Category: Db2 12

This month, I must thank one of my readers who simply asked, “Roy, can you do a newsletter about the Shared Communications Area (SCA) for me please?” Naturally, I replied (after getting two beers from him of course!) So now I wish to delve into the inner workings of the Coupling Facility (CF) and the SCA…

Warning: Scary Stuff Ahead!

In a non-datasharing world, a -DISPLAY GROUP shows this sort of output:                                       

DSN7100I  -DD10 DSN7GCMD
*** BEGIN DISPLAY OF GROUP(........) CATALOG LEVEL(V13R1M504)
                  CURRENT FUNCTION LEVEL(V13R1M504)          
                  HIGHEST ACTIVATED FUNCTION LEVEL(V13R1M504)
                  HIGHEST POSSIBLE FUNCTION LEVEL(V13R1M504) 
                  PROTOCOL LEVEL(2)                          
                  GROUP ATTACH NAME(....)                    
---------------------------------------------------------------------
DB2          SUB                     DB2    SYSTEM    IRLM    
MEMBER   ID  SYS  CMDPREF   STATUS   LVL    NAME      SUBSYS IRLMPROC
-------- --- ---- --------  -------- ------ --------  ----   --------
........   0 DD10 -DD10     ACTIVE   131504 S0W1      IDD1   DD10IRLM
---------------------------------------------------------------------
SPT01 INLINE LENGTH:        32138                                   
*** END DISPLAY OF GROUP(........)                                  
DSN9022I  -DD10 DSN7GCMD 'DISPLAY GROUP ' NORMAL COMPLETION

If that is what you see in your production system, dear reader, then this newsletter is, sadly, not for you!

Hopefully your output actually looks like my little test system:

DSN7100I  -SD10 DSN7GCMD                                             
*** BEGIN DISPLAY OF GROUP(GSD10C11) CATALOG LEVEL(V13R1M504)       
                  CURRENT FUNCTION LEVEL(V13R1M504)                 
                  HIGHEST ACTIVATED FUNCTION LEVEL(V13R1M504)       
                  HIGHEST POSSIBLE FUNCTION LEVEL(V13R1M504)        
                  PROTOCOL LEVEL(2)                                 
                  GROUP ATTACH NAME(SD1 )                           
---------------------------------------------------------------------
DB2          SUB                     DB2    SYSTEM    IRLM          
MEMBER   ID  SYS  CMDPREF   STATUS   LVL    NAME      SUBSYS IRLMPROC
-------- --- ---- --------  -------- ------ --------  ----   --------
MEMSD10    1 SD10 -SD10     ACTIVE   131504 S0W1      JD10   SD10IRLM
MEMSD11    2 SD11 -SD11     ACTIVE   131504 S0W1      JD11   SD11IRLM
---------------------------------------------------------------------
SCA   STRUCTURE SIZE:    16384 KB, STATUS= AC,   SCA IN USE:     3 %
LOCK1 STRUCTURE SIZE:    16384 KB                                   
NUMBER  LOCK ENTRIES:     4194304                                   
NUMBER  LIST ENTRIES:       16354, LIST ENTRIES  IN USE:          81
SPT01 INLINE LENGTH:        32138                                    
*** END DISPLAY OF GROUP(GSD10C11)                                  
DSN9022I  -SD10 DSN7GCMD 'DISPLAY GROUP ' NORMAL COMPLETION

The interesting stuff is from the “SCA   STRUCTURE SIZE:” line down to the “NUMBER  LIST ENTRIES:” line.

Off We Go!

To understand what an SCA is, we need to backtrack a second and first discuss what a CF is?

What is Inside?

The Coupling Facility is de facto *the* central part of data sharing, it contains three objects:

1. A lock structure called LOCK1, where all the table locks using hashes live, and Record Lock Elements (RLE),
2. A list structure that is actually the SCA, which contains a bunch of stuff I will go into later,
3. The Group Buffer pools. Technically, you can run without these but then why are you data sharing if you are not sharing data?

Lock Structure Details

It is called LOCK1 and the system lock manager (SLM) uses the lock structure to control shared Db2 resources like tablespaces and pages and can enable concurrent access to these. It is split internally into two parts: The first part is the Lock Table Entry (LTE), and the second part is a list of update locks normally actually called the Record List Entry (RLE). The default is a 50:50 split of memory. The size of this structure must be big enough to avoid hash contention, which can be a major performance problem. Do not forget that the IRLM reserves 10% of the RLEs for “must complete” processes so you never actually get to use them all!

List Structure Details

This is really the SCA and it contains Member names, BSDS names, Data Base Exception Table (DBET) statuses and recovery information. Typically, at installation time, you pick a number for the INITSIZE (first allocated size of the SCA) from a list of 16 MB , 32 MB, 64 MB or 128 MB. Each of these INITSIZEs then has a SIZE which is typically twice the INITSIZE size as a maximum limit.

Baboom!

IBM write quite happily “Running out of SCA space can cause Db2 to fail” – I can change that to “does” not “can”!

Double Trouble?

