The Online Schema (R)evolution
Ken McDonald
BMC Software
Written by Steven Thomas
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Agenda
• Changing Schemas Online before DB2 10
• Introducing Deferred ALTER in DB2 10 for z/OS
• Deferred ALTER in more detail
• Describe each type of change using new feature
• Including highlighting some of the less obvious side effects
• What’s still missing?
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What is OLS or Online Schema Change?
“The ability to alter characteristics of DB2 objects online,
without the need for an Unload, Drop, Create & Load”
• Before V8 most object changes required the following:
• Analyze the impact of the changes; resolve all conflicts; extract object
definitions and retain authorizations
• Unload the data
• Drop & Recreate the object with the new definitions
• Re-create dependent objects such as indexes, views and triggers
• Re-establish authorizations for the newly created objects
• Reload the data; copy, collect statistics, check RI as needed
• Rebind packages and plans
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Every ALTER has an Impact
• Even Immediate changes require exclusive use of the
Catalog & Directory and can have a performance impact
• DB2 10 Catalog changes will help here – see session A17
• AREO* Status should set alarm bells ringing
• Some require a Utility (usually REORG) to implement
• e.g. Alter Tablespace COMPRESS YES (DB2 9)
• Others make the object inaccessible until action taken
• e.g. Changing PIECESIZE places Index into PSRBD
• Another example for DB2 10 as we’ll see is converting to Hash
• A few even require objects to be stopped
• e.g. Changing Bufferpool for object when Data Sharing (DB2 9)
• I’m not aware of any of these in DB2 10 for z/OS
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Even if an ALTER is effective immediately
• Need to Synchronize Application Code Changes
• Data Type changes can often be accommodated (temporarily)
• New data types have to be compatible and large enough
• Some Numeric changes more difficult due to Indexes
• Renamed columns and tables usually need code changes
• OLS Changes may impact other objects
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Plans, Packages and Dynamic Statement Cache
Dependent Views
Referential Integrity
Check Constraints or other methods of Data Validation
Triggers
Stored Procedures, UDFs & UDTs
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Package Invalidation & Auto Rebind
• Occurs when changing items referenced in Static SQL
• The next invocation causes an Auto Rebind to occur
• Dynamic SQL in Statement Cache invalidated
• Care required with Auto Rebind
• Access path can switch – the index may be unavailable
• Some Statistics may get invalidated and need to be refreshed
• You only get one shot at a Rebind
• If it fails you get an INOPERATIVE Plan or Package
• Watch for timeouts in Catalog after a change to a busy object
Add REBIND explicitly to any activity
that invalidates packages
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Deferred ALTER in DB2 10 for z/OS
• ALTER Statement Validated & Authorization is checked
• Change registered in a new Catalog Table
• SYSIBM.SYSPENDINGDDL
• Tablespace goes into new AREOR State (Advisory)
• Non-restrictive state indicating Changes are Pending
• ALTER statement returns SQLCODE +610
• Regular DML activity continues as normal
• Also get +610 when a more restrictive Pending state set
• Such as RBDP or REORP
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Deferred ALTER in DB2 10 for z/OS
• Changes implemented by next REORG
• SHRLEVEL REFERENCE and CHANGE only
• SHRLEVEL NONE allowed but does not implement changes
• REORG can be at Tablespace or Index level
• REORG TABLESPACE also implements any Index Changes
• Almost all changes require UTS objects
• Except for migration of objects to UTS
• Option to cancel any Pending changes available
• ALTER TABLESPACE ... DROP PENDING CHANGES
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So what can you change?
• Convert older style structures to UTS
• Convert Single Table Segmented TS => UTS PBG
• Convert Single Table Simple TS => UTS PBG
• Convert Classic Partitioned TS => UTS PBR
• Change the following Tablespace Attributes:
• DSSIZE
• SEGSIZE
• MEMBER CLUSTER
• Page Size via Bufferpool (Tablespace and Index)
• Was available for Indexes in DB2 9 but went into RDBP
• Convert Tablespaces to and from HASH access
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Changing Tablespace Types
Source: DB2 10 for z/OS Technical Overview (Redbook)
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Restrictions
• Can’t mix Pending and Immediate ALTER in single SQL
• SQLCODE -20385
• Many immediate ALTERS are not possible while changes
are pending
• Also SQLCODE -20385
• See Manuals for details – almost 2 pages of restrictions!
