Thursday, June 26, 2014

DB2 for z/OS: the Functional Advantages of Native SQL Procedures

I have been a big fan of DB2 for z/OS native SQL procedures ever since the functionality was introduced with DB2 9, back in 1997. I have posted quite a few blog entries on the topic, the first in 1998 (written while I was working as an independent consultant) and the most recent just last month. In these blog entries, and in presentations I've delivered over the past several years (and in discussions generally), I've focused largely on performance aspects of native SQL procedures versus external stored procedures. These performance pluses (native SQL procedures run in the DB2 database services address space, they run under the task of the calling application process, and they are zIIP-eligible when called by a DRDA requester) are important, but lately I've found myself thinking, more and more, about the functional advantages of native SQL procedures. That's the thrust of this blog post.

Right out of the gate with DB2 9 for z/OS (new-function mode) there was some space, functionality-wise, between native and external SQL procedures: a native SQL procedure could include a nested compound statement, while an external SQL procedure could not (a compound SQL statement, typically the heart of a SQL procedure, is a group of SQL statements, set off by BEGIN and END, in which there can be variable declarations and associated assignments, along with SQL logic flow control statements such as IF, WHILE, and ITERATE). What does support for a compound SQL statement within another compound SQL statement mean to a developer? It means that he or she can create SQL procedures that have multi-statement condition handlers, for more sophisticated processing of exception and/or error conditions that might occur in the execution of the procedure.

The functional advantage of native SQL procedure usage advanced further with DB2 10 for z/OS (new-function mode), which allowed a native SQL procedure -- and only a native SQL procedure -- to have input and output parameters of the XML data type (and to specify the XML data type for variable declarations). Before DB2 10 (and even in a DB2 10 or DB2 11 environment, using anything other than a native SQL procedure), getting an XML data value to a stored procedure required serializing the XML document into character string or CLOB (depending on its size) and passing it to the stored procedure (and working with it in the stored procedure) in that form. Yuck.

DB2 11 for z/OS (in new-function mode) delivers two really cool SQL-procedure-only functional enhancements: array parameters and autonomous procedures. More information on these enhancements follow.

Array parameters

An array parameter is a parameter that contains, essentially, a "stack" of values. Before passing an array parameter to a native SQL procedure (or receiving an array as an output parameter of a native SQL procedure, or declaring and using an array variable in a native SQL procedure), you first have to create the array. Why? Because an array in a DB2 for z/OS context is a user-defined data type (UDT). In creating an array, you have two choices: ordinary and associative. In an ordinary array, elements are referenced by their ordinal position within the array (for example, the third element added to an ordinary array would be referenced as value 3 of that array). Elements in an associative array are referenced by user-provided index values; so, if I assigned an index value of 'Home' to a data value in an associative array containing phone numbers, I could reference a person's home number by using the 'Home' index value.

Here is the CREATE statement for an ordinary array:


In the above example statement, CHAR(6) refers to the data type of the values placed in the array (an example would be employee number '089234'), and 20 refers to the number of values that the array can hold (if no value were there, i.e., if [] had been specified instead of [20] after the keyword ARRAY, the maximum number of values that the array could hold would default to the high positive value for the INTEGER data type, which is 2147483647).

The CREATE statement for an associative array would look like this:


As with the ordinary array, the data type after the AS keyword refers to the data values that will be stored in the array ('Eastern State University' could be an example). The second data type specified for the array (and that second data type is your indication that it's an associative array) refers to index values for the array ('Graduate school - Masters' could be one such value). Note that an associative array, unlike an ordinary array, does not have an explicitly or implicitly specified maximum cardinality -- the cardinality of an associative array is based on the number of unique index values used when elements are assigned to the array.

As mentioned, a DB2 for z/OS stored procedure defined with array-type input and/or output parameters (or with references to array variables in the procedure body) must be a native SQL procedure; furthermore, the CALL that invokes such a SQL procedure can come from only two types of program: another SQL procedure language routine (SQL PL is the language in which SQL procedures are coded, and in which compiled SQL scalar functions can be coded) or a Java program that accesses the DB2 for z/OS server via the IBM Data Server Driver for JDBC and SQLJ type 4 driver. That second program type is really important in my eyes. Java programmers regularly work with arrays, and plenty of those folks had requested, prior to DB2 11's debut, the ability to pass an array to, or receive an array from, a DB2 for z/OS stored procedure.

Autonomous procedures

Consider this scenario: you have a transaction for which you want to record some information for each execution, even if the transaction fails before completion and is rolled back by DB2. A rollback would undo any data changes made by the transaction, right? So, how do you persist some information associated with the transaction? With DB2 11, you can do that with an autonomous procedure, which is a type of native SQL procedure. How would this work? Well, the transaction would call the autonomous procedure, and that SQL procedure would do its thing -- inserting, for example, some data into DB2 table T1. Control would then pass back to the caller, and the transaction would do its thing -- updating, let's say, data in table T2. If the transaction fails after updating T2, what happens? DB2 will back out the transaction's change of T2 data, but the insert into T1 performed by the autonomous procedure will not be backed out. Cool, eh?

What makes a DB2 11 native SQL procedure an autonomous procedure? Technically, it's the specification of the AUTONOMOUS option in the associated CREATE PROCEDURE (or ALTER PROCEDURE) statement. AUTONOMOUS would be used in place of the COMMIT ON RETURN option. Completion of a called autonomous procedure will drive a commit, but that commit will "harden" only the data changes made by the autonomous procedure -- it will have NO EFFECT on any data changes made up to that point by the calling program. This is made possible by the fact that the autonomous procedure's DB2 unit of work is independent from that of the calling application process. Because of this independence, locks acquired by DB2 for an application process are not shared with locks acquired for an autonomous procedure called by the application process. It is therefore theoretically possible that an autonomous procedure will encounter lock contention vis-a-vis its caller. That possibility might influence decisions you'd make about the locking granularity that you'd like DB2 to use for a table space (e.g., row versus page), if an autonomous procedure and its caller will change data in the same table.

The future?

Will future versions of DB2 for z/OS introduce other enhancements that fall into the "SQL procedures only" category? We'll have to wait and see about that, but it sure has been interesting to see the progressive augmentation of native SQL procedure functionality just over the past three DB2 versions. I'll be looking for more of the same, and that's one of the reasons that I'm a native SQL procedure advocate: they get better and better.

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