The LOCK1 and the SCA do *not* have to be duplexed but it is very highly recommended to do so, otherwise, you have a single point of failure which defeats the whole point of going data sharing, really.

Death by DBET

The DBET data is, strangely enough, the thing that can easily kill ya!

How So?

Imagine you have the brilliant idea of using COPY YES indexes as you have tested a few and seen that RECOVER INDEX is quicker, better, faster than REBUILD or DROP/CREATE for the critical indexes at your shop.

What Happened Next?

So how do you enable COPY YES at the index level? Just a simple ALTER INDEX xxx.yyy COPY YES is all it takes. But *what*, dear friends, does this ALTER do under the covers? It sets the INDEX to ICOPY status – “Not too bad”, you say as the index is fully available, “just wait until the next COPY and that status is then cleared” – But wait, that is a DBET status! It creates a DBET entry in your SCA… What if you alter 15,000 Indexes to all be COPY YES? Yep – Kiss goodbye to your Db2 sub-system!

Suicide Protection

Some software out there (RealTimeDBAExpert from SEGUS for example!) actually warns you about this and recommends you do this sort of thing in chunks. First the ALTER, then the COPY, one hundred blocks at a time and then the next batch etc.

and then if you do press PF1:

And even then, we have an emergency stop built-in that you can still override but then you must *know* the possible risk involved:

On the other hand, some software is like using SDSF DA when you put a P for “print” by your userid!

Now you know what *not* to do!

Automagic?

Use of the ALLOWAUTOALT(YES) has been discussed for years… on the one hand it automagically adds storage if you are running out, which is a good thing, *but* it also allows other competing systems to decrease storage which can then lead to you running out of storage and losing this Db2 sub-system… nasty, nasty!

Operator command time!

/f <yourirlm>,STATUS,STOR

Gives me:

DXR100I JD11002 STOR STATS
PC: YES  LTEW:  2 LTE:     4M RLE:   16354  RLEUSE:      18        
BB PVT:  1266M  AB PVT (MEMLIMIT):   2160M                         
CSA USE: ACNT:     0K  AHWM:     0K  CUR:  2541K  HWM:  6122K      
        ABOVE 16M:    64   2541K     BELOW 16M:     0      0K      
        AB CUR:               0K     AB HWM:               0K      
PVT USE:   BB CUR:  5684K        AB CUR:     5M                    
           BB HWM:    18M        AB HWM:     5M                    
CLASS   TYPE  SEGS     MEM   TYPE  SEGS     MEM   TYPE  SEGS     MEM
ACCNT    T-1     2      4M    T-2     1      1M    T-3     2      8K
PROC     WRK    14     70K    SRB     5      5K    OTH     4      4K
MISC     VAR    41   7549K    N-V    22    565K    FIX     1     24K

This maps pretty nicely to the – DISPLAY GROUP I just did again:

*** BEGIN DISPLAY OF GROUP(GSD10C11) CATALOG LEVEL(V13R1M504)       
                  CURRENT FUNCTION LEVEL(V13R1M504)                 
                  HIGHEST ACTIVATED FUNCTION LEVEL(V13R1M504)       
                  HIGHEST POSSIBLE FUNCTION LEVEL(V13R1M504)        
                  PROTOCOL LEVEL(2)                                 
                  GROUP ATTACH NAME(SD1 )                           
---------------------------------------------------------------------
DB2          SUB                     DB2    SYSTEM    IRLM          
MEMBER   ID  SYS  CMDPREF   STATUS   LVL    NAME      SUBSYS IRLMPROC
-------- --- ---- --------  -------- ------ --------  ----   --------
MEMSD10    1 SD10 -SD10     ACTIVE   131504 S0W1      JD10   SD10IRLM
MEMSD11    2 SD11 -SD11     ACTIVE   131504 S0W1      JD11   SD11IRLM
---------------------------------------------------------------------
SCA   STRUCTURE SIZE:    16384 KB, STATUS= AC,   SCA IN USE:     3 %
LOCK1 STRUCTURE SIZE:    16384 KB                                   
NUMBER  LOCK ENTRIES:     4194304                                   
NUMBER  LIST ENTRIES:       16354, LIST ENTRIES  IN USE:          18
SPT01 INLINE LENGTH:        32138                                   
*** END DISPLAY OF GROUP(GSD10C11)

Looking at this line in the Modify output:     

PC: YES  LTEW:  2 LTE:     4M RLE:   16354  RLEUSE:      18        

I only have two members so my LTEW (LTE Width) is two bytes, LTE is 4M which is my NUMBER LOCK ENTRIES: 4194304 , RLE is 16354 which is my NUMBER LIST ENTRIES: 16354 and RLEUSE is 18 which is my LIST ENTRIES IN USE: 18. A perfect match – if only they had agreed on a naming convention! ! !

You can also see that my SCA STATUS is AC for ACTIVE and the SCA IN USE is a measly 3%, so no worries for me today!

Finally, you can see that we have 50:50 split as the SCA and the LOCK1 are the same size: 16 MB.