• Generally safer to materialize the Pending Change first
• Must be using Single Table Tablespaces
• Requires UTS unless converting to a UTS
• Primary exception is changing Pagesize for LOB Tablespaces
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How changes are registered
• One or more rows added to SYSIBM.SYSPENDINGDDL
• Each Pending option has one row
• Even if defined in the same SQL statement
• Some changes can only be specified one at a time
• DSSIZE, SEGSIZE and DROP PENDING CHANGES
• MAXPARTITIONS & BUFFERPOOL allowed with other changes
• But you can’t mix Immediate and Pending anyway
• Rows are recorded and applied in the order executed
• For example, conversion to UTS must take place before one of
the other changes such as DSSIZE or SEGSIZE
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SYSIBM.SYSPENDINGDDL
• DBNAME, TSNAME, DBID, PSID, OBJSCHEMA,
OBJNAME, OBJOBID, OBJTYPE Define the object
• STATEMENT_TYPE
Currently always ‘A’ for ALTER
• OPTION_KEYWORD Keyword of Pending Option
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OPTION_VALUE
OPTION_SEQNO
CREATED_TS
ROWID
STATEMENT_TEXT
(e.g. DSSIZE or SEGSIZE)
Value of new option
Sequence in Statement
Timestamp
ROWID for Text Column
Original Statement (2Mb CLOB)
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Some points to be aware of...
• Some changes can be Immediate or Deferred
• e.g. ALTER TABLESPACE.... BP is Immediate unless
• The New Bufferpool uses the same Page size OR
• There’s already a Deferred Action on the Tablespace
• Some changes can cause different restrictive states
depending upon how Base Tablespace is defined
• Changing Index Pagesize causes AREOR if the base object
is a UTS but RBDP if it isn’t
• Both cause an SQLCODE +610
• Could have a huge impact
• No way to tell the difference without a DISPLAY!
• Changes on non-materialized objects are Immediate
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Dropping Pending Changes
• ALTER TABLESPACE .... DROP PENDING CHANGES
• No equivalent for Indexes
• Dropping Pending Changes for Base Tablespace drops them
• Cannot Select what to drop – all or nothing
• One thing I discovered that really surprised me:
• Dropping Pending Changes does NOT remove AREOR!
• Run a REORG to remove this
• or REPAIR ... NOAREORPEND - not recommended
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Implementing the Change
• All outstanding deferred ALTERS for an objects are
implemented by a single REORG of TS or IX
• Tablespace REORG also implements Index changes
• This includes a migration to UTS with other changes
• Not completely clear from Manuals but I’ve tested it!
• The change to UTS must come first if you have one
• Must use SHRLEVEL CHANGE or REFERENCE
• And FASTSWITCH YES
• Note these can also reset REORP in DB2 10
• No Recovery to point before the changes materialized
• You can UNLOAD from old Copies
• Note that the REORG will generate an Imagecopy
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What happens at Materialization?
• Any existing Statistics are invalidated
• REORG collects basic Statistics during Execution
• Default is TABLE ALL INDEX ALL HISTORY ALL
• No COLGROUP, KEYCARD, HISTOGRAM, Extended Indexes
or frequency Statistics where NUMCOLS>1 are collected
• This causes a RC=4 in the REORG
• Dependent Plans and Packages are Invalidated
• You will get one shot at an AUTOREBIND
• Regenerates Dependent Views
• Removes the rows from SYSPENDINGDDL
• Adds new entries into SYSCOPY
• ITYPE = ‘A’ and STYPE = ‘F’, ‘D’, ‘S’ or ‘M’
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Partition Growth
• It’s possible that a UTS PBG object will require additional
partitions when the REORG is executed
• For example, reducing SEGSIZE may result in more Free Pages
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New partitions will be automatically created as required
• Including XML and LOB Auxiliary Objects
• Regardless of the setting of the SQLRULES option
• SQLRULES(DB2) or SQLRULES(STD)
• Remember to check your Backup Strategy if this happens
• Hopefully you use LISTDEF & TEMPLATE which pick this up
automatically but worth checking!
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SYSIBM.SYSPENDINGOBJECTS
• Used by REORG to Store information about any new LOB
or XML objects that need to be created
• Generally caused by Partition Growth
• Stores information about the name and OBID and PSID
that are to be used
• Hopefully this table will usually be empty in your shop!
