Tuesday, May 29, 2007

INSTEAD OF Triggers [DB2 9 for z/OS]

DB2 9 for z/OS introduces a new type of trigger: the INSTEAD OF trigger. The primary usage of INSTEAD OF triggers is to enable views that would not otherwise be updatable to support updates. Typically, a view that consists of multiple base tables cannot be updated. But with an INSTEAD OF trigger this problem can be surmounted. You can code an INSTEAD OF trigger to direct inserts, updates and deletes to the appropriate underlying tables of the view.


With the INSTEAD OF trigger, your application code does not have to include complex algorithms to specify which operations should be performed against views and which should be performed against base table. Instead, all actions are performed against the view and the activated trigger determines which underlying base tables are to be impacted.


So, you might choose to code an INSTEAD OF trigger on a view over a join to allow modifications on the view to go through to the underlying base tables joined in that view. Or you can encode and decode data within a view: for example, the view could contain the decryption functions while the INSTEAD OF triggers use the encryption functions to ensure security.


Only one INSTEAD OF trigger is allowed for each type of operation on a given subject view. That is, one for inserts, one for updates, and one for deletes. Therefore, you can have a grand total of three INSTEAD OF triggers per view.


DB2 executes the triggered-action instead of the insert, update, or delete operation on the subject view. Neither the WHEN clause nor the FOR EACH STATEMENT clause are allowed in INSTEAD OF triggers.


Furthermore, there are some restrictions on the view in order for an INSTEAD OF trigger to be allowed. First of all, the view must exist at the current server when the INSTEAD OF trigger is created. Additionally, none of the following are permitted for a view to have an INSTEAD OF trigger:

  • the WITH CASCADED CHECK option
  • a view on which a symmetric view has been defined
  • a view that references data encoded with different encoding schemes or CCSID values
  • a view with a ROWID, LOB, or XML column (or a distinct type that is defined as one of these types)
  • a view with a column based on an underlying column defined as an identity column, security label column, or a row change timestamp column
  • a with a column that is defined (directly or indirectly) as an expression
  • a view with a column that is based on a column of a result table that involves a set operator
  • a view with any columns that have field procedures
  • a view where all of the underlying base tables are DB2 Catalog tables or created global temporary tables
  • a view that has other views dependent on it

One way to think of INSTEAD OF triggers is that they contain the inverse of the logic in the body of the view. If the view joins tables, the trigger should break the join apart to modify the correct data. If the view decrypts columns, the INSTEAD OF trigger should encrypt the columns. Etc.


Let’s take a look at an example to better understand the INSTEAD OF trigger. First, we create a view that joins the EMP and DEPT tables:



CREATE VIEW EMP_DEPT (EMPNO, FIRSTNME, MIDINIT, LASTNAME,
PHONENO, HIREDATE, DEPTNAME)
AS SELECT EMPNO, FIRSTNME, MIDINIT, LASTNAME, PHONENO,
HIREDATE, DEPTNAME
FROM EMP, DEPT
WHERE EMP.WORKDEPT = DEPT.DEPTNO;



OK, so far, so good. But since this view is a join it is not updateable. Let’s fix this by coding up some INSTEAD OF triggers. First, we’ll take care of inserts:




CREATE TRIGGER E_D_ISRT
INSTEAD OF INSERT ON EMP_DEPT
REFERENCING NEW AS NEWEMP
FOR EACH ROW INSERT INTO EMPLOYEE
(EMPNO, FIRSTNME, MIDINIT, LASTNAME, WORKDEPT, PHONENO, HIREDATE)
VALUES
(EMPNO, FIRSTNME, MIDINIT, LASTNAME,
COALESCE
((SELECT DEPTNO FROM DEPT AS D WHERE D.DEPTNAME = NEWEMP.DEPTNAME),
RAISE_ERROR('70001', 'Unknown dept name')
),
PHONENO, HIREDATE);



An insert against the view would not be inserting a new department, so we will be inserting data into the EMP table. If the department does not exist, we’ll raise an error. Next we’ll consider updates:




CREATE TRIGGER E_D_UPD
INSTEAD OF UPDATE ON EMP_DEPT
REFERENCING NEW AS NEWEMP OLD AS OLDEMP
FOR EACH ROW
BEGIN ATOMIC
VALUES(CASE WHEN NEWEMP.EMPNO = OLDEMP.EMPNO THEN 0
ELSE RAISE_ERROR('70002', 'Must not change EMPNO') END);
UPDATE EMP AS E SET
(FIRSTNME, MIDINIT, LASTNAME, WORKDEPT, PHONENO, HIREDATE)
= (NEWEMP.FIRSTNME, NEWEMP.MIDINIT, NEWEMP.LASTNAME,
COALESCE((SELECT DEPTNO FROM DEPT AS D
WHERE D.DEPTNAME = NEWEMP.DEPTNAME),
RAISE_ERROR ('70001', 'Unknown dept name')),
NEWEMP.PHONENO, NEWEMP.HIREDATE)
WHERE NEWEMP.EMPNO = E.EMPNO;
END








