filter/access predicates:

Plan hash value: 3261981189
------------------------------------------------------------------------------------------------------
| Id  | Operation                         | Name                 | Rows  |Bytes| Cost   |Pstart|Pstop|
------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                  |                      |       |     | 67841  |      |     |
|   1 |  SORT ORDER BY                    |                      |     8 | 920 | 67841  |      |     |
|   2 |   VIEW                            | VW_PMTP              |     8 | 920 | 67840  |      |     |
|   3 |    UNION-ALL                      |                      |       |     |        |      |     |
|*  4 |     FILTER                        |                      |       |     |        |      |     |
|   5 |      NESTED LOOPS                 |                      |     4 | 280 | 28490  |      |     |
|*  6 |       TABLE ACCESS BY INDEX ROWID | W_TBLP               |     1 |  14 |     3  |      |
|*  7 |        INDEX UNIQUE SCAN          | PK_W_TBLP            |     1 |     |     2  |      |     |
|   8 |       PARTITION RANGE ALL         |                      |     4 | 224 | 28487  |    1 | 1580|
|*  9 |        TABLE ACCESS BY LOC INd RID| W_PAYMENT            |     4 | 224 | 28487       1 | 1580|
|* 10 |         INDEX RANGE SCAN          | IDX_W_P_TRANS        | 66936 |     |  3345  |    1 | 1580|
...

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

   6 - filter("PLY"."ACCOUNTID"=:P3)
   7 - access("PLY"."PERS_ID"=:P2)
   9 - filter(("PT"."TYPE"=1184769 AND INTERNAL_FUNCTION("PT"."MODIFIED")>=:P0 AND INTERNAL_FUNCTION ("PT"."MODIFIED")<:P1

As you can see, the view is translated into a simple two-table join {and unioned with a second two-table join, which I removed again for clarity, but if anyone wants the full plan and statement, email me} where:

  • one table is accessed on a unique index (via bind variable P2).
  • Row filtered for P3
  • This table is then joined to a Partitioned table in a nested loop 
  • The table being accessed via a local index range scan.

At first glance, the plan may look fine, but Look at the pstart and pstop. 1 to 1580. That is every partition in the table.
The predicate information shows that the INTERANAL_FUNCTION(“PT”.”MODIFIED”) column is being compared to P0 and P1.

The partition key on the table is:-

>@chk_patr_key
Name of Table : W_PAYMENT

TAB_NAME                       OBJECT_TYP PSN COLUMN_NAME
------------------------------ ---------- --- -----------
USER.W_PAYMENT                 TABLE       1 MODIFIED

ie the column that is being flitered by. Why no partition pruning?

Partition pruning is not occuring because of the “INTERNAL_FUNCTION” being applied to that column. The CBO is not able to understand how the result of a function will match to the partition values of the column. Not even it’s own, internally used function :-P

For the above, the “date” bind variables were defined as

p0 timestamp := to_timestamp(’01-FEB-2010 00:00′,’DD-MON-YYYY HH24:MI’);
p1 timestamp := to_timestamp(’03-FEB-2010 00:00′,’DD-MON-YYYY HH24:MI’);

I then altered the code such that the bind variables were defined as dates.

p0 date := to_date(’01-FEB-2010′,’DD-MON-YYYY’);
p1 date := to_date(’03-FEB-2010′,’DD-MON-YYYY’);

The plan and significant predicates were now:

Plan hash value: 346162108
------------------------------------------------------------------------------------------------------
| Id  | Operation                         | Name                 | Rows  |Bytes| Cost   |Pstart|Pstop|
------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                  |                      |       |     | 67841  |      |     |
|   1 |  SORT ORDER BY                    |                      |     4 | 460 | 67841  |      |     |
|   2 |   VIEW                            | VW_PMTP              |     4 | 460 | 67840  |      |     |
|   3 |    UNION-ALL                      |                      |       |     |        |      |     |
|*  4 |     FILTER                        |                      |       |     |        |      |     |
|   5 |      NESTED LOOPS                 |                      |     2 | 140 | 28490  |      |     |
|*  6 |       TABLE ACCESS BY INDEX ROWID | W_TBLP               |     1 |  14 |     3  |      |     |
|*  7 |        INDEX UNIQUE SCAN          | PK_W_TBLP            |     1 |     |     2  |      |     |
|   8 |       PARTITION RANGE ALL         |                      |     2 | 112 | 28487  |  KEY | KEY |
|*  9 |        TABLE ACCESS BY LOC INd RID| W_PAYMENT            |     2 | 112 | 28487  |  KEY | KEY |
|* 10 |         INDEX RANGE SCAN          | IDX_W_P_TRANS        | 66936 |     |  3345  |  KEY | KEY |
...

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

   6 - filter("PLY"."ACCOUNTID"=:P3)
   7 - access("PLY"."PERS_ID"=:P2)
   9 - filter(("PT"."TYPE"=1184769 AND "PT"."MODIFIED">=:P0 AND "PT"."MODIFIED"<:P1

The expected rows and bytes drop but the key difference in the plan is Pstart and Pstop are now KEY KEY. ie the CBO will evaluate at run time the values coming back from the Table access of W_TBLP and partition prune the access of the table.

In this situation, the original query took over 30 minutes to return. The new code took 18 seconds.

I tried one final test. I set the bind variables to be varchar2 so that implicit data conversion would occur:
p0 varchar2(20) := ’01-FEB-2010 00:00′;
p1 varchar2(20) := ’03-FEB-2010 00:00′;

With VARCHAR2 the CBO was able to do the implicit conversion without the INTERNAL_FUNCTION and the plan/filter predicates were exactly as for native dates.

It would seem this issue with INTERNAL_FUNCTION blights timestamp to date implicit data conversion but not some other implicit data conversions. Without testing further, I can’t be sure which.

This is a documented feature by Oracle. Unusually (and much to their credit) Oracle even document the potential negative impact of this particular construct with timestamps and dates. See this section in chapter 5 of the Oracle 10g data warehousing guide.

I better balance the fact I linked to an inaccurate posting on the subject with a few good ones.
This is a nice one from McLaud on the OraFAQ site where he has come across it as a problem and with help shows how it is due to implicit conversion of a timestamp from Hibernate.
This blog on the issue arising with Java is a nice example of the fix (use a date to hold a date, not a timestamp).

Finally, this link is to an older but very, very valid rant by Tom Kyte about implicit data conversion and using the correct data type for your data. {Don’t go using numbers or varchars to hold dates, just don’t, and similarly you should avoid implicit data conversion}.

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