One more line of interest:                  

BB PVT:  1266M  AB PVT (MEMLIMIT):   2160M       

Here you can see the calculated MEMLIMIT for the IRLM, which for my test system is very low, but you should check that the number is good for your site as the range is now much bigger!

Panic Time?

I would start to get seriously sweaty if the SCA IN USE ever got near 70%.

And?

I would not use ALLOWAUTOALT(YES) and allocate 256MB or, if forced, I would go with INITSIZE 128 MB and SIZE 256 MB and then ALLOWAUTOALT(YES).

Lock it Down!

The Max Storage for locks is between 2,048 MB and 16,384 PB with a default of 2,160 MB that I have in the PVT (MEMLIMIT) output above.

Locks per table(space) (NUMLKTS) is 0 to 104,857,600 with a default of 2,000 in Db2 12 and 5,000 in Db2 13. If this number is exceeded then lock escalation takes place unless it is zero in which case there is no lock escalation. As IBM nicely put it “Do not set the value to 0, because it can cause the IRLM to experience storage shortages”.

Locks per user (NUMLKUS) is also from 0 to 104,857,600 with a default of 10,000 in Db2 12 and 20,000 in Db2 13. 0 means no limit. IBM do not recommend 0 or a large number here unless it is really required to run an application. Db2 assumes that each lock is approximately 540 bytes of memory. Here you can also drain your IRLM until it runs out of ACCOUNT T-1 storage space:

DXR175E xxxxxxxx IRLM IS UNABLE TO OBTAIN STORAGE – PVT

This is not a message you ever want to see on the master console! Just do the math on your locks and your memory size!

The Bachelor Problem – Fear of Commitment

You must get the developers to COMMIT or not use row-level locking everywhere.

SCA DBET Calculation

The lock size for each DBET entry is approximately 1,864 bytes from my tests in Db2 12 FL 510. How did I determine that, you ask?

What I did, was multiple ALTER INDEX aaa.bbb COPY YES SQLs until the “SCA in use” percentage changed from 4% to 5%, and then I kept doing ALTERs and DISPLAYs until the percentage changed from 5% to 6%. It took me exactly 90 ALTERs and with an SCA size of 16,384KB, that gives me a size of around 1,864 bytes per DBET entry. Use this as a Rule Of Thumb for your DBET inducing ALTERs! It gives me a hard limit of about 6,000 Alters.

I hope you found this little discourse into the world of the Coupling Facility and SCA useful!

TTFN,

Roy Boxwell

This month is all about going parallel! In the Db2 world, we have had the ability to run SQLs using parallel processing for decades. It started off a bit wobbly and most people didn’t use it, or even like it, but these days it is extremely useful for certain cases.

Sort Yourself Out

Sort is very important here. If you can do a parallel sort with only one extra parallel task it will halve your elapsed time… if you can add more tasks it takes even less elapsed. Naturally, you do not save on CPU here, in fact it will probably “cost” more, but you are trading CPU for elapsed time.

Ground Rules

IBM states in the documentation that parallel processing is “Only for partitioned objects” but then they mention that even non-partitioned objects can benefit, as the access to the non-clustering index and the data can be done in parallel… which is not helpful if you only have clustering index access, of course!

Bells and Whistles!

There are quite a few things to adjust and play with on the road to parallel processing!

No Way!

If you declare your cursor as WITH HOLD and with isolation level RR or RS then there is *no* CPU parallelism allowed at all but you can still get parallel sorts.

ZPARM Time

First up is CDSSRDEF (CURRENT DEGREE) where the IBM recommendation must be read:

The ZPARM INDEX_IO_PARALLELISM should get an honorable mention here as it was there right up until Db2 11 and it is, in fact, still within the Db2 12 and 13 indexes at the end of the installation guide pdf, but it has thrown off its mortal coil…

INDEX_IO_PARALLELISM

Specifies whether I/O parallelism is enabled for index insertion.

Acceptable values: YES, NO

Default: YES

DSNZPxxx: DSN6SPRM INDEX_IO_PARALLELISM

Security parameter: No

YES I/O parallelism is enabled for index processing. I/O parallelism allows concurrent insert operations on multiple indexes and can reduce I/O wait time when many indexes are defined in a table.

NO I/O parallelism is disabled for index processing.

Naturally, YES is what it should be set to!

To round out the dearly departed, there was also this one:

PARALLELISM EFFICIENCY field (PARA_EFF subsystem parameter)

Controls the efficiency that DB2 assumes for parallelism when DB2 chooses an access path. Valid values are integers 0 – 100. The integer represents a percentage efficiency.

It came in Db2 9 with PM16020 and started life with a default value of 100 before getting a default change to 50 in Db2 10. It got deprecated in Db2 12 and was removed in Db2 13.

Then PARAMDEG with a handy Top Tip:

MAX DEGREE field (PARAMDEG subsystem parameter)

The PARAMDEG subsystem parameter specifies the maximum degree of parallelism for a parallel group. When you specify a non-zero value for this parameter, you limit the degree of parallelism so that Db2 cannot create too many parallel tasks that use virtual storage.

