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Don Burleson Blog 







Inside the pickler fetch

Oracle Database Tips by Donald BurlesonMay 16, 2015

Question:  I don't understand the explain plan step "collection iterator pickler fetch.  I am using this query:

SELECT p.order_number, p.shipping_date,, o.line_number, o.quantity, o.products.product_number, o.products.description
FROM purchaseorder_t p, TABLE(p.orderlineitem) o;

I generated the explain plan command for the above query and found that Oracle using the COLLECTION ITERATOR (PICKLER FETCH).

Execution Plan
0 SELECT STATEMENT Optimizer=ALL_ROWS (Cost=220564 Card=81680000 Bytes=4900800000)
1 0 NESTED LOOPS (Cost=220564 Card=81680000 Bytes=4900800000)
2 1 TABLE ACCESS (FULL) OF 'PURCHASEORDER_T' (TABLE) (Cost=180 Card=10000 Bytes=580000)



Answer:  In general, a pickler fetch is the process of converting a packed array into a displayable format.  For more details on Oracle object-oriented database, see my notes on Oracle objects.

Oracle consultant Steve Adams notes that the collection iterator with a pickler fetch is used to un-pack object type table columns:

"Pickling is serializing arbitrary object-oriented data structures.

That is, converting them into a byte stream for storage, transmission over a network or iterative navigation as in this case.  It is a more complex process than you might imagine.  If you want more information, search the web for information on Python's pickle module.

I guess this is the way Oracle dereferences the embedded collections?"

The execution plan step "COLLECTION ITERATOR (PICKLER FETCH)" is the optimizer plan operation showing use of the dbms_pickler package, a PL/SQL package which is used to get TDS (Type Descriptor Source) of a object. This is required to know the contents of the given ADT (ie., attributes in a Abstract Data Type).  During a pickler fetch, the dbms_pickler package is called to convert the array data type to a byte stream that can be displayed.

Starting in Oracle8i (8.1.5), the pickler fetch and collection iterator operations were introduced, and new CBO access methods that were added to Oracle SQL to manage "objects" (0NF tables, e.g. nested tables, varray tables, tables with abstract data type columns). 

The pickler code is very new in Oracle8i and there are several reported memory leak issues with pickler fetches, even in Oracle 11g.   As for performance, the collection iterator operation is used to extract the embedded repeating group from the row.

Remember, rows with varray items are stored in-line so that a fetch for the data block that contains the row will also contain the repeating groups. Hence, the performance of tables with varray columns is comparable to standard row select statements.

OPTIONS                        OBJECT_NAME                      POSITION
------------------------------ ------------------------------ ----------
FULL                           EMP                                     1


Related notes on object-oriented Oracle tables:



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