Finally we take care of deletions:




CREATE TRIGGER E_D_DEL
INSTEAD OF DELETE ON EMP_DEPT
REFERENCING OLD AS OLDEMP
FOR EACH ROW
DELETE FROM EMP AS E WHERE E.EMPNO = OLDEMP.EMPNO;


Using an INSTEAD OF trigger, each requested modification operation made against the view is replaced by the trigger logic. The trigger performs the insert, update, or delete on behalf of the view. No application changes are required because the code is in the trigger which resides in the database.


If you want to read more about INSTEAD OF triggers, I recommend this quite extensive article (albeit for DB2 LUW) out on the IBM Developer Works web site.


Thursday, May 24, 2007

LOB Enhancements [DB2 9 for z/OS]

IBM focused their attention on improving DB2’s ability to store and manage LOB data in Version 9. As anyone who has tried to use LOBs in a previous version of DB2 knows, the usability limitations were troublesome. But with Version 9, IBM chips away at some of the more annoying LOB limitations.

FETCHing LOBs

Prior to Version 9, there were two methods you could deploy in your programs to fetch LOB data:

  • Fetching data into a pre-allocated buffer
  • Using a LOB locator to retrieve a handle on the data.

Both methods have their issues. Fetching data into a preallocated buffer can cause virtual storage constraint problems, especially for larger LOBs. On the other hand, using LOB locators that commit infrequently or do not explicitly free the locators can use considerable amounts of DB2 resources.

V9 introduces a new clause, WITH CONTINUE, for use on your FETCH statements. By coding your program to use WITH CONTINUE you can retrieve LOB columns in multiple pieces without using a LOB locator, and continue a FETCH operation to retrieve the remaining LOB data when truncation occurs. (Note: this method can be used with XML data, too.) You will have to manage the buffers and reassemble the pieces of data in your application program.

So, by specifying WITH CONTINUE on your FETCH statement you tell DB2 to allow subsequent FETCH CURRENT CONTINUE operations. These will allow you to access the remaining truncated LOB (or XML) column after the initial FETCH. If truncation occurs, DB2 will remember the truncation position and will not discard the remaining data. DB2 will return the total length that would have been required to hold all of the data of the LOB or XML column. This will either be in the first four bytes of the LOB host variable structure or in the 4 byte area that is pointed to by the SQLDATALEN pointer in the SQLVAR entry of the SQLDA for that host variable.

File Reference Variables

DB2 V9 adds support for a LOB file reference variable. A file reference variable is a host variable that is defined in a host language to contain the file name that directs file input and output for a large object (LOB).

Using file reference variables, large LOB values can be inserted from a file or selected into a file rather than a host variable. This means that your application program does not need to acquire storage to contain the LOB value. File reference variables also enable you to move LOB values from the DBMS to a client application or from a client application to a database server without going through the working storage of the client application.

LOBs and Utilities

The manner in which DB2 handles LOBs in utility processing has also been improved in DB2 V9.

Loading and unloading LOBs has been improved. For LOAD, an input field value can contain the name of the file that contains a LOB column value. The LOB column value will then be loaded from that file. For UNLOAD, you can store the value of a LOB column in a file and record the name of the file in the unloaded record in the base table.

What about REORG? Well, prior to V9 you could not access LOB data during a REORG. And a REORG did not reclaim physical space from the LOB data set because LOBs were moved within the existing LOB table space. V9 fixes these problems. During a REORG (in V9), the original LOB table space is drained of writers. All LOBs are then extracted from the original data set and inserted into a shadow data set. When this operation is complete, all access to the LOB table space is stopped (the readers are drained) while the original data set is switched with the shadow data set. At this point, full access to the new data set is enabled, and an inline copy is taken to ensure recoverability of data.

Additionally, both CHECK LOB and CHECK DATA have SHRLEVEL REFERENCE and SHRLEVEL CHANGE options. Using these you can minimize downtime. For LOBs, CHECK DATA checks for consistency between a base table space and the corresponding LOB or XML table spaces. The CHECK LOB utility identifies structural defects in the LOB table space and any invalid LOB values. Running CHECK (DATA or LOB) with SHRLEVEL REFERENCE indicates that applications can read from but cannot write to the index, table space, or partition that is to be checked. SHRLEVEL CHANGE means that applications can read from and write to the index, table space, or partition that is to be checked.

Performance

Finally, DB2 V9 provides several performance improvements related to LOBs. Using file reference variables or WITH CONTINUE to “chunk” read LOBs can improve performance over using locators.