Acceptable values: 0 – 254

Default: 0

Update: option 30 on panel DSNTIPB

DSNZPxxx: DSN6SPRM PARAMDEG

0 Specifies no limit to the maximum degree of parallelism that Db2 chooses based on the cost estimate for the query and the system configuration, in particular the number of processors online. Db2 counts both general purpose and zIIP processors equally, and applies further adjustment to determine the degree to use.

1 – 254 Specifies the maximum degree of parallelism that Db2 uses. When optimization hints for parallelism are used, the value of the PARAMDEG subsystem parameter does not limit the degree of parallelism at bind time. However, the value of the PARAMDEG subsystem parameter is enforced at execution time. So, if the value of the PARAMDEG subsystem parameter is lower than the degree of parallelism that is specified at bind time, the degree of parallelism is reduced at execution time.

Tip: For systems with more than two zIIP processors configured, use the number of zIIP processors as the starting value, and then adjust as needed for your response time requirements.

Basically, set this value to be one or two times the number of online available CPUs but take into account the ZiiPs.

Then its new DPSI baby brother:

MAX DEGREE FOR DPSI (PARAMDEG_DPSI subsystem parameter)

The PARAMDEG_DPSI subsystem parameter specifies the maximum degree of parallelism that you can specify for a parallel group in which a data partitioned secondary index (DPSI) is used to drive parallelism.

A DPSI is a non-partitioning index that is physically partitioned according to the partitioning scheme of the table. When you specify a value of greater than 0 for this parameter, you limit the degree of parallelism for DPSIs so that Db2 does not create too many parallel tasks that use virtual storage.

Acceptable values: 0-254, DISABLE

Default: 0

Update: option 30 on panel DSNTIPB

DSNZPxxx: DSN6SPRM PARAMDEG_DPSI

Data sharing scope: All members use the same setting

0 Specifies that Db2 uses the value that is specified for the PARAMDEG subsystem parameter, instead of PARAMDEG_DPSI, to control the degree of parallelism when DPSI is used to drive parallelism. This is the default value for the field.

1 Specifies that Db2 creates multiple child tasks but works on one task at a time when DPSI is used to drive parallelism.

2-254 Specifies that Db2 creates multiple child tasks and works concurrently on the tasks that are specified. The number of specified tasks may be larger or smaller than the number of tasks as specified in PARAMDEG. When PARAMDEG is set to 1, the rest of the query does not have any parallelism.

DISABLE Specifies that Db2 does not use DPSI to drive parallelism. Parallelism might still occur for the query if PARAMDEG is greater than 1.

This is for fine-tuning the DPSI use case. Remember, you can have 4096 partitions and so it could well be that a query goes crazy if it sees the ability to go massively parallel. Here you can limit, or even inhibit, that from happening. Only use if you have been bitten by a rogue SQL!

Then we get to Utility parallel processing which is not for all Utilities – but REORG TABLESPACE and COPY are there!

MAX UTILS PARALLELISM field (PARAMDEG_UTIL subsystem parameter)

The PARAMDEG_UTIL subsystem parameter specifies the maximum number of parallel subtasks for some utilities.

PARAMDEG_UTIL affects the following utilities:

• REORG TABLESPACE

• REBUILD INDEX

• CHECK INDEX

• UNLOAD

• LOAD

• COPY

• RECOVER

Acceptable values: 0 – 32767

Default: 99

Update: option 34 on panel DSNTIPB

DSNZPxxx: DSN6SPRM PARAMDEG_UTIL

0 No additional constraint is placed on the maximum degree of parallelism in a utility.

1 – 32767 Specifies the maximum number of parallel subtasks for all affected utilities.

Interesting default huh? In Db2 11 it was actually zero!

LOAD must get a special mention here, as back in Db2 11 it got a new keyword PARALLEL
( nnn ) which enabled parallel loading from a *single* input file (Initially not for PBGs but then that was allowed in Db2 12). This little chestnut has often been forgotten.

Further into the guts of REORG is this little one:

REORG LIST PROCESSING field (REORG_LIST_PROCESSING subsystem parameter)

The REORG_LIST_PROCESSING subsystem parameter specifies the default setting for the PARALLEL option of the Db2 REORG TABLESPACE utility.

Acceptable values: PARALLEL, SERIAL

Default: PARALLEL

Update: option 37 on panel DSNTIPB

DSNZPxxx: DSN6SPRM REORG_LIST_PROCESSING

PARALLEL The default value PARALLEL specifies that the REORG TABLESPACE utility is to use a default PARALLEL YES option when the PARALLEL keyword is not specified in the utility control statement. The PARALLEL YES option specifies that the REORG TABLESPACE utility is to process all partitions that are specified in the input LISTDEF statement in a single execution of the utility.

SERIAL Specifies that the REORG TABLESPACE utility is to use a default PARALLEL NO option when the PARALLEL keyword is not specified in the utility control statement. The PARALLEL NO option specifies that each partition that is specified in the input LISTDEF statement is to be processed in a separate execution of the utility.

I would also happily stick with the default unless you have experienced serious problems with VTS or some such.

Don’t Forget the Bufferpool, Stupid!

