When to filter on table

Question

I have a table that contains a XML field.
To make things easier, a view is created with extracted XML fields.
This is deemed to be an expensive operation.
A bunch of table is then joined with the view on a filter condition.
My question is:
Does every XML row gets extracted when doing the join?
In other word, should put the filter condition on the view first.
create view XML as
select
id,extractvalue(xml,'/COMPONENT/START_DATE/DATE') startdate from trans
select * from XML x join product p on x.id = p.id

Answer 1

Look at this test:
SQL> select * from trans;
        ID XML
         1
           <COMPONENT>
             <START_DATE>
                <DATE>2009-12-31</DATE>
             </START_DATE>
           </COMPONENT>
         2
           <COMPONENT>
             <START_DATE>
                <DATE>2010-12-31</DATE>
             </START_DATE>
           </COMPONENT>
         3
           <COMPONENT>
             <START_DATE>
                <DATE>2011-12-31</DATE>
             </START_DATE>
           </COMPONENT>
SQL> create view XML as
  2  select
  3  id,extractvalue(xml,'/COMPONENT/START_DATE/DATE') startdate from trans
  4  ;
View created.
SQL> select * from xml;
        ID STARTDATE
         1 2009-12-31
         2 2010-12-31
         3 2011-12-31
SQL> create index trans_ind on trans(id);
Index created.
SQL> set autot on exp
SQL> select * from xml
  2  where id=2;
        ID STARTDATE
         2 2010-12-31
Execution Plan
Plan hash value: 128389523
| Id  | Operation                   | Name      | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT            |           |     1 |  2015 |     2   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| TRANS     |     1 |  2015 |     2   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN          | TRANS_IND |     1 |       |     1   (0)| 00:00:01 |
Predicate Information (identified by operation id):
   2 - access("ID"=2)
Note
   - dynamic sampling used for this statement
SQL> create table product (id number);
Table created.
SQL> insert into product values (2);
1 row created.
SQL> select * from XML x join product p on x.id = p.id;
        ID STARTDATE                    ID
         2 2010-12-31                    2
Execution Plan
Plan hash value: 3807437055
| Id  | Operation                   | Name      | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT            |           |     1 |  2028 |     4   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| TRANS     |     1 |  2015 |     1   (0)| 00:00:01 |
|   2 |   NESTED LOOPS              |           |     1 |  2028 |     4   (0)| 00:00:01 |
|   3 |    TABLE ACCESS FULL        | PRODUCT   |     1 |    13 |     3   (0)| 00:00:01 |
|*  4 |    INDEX RANGE SCAN         | TRANS_IND |     1 |       |     0   (0)| 00:00:01 |
Predicate Information (identified by operation id):
   4 - access("ID"="P"."ID")
Note
   - dynamic sampling used for this statementAs you can see in the last execution plan, the query has been rewrote by the optimizer and it has fist of all selected the row to return (id=2) from the index and then is gone to read data from the TRANS table.
So the extracvalue function has been evaluated only on one row.
Max
[My Italian Oracle blog|http://oracleitalia.wordpress.com/2010/01/17/supporto-di-xml-schema-in-oracle-xmldb/]
Edited by: Massimo Ruocchio on Jan 18, 2010 5:44 PM
Adjusted layout