And as we all know, removing locks can improve performance. DB2 V9 eliminates LOB locking for space allocation, as well as for insert, delete, update, and select operations. Additionally, a LOB lock is no longer required to serialize the consistency between the value of the LOB and the column of the base row for an uncommitted read operation.

Monday, May 21, 2007

New OLAP Capabilities [DB2 9 for z/OS]

DB2 9 for z/OS offers several new SQL improvements focused on improving OLAP functionality. The OLAP functions provide the ability to return ranking, row numbering, and existing aggregate function information as a scalar value in the result of a query. You can include OLAP specifications in an expression, in a select-list, or in the ORDER BY clause of a select-statement.


The result to which the OLAP specification is applied is the result table of the innermost subselect that includes the OLAP specification. OK, at this point you might well be asking, “So, what are these new OLAP things and what can they do for me?” Let’s take them one at a time.


First up, we have the RANK and DENSE_RANK functions. These functions specify that the ordinal rank of a row within the specified window is computed. Rows that are not distinct with respect to the ordering within the specified window are assigned the same rank. Here is a quick example:


SELECT
EMPNO,
LASTNAME,
FIRSTNAME,
SALARY+BONUS+COMM AS TOTAL_COMP,
RANK() OVER(ORDER BY SALARY+BONUS+COMM DESC) AS RANK_COMP
FROM EMP
WHERE SALARY+BONUS+COMM > 30000
ORDER BY LASTNAME;


This query will rank employees who have total compensation greater than $30,000, but order the results by last name. This allows you to rank data differently than the order in which it is presented.

You can define the results of ranking with gaps in the sequential rank numbering by using the RANK specification, or without gaps, by using the DENSE_RANK specification.

The difference between the two can be a bit difficult to decipher at first. Think of it this way: RANK specifies that the rank of a row is defined as 1 plus the number of rows that strictly precede the row. So, if two or more rows are not distinct you will get gaps in the ranking. With DENSE_RANK the rank of a row is defined as 1 plus the number of preceding rows that are distinct with respect to the ordering. In this case there will be no gaps in the sequential rank numbering. Consider the following data:


EMPNO LASTNAME FIRSTNAME SALARY BONUS COMM
----- -------- --------- ------ ------ ------
100 MULLINS CRAIG 500000 100000 400000
200 SHAW DONNA 25000 10000 0
300 ABBOTT RANDY 700000 300000 0
400 WISER BUD 10000 0 0
500 GREEN RACHEL 40000 2000 5000



The results of the previous query run against this data would look like this:


300 ABBOTT RANDY 1000000 1
500 GREEN RACHEL 47000 3
100 MULLINS CRAIG 1000000 1
200 SHAW DONNA 35000 4
400 WISER BUD 10000 5


Note that both ABBOTT and MULLINS earn the most, but the amount is the same, so they share the number one ranking. As this is not a dense rank, the next rank value is 3, and not 2.


The next OLAP specification introduced by DB2 9 for z/OS is ROW_NUMBER. ROW_NUMBER specifies that a sequential row number is computed for the row that is defined by the ordering, starting with 1 for the first row. If the ORDER BY clause is not specified in the window, the row numbers are assigned to the rows in an arbitrary order, as the rows are returned. This satisfies an often-requested capability to simply assign a number to the result rows of a query. Row numbers also enable easy formulation of queries for computing histogram statistics and they enable formation of other OLAP specifications (for example, moving sums, moving averages, and so on).


Here is a sample query using ROW_NUMBER:



SELECT
ROW_NUMBER() OVER(ORDER BY WORKDEPT, LASTNAME) AS NUMBER,
LASTNAME,
SALARY
FROM EMP
ORDER BY WORKDEPT, LASTNAME;


The result of a RANK, DENSE_RANK, and ROW_NUMBER specification is BIGINT, and the result cannot be null.


One more thing to consider for the new OLAP features is the ability to partition results for the OLAP specification. This is specified using the PARTITION BY clause with a partitioning-expression, which is an expression used to define the partitioning of the result table. Each column name that is referenced in a partitioning-expression must unambiguously reference a column of the result table of the subselect that contains the OLAP specification. A partitioning-expression cannot include a scalar-fullselect or any function that is not deterministic or has an external action.


Here is an SQL example using partitioning:



SELECT
WORKDEPT,
EMPNO,
LASTNAME,
FIRSTNAME,
EDLEVEL,
DENSE RANK() OVER
(PARTITION BY WORKDEPT ORDER BY EDLEVEL DESC) AS RANK_EDLEVEL
FROM EMP
ORDER BY WORKDEPT, LASTNAME;


This SQL ranks departments according to their education level. And because DENSE_RANK is specified multiple employees with the same rank in the department will not increase the next ranking.