BUFFERPOOLs play a major role. The setting of VPSEQT (Virtual bufferpool sequential steal threshold) and VPPSEQT (Virtual bufferpool parallel sequential threshold) might well both have to be raised. Remember that the VPPSEQT is a percentage of the VPSEQT available pages. The bufferpool size itself might well have to be raised (VPSIZE) if you do not see an improvement in the degree of parallelism.

All Ready?

So, you have set, checked, reviewed, changed the ZPARMS and are ready to go?

Where’s the ON Switch?

For Static SQL just bind or rebind with DEGREE(ANY), for dynamic SQL issue a

SET CURRENT DEGREE = ‘ANY’ ;

CDSSRDEF Zparm has the default for this parameter.

If you bind with isolation level CS, then also try and make sure you use CURRENTDATA(NO) as well. This helps performance anyway and also aids Db2 in working with ambiguous cursors. Explicit read-only is always better!

For sorts, where the big elapsed time gains come from, make sure you have sufficiently sized work files allocated! Here, even the WITH HOLD and isolation RR or RS can benefit.

Gotchas?

Always at least one problem isn’t there? If you do DEGREE(ANY) you can expect the EDMPOOL usage to go up between 50% and 70% due to run-time structures. Check your SYSPACKAGE AVGSIZE before and after the BIND/REBIND if you are worried. Always monitor this pool and make sure it is correctly sized for your workload!

Naturally, the CPU might well go up but you should see a good drop in elapsed times and, as far as sort is concerned, you might actually manage a successful parallel sort in a normally constrained sub-system!

Where’s the OFF Switch?

For Static SQL just bind or rebind with DEGREE(1), for dynamic SQL issue a

SET CURRENT DEGREE = ‘1’;

This is also the default value but what if someone changed your default?

If you shrink the VPPSEQT to 0 that will disable all parallel access for objects in that bufferpool.

Insert rows in the resource limit tables – Not recommended as this is a lot of work!

Are You Running in Parallel then? EXPLAIN is Your Friend!

There are a few “access patterns” that allow CP parallelism and they are all documented in the Managing Performance book – “Checklist of query restrictions for query CP parallelism” table.

How do You Check?

The columns of interest in the PLAN_TABLE are the ACCESS_DEGREE and JOIN_DEGREE. If either of these two is not NULL then you are using, or hoping to use, parallel processing! The moment you have more than one table then four other columns become interesting: ACCESS_PGROUP_ID, JOIN_PGROUP_ID, SORTN_PGROUP_ID and SORTC_PGROUP_ID. PGROUP is short for PARALLEL GROUP and for a given PLANNO step they all have the same number. Finally, the PARALLELISM_MODE reflects which type you are using which, these days, can *only* be ‘C’ for Query CP parallelism (Came in DB2 V4). It used to also have the values ‘I’ for parallel I/O operations (Came in DB2 V3)  and ‘X’ for Sysplex query parallelism (Came in DB2 V5) but they were deprecated in Db2 9 and are now both dead and buried!

Which SQLs are Best for Parallel Processing?

SQLs that are I/O intensive and scan lots of pages while returning just a few rows, SQLs that have lots of aggregate functions, and naturally SQLs that require Sort – all of these are good candidates.

Trial it DUMMY!

Best thing to do is a trial rebind to a dummy collection of all your SQL in a sandbox system with production statistics copied over and with parallel processing enabled (Do not forget the bufferpools!). This will then quickly reveal which queries could indeed benefit from going parallel and enable you to activate it only at the package/SQL level that you require in production.

Apply and Test

Once you have your candidate list, and I hope it is not that long, you can simply enable it all in production and do a live run through and review. First with EXPLAIN and then really live.

Remember that it is not good for *all* SQLs but it can really help when it hits the spot!

My Favorite Table Ever!

Db2 11 introduced an automatic five level control system for parallel queries:

Level 1 OK: Query runs with planned parallel degree

Level 2 Mild warning: Reduce parallel degree by ¼

Level 3 Moderate warning: Reduce parallel degree by ½ or to degree 2

Level 4 Severe warning: Reduce to sequential run

Level 5 Melt down: Reduce to sequential run

Level 5 is sub-optimal!

The original table is here: (Do a search for “Melt”)

https://www.redbooks.ibm.com/redbooks/pdfs/sg248222.pdf

How to Tame the Beast?

For static SQL, just bind/rebind with DEGREE(1) to switch off or DEGREE(ANY) to switch on.

For dynamic SQL, if you cannot add the SET CURRENT DEGREE = ‘ANY’ and RLF tables do not work for you then the only way is to assign the tables in question to their own bufferpools and set the VPPSEQT to a value, or leave at default 50, and for *all* other bufferpools set the VPPSEQT to 0 which switches off parallel processing.

Another way of handling dynamic, but a bit over the top for my taste, is a new data-sharing member where the ZPARM CDSSRDEF is set to ‘ANY’. Then any dynamic work that should be allowed to go parallel is simply routed to just this member.

Whaddya all think?

Going to start testing out parallel processing anytime soon?