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Changing Simple/Segmented TS to PBG
• ALTER TABLESPACE... MAXPARTITIONS n
• Can also change MAXPARTITIONS for a PBG UTS
• Changes made when Materialized:
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SYSTABLESPACE column TYPE = ‘G’
SEGSIZE inherited if present but set to at least 32
DSSIZE set to 4
MEMBER CLUSTER inherited
If LOCKSIZE was TABLE it is changed to TABLESPACE
• Partition Growth may occur on implementation
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Changing Table Controlled Partitioned to PBR
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ALTER TABLESPACE... SEGSIZE n
Object must be Table Controlled not Index Controlled
Can also change SEGSIZE of any UTS object this way
Must be the only option specified on the ALTER
• OK to issue multiple ALTERS and implement together
• Changes made when Materialized:
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SYSTABLESPACE column TYPE = ‘R’
MEMBER CLUSTER inherited
Number of Partitions is inherited
FREEPAGE adjusted to 1 below SEGIZE if necessary
• May result in partition growth on implementation
• If DSSIZE = 0 then it is reset to the maximum allowed
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Changing Pagesize of a UTS Tablespace
• ALTER TABLESPACE... BUFFERPOOL BPxxx
• Change of page size only supported for UTS & LOB
• Not supported for XML objects
• Can use Bufferpool with the same page size for non-UTS
• Bufferpool must be valid and defined (VPSIZE > 0)
• Datasets required may increase or decrease
• Empty Partitions are not removed – allocated empty
• Immediate change if the Pagesize is identical
• Exception if Conversion to UTS is pending
• Implemented at next dataset Open (usually a START)
• In DB2 10 you no longer need to Stop objects across the Group
prior to changing the Bufferpool in a Data Sharing Environment
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Changing Pagesize of an Index
• ALTER INDEX... BUFFERPOOL BPxxx
• Supported in DB2 9 but object placed into RBDP
• Improved in DB2 10 for Indexes based on UTS objects
• For Standard indexes and those based on Auxiliary (LOB) or XML
tables defined on base UTS objects
• These are treated as Deferred ALTERs
• All others remain as they were using RBDP status
• You get SQLCODE +610 either way
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Changing DSSIZE of a UTS object
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ALTER TABLESPACE ... DSSIZE n
Change of page size only supported for UTS & LOB
Must be the only option specified on the ALTER
DSSIZE must be valid
• Based on Page Size and Number of Partitions
• Decreasing DSSIZE may result in Partition Growth
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Setting MEMBER CLUSTER for a UTS object
• ALTER TABLESPACE ... MEMBER CLUSTER YES
• Member Cluster was not supported for UTS in DB2 9
• This feature allows you to add it if necessary using a
Deferred Alteration
• Similar to the rest and only added here for completeness
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Converting Tables to Hash organization
• ALTER TABLE ... ADD ORGANIZE BY HASH
UNIQUE (col,col...) HASH SPACE nnnnn
• Also amend Hash objects using the following:
• ALTER TABLE ... ALTER ORGANIZATION
SET HASH SPACE nnnnn
• Restrictions:
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Column(s) must be defined as NOT NULL
Table cannot be APPEND(YES) or MEMBER CLUSTER
Tablespace must be UTS
Table cannot be a Global Temporary Table
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Converting to Hash
• When you issue this SQL
• SQLCODE +610 as normal
• Creates the Overflow Index which is placed into PSRBD
• Object in placed in AREOR
• This situation only allows UPDATE & DELETE statements
• No INSERT due to the Dependent Index in PSRBD
• Even though the Hash organization is not being used!
• Correct by Rebuilding Index or REORG Tablespace
• With REBUILD then INSERT is allowed but TS will still not be Hash
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Implementing Hash Organization
• REORG required to Implement Hash organization
• Care required with AUTOESTSPACE if adding more data
• Estimates the size required for the Hash Space
• If you expect to add more data you may get more use of the
overflow space than you want for good performance
• Unique Index on Hash Key still exists but is redundant
• You can Drop this whenever you wish
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Removing Hash Organization
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ALTER TABLE ... DROP ORGANIZATION
Reverts Table back to regular UTS PBR or PBG
This places object into REORP which is restrictive!
Also drops the Hash Overflow Index when executed
• If you don’t have a Unique Key you might want to add one before
you run the REORG to implement
• Otherwise there will be no way check for duplicate rows
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Immediate ALTERs added in DB2 10 for z/OS
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LOB Inline length and default
VERSIONING
ACCESS CONTROL
MASK & PERMISSION
TRIGGER and FUNCTION – SECURED attribute
TIMESTAMP Precision and Timezone
BUFFERPOOL PGSTEAL NONE (in memory objects)
MAXPARTITIONS for PBG objects
INDEX include columns
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Schema Items that still cannot be changed
• ALIAS
• AUXILIARY TABLE
• Including Drop – handled via Base Table
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DISTINCT TYPE
GLOBAL TEMPORARY TABLE
SYNONYM
TRIGGER
• There is an ALTER TRIGGER but it only allows (NOT) SECURED
• VIEW
• There is an ALTER VIEW but it only allows REGENERATE
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Wouldn’t it be nice if we could....?
• Alter Table Columns and other common schema changes
using a Deferred Syntax in advance
• Reorder and Remove columns
• And have them implemented by a REORG when we can
plan our Binds and Stats in a more controlled fashion
• Please bear in mind I don’t work for IBM and have no
access to future plans so this wish list should in no way
be taken as an indication of what’s likely to appear in a
future release of DB2 
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Ken Mc Donald
BMC Software
ken_mcdonald@bmc.com