Sometimes RANK, DENSE_RANK, and ROW_NUMBER are called window functions. An OLAP specification is not valid in a WHERE, VALUES, GROUP BY, HAVING, or SET clause. An OLAP specification cannot be used as an argument of an aggregate function. When invoking an OLAP specification, a window is specified that defines the rows and order of the rows over which the function is applied.

Tuesday, May 15, 2007

Implicitly Hidden Columns [DB2 9 for z/OS]

Another nice new feature deep in the bowels of DB2 9 for z/OS is the ability to hide columns from the SELECT * statement. As far back as anyone can remember the advice has been given to avoid using SELECT * in application programs. But I still see it every now and then.

Now don't get me wrong, SELECT * is a nice shorthand when you are writing quick & dirty SQL using SPUFI or some other ad hoc SQL tool. But it does not belong in your application programs because a subsequent ALTER to add a column will cause the program to fail because there are now more columns being returned than the programmer coded into the program.

"OK," you may be asking, "so what? I thought you were writing about DB2 9 here?" Fair enough. DB2 9 for z/OS adds the ability to code IMPLICITLY HIDDEN on the column specification of a CREATE or ALTER TABLE statement. By coding IMPLICITYLY HIDDEN, the column will not be visible in the result of a SQL statement unless you explicitly refer to the column by name. So, SELECT * will not return any implicitly hidden columns to the result set.

For example, in the following table C2 is implicitly hidden and will not be returned as the result of SELECT *:

CREATE TABLE T1
(C1 SMALLINT NOT NULL,
C2 CHAR(10) IMPLICITLY HIDDEN,
C3 TIMESTAMP)
IN DB.TS;

This has some obvious beneifts. First of all, if you are one of those shops where programmers did not follow the no SELECT * in programs rule, then you can simply add every new column with the IMPLICITLY HIDDEN attribute and those SELECT * statements will keep on running because they won't see the new columns.

Or, you might want to take a more comprehensive approach, and specify every column (except one, perhaps the key) of every new (or modified) table as IMPLICITLY HIDDEN. If every column except the key is hidden, then a SELECT * won't return anything except the key - you'll have to explicitly specify all of those other columns to get them into your result sets. Of course, this negates the ability to use SELECT * for quick & dirty SPUFI queries because implicitly hidden columns will be hidden there, too.

There are a few caveats on the usage of IMPLICITLY HIDDEN. You cannot specify IMPLICITLY HIDDEN for a column that is defined as a ROWID, or a distinct type that is based on a ROWID. Additionally, IMPLICITLY HIDDEN must not be specified for all columns of a table.

Monday, May 14, 2007

Index on Expressions [DB2 9 for z/OS]

DB2 9 for z/OS offers, for the first time, the ability to create an index on data that is not technically in the table. At this point, you may well be asking “What does that mean?” and “Why would I do that?” I’ll attempt to answer both of those questions.

Basically, the neat new feature is an extension to the CREATE INDEX statement that lets you create an index on an expression. That begs the question of just what is an expression… Well, an expression can be as simple as a column reference, or it can be a built-in function invocation or even a more general expression, with certain restrictions.

The expression that is being indexed is referred to as the key-expression. The maximum length of the text string of each key-expression is 4000 bytes after conversion to UTF-8. Each key-expression must contain as least one reference to a column of the table named in the ON clause of the index. Referenced columns cannot be LOB, XML, or DECFLOAT data types nor can they be a distinct type that is based on one of those data types. Referenced columns cannot include any FIELDPROCs nor can they include a SECURITY LABEL. Furthermore, the key-expression cannot include any of the following:

  • subquery
  • aggregate function
  • not deterministic function
  • function that has an external action
  • user-defined function
  • sequence reference
  • host variable
  • parameter marker
  • special register
  • CASE expression
  • OLAP specification

Unlike a simple index, the index key of an index on expression is composed by concatenating the result (also known as a key-target) of the expression that is specified in the ON clause. An index that is created on an expression lets a query take advantage of index access (if the index is chosen by the optimizer) and avoid a table space scan.

Perhaps an example would help to clarify this topic. Consider this sample DDL:

CREATE INDEX XUPLN
ON EMP
(UPPER(LAST_NAME))
USING STOGROUP DSN8G910
PRIQTY 360 SECQTY 36
ERASE NO
COPY YES;

This example would create an index on the LAST_NAME column of the EMP table, but would index on the data after applying the UPPER function. This is useful if you store the data in mixed case but submit queries with parameter markers (or host variables) in upper case only. By indexing the data as upper case the index better matches your queries.

Query performance can be enhanced if the optimizer chooses that index. When you use an index on an expression, the results of the expressions are evaluated during insertion time or during an index rebuild and are kept in the index. If the optimizer chooses to use that index, the predicate is evaluated against the values that are stored in the index. As a result, run-time performance overhead is eliminated.

This is a nice new feature of DB2 9 for z/OS that you should consider if you have queries that search on expressions.