I’d love to hear from you!

TTFN

Roy Boxwell

Feedback:

One of my readers mentioned that the primary reason they use parallelism is to increase zIIP offload which offers not just elapsed reduction but also saves the cost of the cpu as well.

Naturally, if you get this, then you are really laughing all the way to the bank!

Aren’t I supposed to be wishing Happy New Year there? Not this month! One of my readers asked me a question about SQLCODEs and it opened up a quite fascinating can of worms!

What is the SQLCODE?

Anyone reading this blog should already know exactly what the SQLCODE is, however, for those of you out there using Google in 2045, the definition is, at least in COBOL:

01 SQLCA.
   05 SQLCAID     PIC X(8).
   05 SQLCABC     PIC S9(9) COMP-5.
   05 SQLCODE     PIC S9(9) COMP-5.
   05 SQLERRM.
      49 SQLERRML PIC S9(4) COMP-5.
      49 SQLERRMC PIC X(70).
   05 SQLERRP     PIC X(8).
   05 SQLERRD     OCCURS 6 TIMES
                  PIC S9(9) COMP-5.
   05 SQLWARN.
      10 SQLWARN0 PIC X.
      10 SQLWARN1 PIC X.
      10 SQLWARN2 PIC X.
      10 SQLWARN3 PIC X.
      10 SQLWARN4 PIC X.
      10 SQLWARN5 PIC X.
      10 SQLWARN6 PIC X.
      10 SQLWARN7 PIC X.
   05 SQLEXT.
      10 SQLWARN8 PIC X.
      10 SQLWARN9 PIC X.
      10 SQLWARNA PIC X.
      10 SQLSTATE PIC X(5).

The third field within the SQLCA is the SQLCODE, which is a four-byte signed integer and it is filled after every SQL call that a program/transaction makes.

All Clear So Far?

So far so good! In the documentation from IBM is this paragraph about how to handle and process SQLCODEs:

SQLCODE

Db2 returns the following codes in SQLCODE:

• If SQLCODE = 0, execution was successful.

• If SQLCODE > 0, execution was successful with a warning.

• If SQLCODE < 0, execution was not successful.

SQLCODE 100 indicates that no data was found.

The meaning of SQLCODEs, other than 0 and 100, varies with the particular product implementing SQL.

Db2 Application Programming and SQL Guide

So, every programmer I have ever talked to checks if the SQLCODE is 0 – Green! Everything is fine, if the SQLCODE is negative – Bad message and ROLLBACK, if the SQLCODE is +100 – End of cursor or not found by direct select/update/delete – normally 100% Ok, everything else issues a warning and is naturally very dependent on the application and business logic!

What’s Wrong With This Picture?

Well, sometimes zero is not really zero… I kid you not, dear readers! The sharp-eyed amongst you, will have noticed the last bytes of the SQLCA contain the SQLSTATE as five characters. Going back to the documentation:

An advantage to using the SQLCODE field is that it can provide more specific information than the SQLSTATE. Many of the SQLCODEs have associated tokens in the SQLCA that indicate, for example, which object incurred an SQL error. However, an SQL standard application uses only SQLSTATE.

So, it still seems fine, but then it turns out that SQLCODE 0 can have non-all zero SQLSTATEs!

New in the Documentation

At least for a few people this is a bit of a shock. From the SQLCODE 000 documentation:

SQLSTATE

• 00000 for unqualified successful execution.
• 01003, 01004, 01503, 01504, 01505, 01506, 01507, 01517, or 01524 for successful execution with warning.

Say What?

Yep, this is simply stating that you have got an SQLCODE 0 but up to nine different SQLSTATEs are possible… This is not good! Most error handling is pretty bad, but now having to theoretically add SQLSTATE into the mix makes it even worse!

What are the Bad Guys Then?

01003 Null values were eliminated from the argument of an aggregate function.

01004 The value of a string was truncated when assigned to another string data type with a shorter length.

01503 The number of result columns is larger than the number of variables provided.

01504 The UPDATE or DELETE statement does not include a WHERE clause.

01505 The statement was not executed because it is unacceptable in this environment.

01506 An adjustment was made to a DATE or TIMESTAMP value to correct an invalid date resulting from an arithmetic operation.

01507 One or more non-zero digits were eliminated from the fractional part of a number used as the operand of a multiply or divide operation.

01517 A character that could not be converted was replaced with a substitute character.

01524 The result of an aggregate function does not include the null values that were caused by evaluating the arithmetic expression implied by the column of the view.

Not Good!

From this list the 01004, 01503, 01506 and especially 01517 just jump right out and scream at you! Here in Europe, we have a right to have our names or addresses correctly written and, in Germany with all the umlauts, it can get difficult if you then have a 01517 but SQLCODE 0 result!

I hope you don’t find this newsletter too unsettling as, after all, Db2 and SQL normally works fine, but I do think that these SQLSTATEs should really have warranted a positive SQLCODE when they were first created…

What do you all think?

