Skip to main content

Weak Ref Cursor Types: Do I ever need to declare my own? No!

This question just came in via Twitter DM, so I thought it could do with a blog answer.

Ref cursor types are the datatypes from which we declare cursor variables. A cursor variable is, well, just that: a variable pointing back to a cursor/result set.

Some really nice aspects of cursor variables:

  • you can associate a query with a cursor variable at runtime (useful with both static and dynamic SQL); 
  • you can pass the cursor variable as a parameter or function RETURN value. Specifically: you can pass a cursor variable back to a host language like Java for consumption.
Check out the cursor variable documentation. 

Explore this LiveSQL script on cursors, including multiple examples of cursor variables.

OK, to get to it, then:

Before you can declare a cursor variable, you need to have a ref cursor type defined. There are two, ahem, types of types: strong and weak. 

With a strong type, you include a RETURN clause that specifies the number and datatypes of expressions returned by the query associated with the cursor variable. 

With a weak type, there is no RETURN. You can associate a variable based on a weak type with any SELECT statement.

PL/SQL offers a pre-defined weak ref cursor type: SYS_REFCURSOR. And (now we are getting to the answer to the question in the title of this post) that's the only weak ref cursor type you'll ever need. 

Here's a package specification that shows both usages:

CREATE OR REPLACE PACKAGE refcursor_pkg 
IS 
   /* Use this "strong" REF CURSOR to declare cursor variables whose 
      queries return data from the endangered_species table. */ 
 
   TYPE endangered_species_t IS REF CURSOR 
      RETURN endangered_species%ROWTYPE; 
 
   FUNCTION filtered_species_cv (filter_in IN VARCHAR2) 
      RETURN endangered_species_t; 

   /* User-defined weak REF CURSOR type . BUT YOU SHOULD NOT DO THIS! */
 
   TYPE weak_t IS REF CURSOR;
  
   FUNCTION data_from_any_query_cv (query_in IN VARCHAR2) 
      RETURN weak_t; 
 
   /* INSTEAD, just use SYS_REFCURSOR! */ 

   FUNCTION data_from_any_query_cv2 (query_in IN VARCHAR2) 
      RETURN SYS_REFCURSOR; 
END refcursor_pkg;

Remember: every line of code you write needs to be tested and maintained. Every bit of code you manage to avoid writing gives you more time to test and maintain - and enhance - the code you have to write.

So don't declare you own weak REF CURSOR types!

Labels

Labels:

Comments

Post a Comment

Popular posts from this blog

Running out of PGA memory with MULTISET ops? Watch out for DISTINCT!

A PL/SQL team inside Oracle made excellent use of nested tables and MULTISET operators in SQL, blending data in tables with procedurally-generated datasets (nested tables).  All was going well when they hit the dreaded: ORA-04030: out of process memory when trying to allocate 2032 bytes  They asked for my help.  The error occurred on this SELECT: SELECT  *    FROM header_tab trx    WHERE (generated_ntab1 SUBMULTISET OF trx.column_ntab)       AND ((trx.column_ntab MULTISET             EXCEPT DISTINCT generated_ntab2) IS EMPTY) The problem is clearly related to the use of those nested tables. Now, there was clearly sufficient PGA for the nested tables themselves. So the problem was in executing the MULTISET-related functionality. We talked for a bit about dropping the use of nested tables and instead doing everything in SQL, to avoid the PGA error. That would, however require lots of wo...
4 comments
Read more

How to Pick the Limit for BULK COLLECT

This question rolled into my In Box today: In the case of using the LIMIT clause of BULK COLLECT, how do we decide what value to use for the limit? First I give the quick answer, then I provide support for that answer Quick Answer Start with 100. That's the default (and only) setting for cursor FOR loop optimizations. It offers a sweet spot of improved performance over row-by-row and not-too-much PGA memory consumption. Test to see if that's fast enough (likely will be for many cases). If not, try higher values until you reach the performance level you need - and you are not consuming too much PGA memory.  Don't hard-code the limit value: make it a parameter to your subprogram or a constant in a package specification. Don't put anything in the collection you don't need. [from Giulio Dottorini] Remember: each session that runs this code will use that amount of memory. Background When you use BULK COLLECT, you retrieve more than row with each fetch, ...
4 comments
Read more

PL/SQL 101: Save your source code to files

PL/SQL is a database programming language. This means that your source code is compiled into  and executed from within the Oracle Database. There are many fantastic consequences of this fact, many of which are explored in Bryn Llewellyn's Why Use PL/SQL? whitepaper. But this also can mean that developers see the database as the natural repository for the original source code , and this is a bad mistake to make. It's not the sort of mistake any JavaScript or Java or php developer would ever make, because that code is not compiled into the database (well, you can  compile Java into the database, but that's not where 99.99% of all Java code lives). But it's a mistake that apparently too many Oracle Database developers make. So here's the bottom line: Store each PL/SQL program unit in its own file . Use a source code control system to manage those files. Compile them into the database as needed for development and testing. In other words: you should never kee...
16 comments
Read more