TTFN,

Roy Boxwell

Update:

One of my readers wonders how practicle these “errors” are. A good point, and so here is a nice and easy recreate for the 01003 problem:

CREATE TABLE ROY1 (KEY1   CHAR(8) NOT NULL,                 
                   VALUE1 INTEGER ,                         
                   VALUE2 INTEGER )                         
;                                                           
INSERT INTO ROY1 (KEY1)               VALUES ('A') ;        
INSERT INTO ROY1 (KEY1)               VALUES ('AA') ;       
INSERT INTO ROY1 (KEY1,VALUE1)        VALUES ('B', 1) ;     
INSERT INTO ROY1 (KEY1,VALUE1,VALUE2) VALUES ('BB', 1 , 1) ;
INSERT INTO ROY1 (KEY1,VALUE1)        VALUES ('C', 2) ;     
INSERT INTO ROY1 (KEY1,VALUE1,VALUE2) VALUES ('C', 2 , 2) ; 
SELECT * FROM ROY1                                          
;                                                           
CREATE VIEW ROYVIEW1 AS                                     
(SELECT AVG(VALUE1) AS AVGVAL1, AVG(VALUE2) AS AVGVAL2      
 FROM ROY1)                                                 
;                                                           
SELECT * FROM ROYVIEW1                                      
;                                                           

This set of SQL ends up with these outputs:

---------+---------+---------+--------
KEY1           VALUE1       VALUE2    
---------+---------+---------+--------
A         -----------  -----------    
AA        -----------  -----------    
B                   1  -----------    
BB                  1            1    
C                   2  -----------    
KEY1           VALUE1       VALUE2    
---------+---------+---------+--------
C                   2            2    
DSNE610I NUMBER OF ROWS DISPLAYED IS 6

---------+---------+---------+---------+---------+---------+-----
    AVGVAL1      AVGVAL2                                                
---------+---------+---------+---------+---------+---------+-----
          1            1                                                
DSNT400I SQLCODE = 000,  SUCCESSFUL EXECUTION                           
DSNT418I SQLSTATE   = 01003 SQLSTATE RETURN CODE                        
DSNT415I SQLERRP    = DSN SQL PROCEDURE DETECTING ERROR                 
DSNT416I SQLERRD    = 0 0  0  -1  0  0 SQL DIAGNOSTIC INFORMATION       
DSNT416I SQLERRD    = X'00000000'  X'00000000'  X'00000000'  X'FFFFFFFF'
         X'00000000'  X'00000000' SQL DIAGNOSTIC INFORMATION
DSNT417I SQLWARN0-5 = W,,W,,, SQL WARNINGS                              
DSNT417I SQLWARN6-A = ,,,,   SQL WARNINGS                               
DSNE610I NUMBER OF ROWS DISPLAYED IS 1                                  

This is not so good if you ask me…

Ahhh! What a great time I just had in sunny ol’ Toronto. I was extremely happy to be back at the Central Canada Db2 Users Group (CCDUG) meeting for 2023 held at the BMO Institute For Learning. It has always been a fantastic venue for presenters and attendees, and this year we had the added bonus of a “Coyote” warning… fun fun fun!

The CCDUG is a two day, three track mini-IDUG if you like, and the organizers added onto the second day an AppDev track as well. So you had Db2 for z/OS, Db2 for LUW, IMS and AppDev – A very nice mix of things indeed!

Just the Facts, Ma’am!

I am mainly a z/OS style of guy (old school!) and so went for the z/OS track of course! It all started, as normal, with a keynote from Michael Kwok that was all about “accidental discoveries” and how these often have had a tremendous impact on the world. Examples included Penicillin and Microwave cookers. It also included Db2!

He highlighted the environmental factors required for innovation to succeed:

01 Have an open mind

02 Use Strategy as a filter

03 Commit

Open Minded?

For the first point, Penicillin, having an open mind was important when Alexander Fleming, in September 1928, noticed the effect that a mold had on his old petri dishes. It was having an open mind that enabled him to really see the very first antibiotic in action. Sadly, it took another 14 years before it was actually used in a pure form and yet another year before mass production started in the second world war.

Strategy is Everything!

Using Strategy as a filter was highlighted by looking at how Bill Gates took advantage of the CP/M creators Digital Research people’s failure to meet and agree a deal with IBM to discuss a new OS for the IBM 5150 PC. IBM then contacted Microsoft, who agreed to write PC-DOS which evolved into MS-DOS which basically started the modern PC world we all now know today.

Commit is Not Just a Db2 Verb!

Commit was all about the invention of Microwave Cookers! Percy Spencer working at Raytheon in 1945 was working with Radar sets using high energy magnetrons (A British invention, again in the second world war) and he noticed that a chocolate bar in his pocket melted. It took an awful lot of commitment to eventually get the product to market and it first appeared in 1947 as the “Radarange” at $5000, 3kw and was water-cooled!

Oh oh! IBM at Work…

Into these three ideas comes IBM! Back in the 60’s and 70’s, data centers started storing more and more data and Tedd Codd wrote down the rules of the Relational Data Model in his seminal paper “A Relational Model of Data for Large Shared Data Banks” in June 1970. Now back in those days, IBM was not exactly well known as being Open Minded, Strategic or Committed!

Then, a few extraordinary individuals pulled it all together to actually get what was called System R, the granddaddy of all Relational Databases, into existence, starting in 1974 and got its first customer in 1977. It still took another six years to actually arrive at DB2 in 1983, and ten years later, we got DB2 for LUW as well. The actual GA date of DB2 1.1 was the 2nd of April 1985.

So, basically, Db2 was an accidental discovery!

Then it Begins!

The sessions at CCDUG then all kicked off, so here is my list of the ones I either attended or read up afterwards (naturally, I cannot attend every session!) All are linked directly to the CCDUG website and I thank them for allowing public access!

Database Administration Enhancements of Db2 13 for z/OS from Robert Catterall. An excellent run through everything you need to know about Db2 13 including more PROFILE support. This is probably one of the most under-used Db2 features out there.

Db2 for z/OS 101: Buffer Pools and Group Buffer Pools from Tori Felt and Keziah Knopp. Was a highly entertaining 101 level intro to the complex world of Buffer Pools, Managers and disk usage within Db2 – Highly recommended as these little details easily get lost nowadays!

It’s AI Jim, but not as we know it! from me. Obviously, a fantastic presentation all about AI – truth or hype? 😄

Analysis of DEADLOCK IFCID 172 in SMF with no special tool and low cost from Xiaodong Ma. This was all about reading the bits and bytes from the IFCID to find *all* the blockers and waiters – not just the first two you get in the DSNT375I messages.

Who is afraid of DDF? from Toine Michielse was all about controlling your DDF workload simply and easily. Use of PROFILE is also recommended …

End of Day One

Then we had a very pleasant evening with the Vendor Expo and prize draws before retiring to the bar to drink some cold beverages and play pool!

Next day, bright-eyed and bushy-tailed we started off again.

Securing Db2 for z/OS Data: Encryption and Much More from Robert Catteral which contained a ton of great information all about encrypting and securing your data and communications – Great stuff!

An Audit a day keeps the lawyers at bay! from my colleague Ulf Heinrich listed out all the requirements for auditing a Db2 system from the auditor’s perspective. Including real-world examples and ending with a “how it looks in Zowe” bonus preview!

Db2 for z/OS Administrative and Developer tools strategy: Customer feedback, roadmap, and future direction from Sowmya Kameswaran was all about the ongoing project to unify the user experience in Db2. This includes the IBM Unified Management Server for z/OS, IBM Db2 Administration Foundation for z/OS, IBM Db2 DevOps Experience for z/OS, IBM Query Workload Tuner for z/OS , IBM Db2 Analytics Accelerator Administration services for z/OS , Zowe and IBM Db2 for z/OS Developer Extension! A ton of stuff here!

Partition By Range (PBR) and Relative Page Number (RPN) explained from Frank Rhodes drilled down into the nitty gritty details of PBG, PBR and the seven-byte RID!

Db2 For z/OS and Unicode – What you need to know from Chris Crone went into details about what exactly you need to know about UNICODE and how it affects you, your data, and your SQL applications! COLLATION_KEY is the really good one here!

Db2 for z/OS System Profile Monitoring: Overview and Db2 13 Enhancements from Mark Rader contained all the great new stuff in Db2 13 regarding PROFILE usage. Once again well worth a read as it is the best way to control DDF. Slide 15 is the highlight for me!

On the Application development front there were also a whole bunch of great presentations:

Db2 SQL and SQL PL – A Journey Through Db2 12 and 13 Functions Levels from Chris Crone where he runs through the new SQL and SQLPL stuff from Db2 12 before showing all the new stuff in Db2 13 as well.

Db2 for z/OS: REST and Hybrid Cloud from Tori Felt and Keziah Knopp was a huge presentation all about RESTful APIs and all the fun and games involved with them… Covered here was all the stuff on GET, POST, PUT etc. etc., also JSON, z/OS Connect et al!

Db2 Hot topics from Progressive Insurance from Bob Vargo and Dustin Ratliff. They discussed the challenges of how to get SQL Data Insights up and running and then got into FTB and PROFILE usage again (I love PROFILEs, have I mentioned this before?)

Explain explained from Toine Michielse. Does what it says on the side of the can! He shows, and explains, all the available access paths so that everyone is “on the same page” afterwards.

Bringing Db2 for z/OS-Based Applications Into the Modern Age from Robert Catterall where he showed the modern new way to access the data using REST services with plus and minus points of them. Then he went on to highlight the “new features” in Db2 for z/OS like System and Business time or XML columns and tables and finished off with a review of the advantages that SQL PL and Native SQL Procedures can give you.

DB2 SQL – go beyond the usual – My current TOP 40 SQL tips, tricks, and opinions from Brian Laube where he discussed all of the tricks and secrets he has learned over the years. Very worthy of a good read!

That’s all folks! I did not even mention all of the LUW or the IMS ones – Feel free to look for them as well if you are interested.

As always, a fantastic conference and I am already looking forward to the next one in 2024!

TTFN,

Roy Boxwell