A Primer on Multidimensional Clustering for UDB LUW

A Primer on Multidimensional

Clustering for UDB LUW

2

The DB2 Optimizer asks him for the best access path.

He wrote an improved version of the DB2 Optimizer, using only 9 lines of code.

He can type all SQL syntax with 100% accuracy from memory.

He taught his dog to prefetch, so that when he throws one ball, the dog returns with 32.

He had the "Backspace" and "Delete" keys permanently removed from his keyboard.

His sysadmins call him daily to ask if they can give him more disk space.

He has never had a Network Security firewall rule refuse him access.

On a slow day, he will reorg large tables completely in his mind.

He once made an SQL statement run faster just by staring at it.

He has never clicked on the “undo” arrow.









































A brief bio…• 29 years IT, 19 years of DBA experience

o UDB LUW on AIXo DB2/ZOSo Oracle

• Longest query I ever tuned was over 4 feet long when printed out

• Favorite saying: “Even a blind squirrel finds a nut once in a while”

Agenda• What is clustering?• What is multidimensional clustering (MDC)?• Some design guidelines for MDC

Left Outer Join

JOIN

L

T E

F

Backup

back

Hint: The most important thing to a DBA

Create table in tablespace

tablecreatetablespace

What is Clustering?• Physical sequence of rows in a DB2 table.• Determined by defining one index as the

“clustering index”.• As rows are inserted, DB2 attempts to put them

in correct clustering location• During Reorg, rows are sorted in clustering order

before reloading back into table• Is the table clustered or is the index clustered??

9

Regular non-clustering indexes

Table

IndexOn Region

Index onYear

10

Clustering Index

Table

Clustering IndexOn Region

Index onYear

11

Why are reads faster when a table is clustered??

• The first I/O reads a page into memory which contains many rows with the same key or a range of key valueso Example: App needs 500 rows for a given region….

• If the DBMS knows that it will need to fetch several or many consecutive pages, then it can begin “prefetching” extents (multiple pages) into memory before application needs it

18 IOs 3 IOsvs vs 1 IO

Sequential Prefetch• “Holy Grail” when accessing large numbers of

rows• Significant reduction in I/O• Physical reads vs Logical reads• Tablespace Page size (bytes)• Tablespace Extent size (pages)• Tablespace Prefetch size (pages)

How UDB uses clustering

• Sequential prefetch is turned on if UDB determines cost savings

• Clustered data makes it more likely for sequential prefetch to be turned on

• Optimizer looks at clusterratio and clusterfactor (on syscat.indexes)

• Sequential detection can be turned on dynamically during query execution

So what’s the shortfall with Clustering?

• Clustering deteriorates over time (probably) – requiring reorgs

• Record based indexes with a pointer for every single record, so can become very large in size

• Only get one choice for the clustering index.• If Joe needs the table clustered by timestamp and

Bill needs it clustered by policy #, one of them will probably be unhappy.

Partitioned Database

DA TA BA SE

MultiDimensional Clustering

DimensionalclusteringDimensionalclusteringDimensionalclustering

MultiDimensional Clustering (MDC)

• What if your data could be physically sequenced in more than one way at the same time??

• Great in theory, but how do you make this happen in real life on a real table??


• Data is physically grouped together by “dimensions” into separate blocks, or extents

• Each page belongs to exactly one block• All blocks are of equal size• Tablespace Page size (bytes)• Tablespace Extent size (pages)• Tablespace Prefetch size (pages)

What is an extent?• An extent is a set of contiguous data pages on

disk, specified at tablespace creation time.• Physical size of an extent determined by:

o Extent Size (# of pages)o Page Size (kb)

Color

Year

Age

2002 2003 2004

1

2

3

Red

Blue

Green

MDC with Three Dimensions

What is a (logical) cell?

• Contains all rows for a unique combination of dimension values

• Physically made up of one or more blocks (extents)

• Blocks are only allocated for logical cells which actually have records for a given combination of dimension values

Color

Year

Age

2002 2003 2004

1

2

3

Red

Blue

Green

2002, Red, 1

A “Cell”

What is a Slice?• A slice is a set of blocks having a particular

dimension key.

Color

Year

Age

2002 2003 2004

1

2

3

Red

Blue

Green

2002, Red, 1 2003, Red, 1 2004, Red, 1

A Red “Slice” of the Color

Dimension

Color

Year

Age

2002 2003 2004

1

2

3

Red

Blue

Green

2004, Red, 1

2004, Blue, 1

2004, Green, 1

A 2004 “Slice” of the Year

Dimension

Color

Year

Age

2002 2003 2004

1

2

3

Red

Blue

Green

2004, Red, 1

2004, Blue, 1

2004, Green, 1

2003, Red, 1

2003, Blue, 1

2003, Green, 1

2002, Red, 1

2002, Blue, 1

2002, Green, 1

A 1 “Slice” of the Age

Dimension

How MDC works

• Rows are organized in extents based upon dimensions

Dimension BlockIndex onColor

Red Red

Red Red Red

Blue

Blue BlueBlueBlue

BlueBlue

GreenGreenGreen

Green Green Green

2000

2000

2000

2000

2000

2000

2000

2001

2001

2001

2001

2001

2002

2002

2002

2002

2003

2003

Dimension BlockIndex on Year


• MDC introduces indexes that are block-based – much smaller than record-basedo A pointer for each block instead of a pointer for

each row• MDC allows a table to be physically clustered on

more than one key or dimension • MDC table is able to maintain and guarantee

clustering over all dimensions automatically and continuously

MDC Indexes• A dimension block index is automatically created

for each dimension specified• A composite block index is automatically created

containing all columns across all dimensions• Composite index used to maintain clustering• Much lower overhead for logging

Creating an MDC table

Create table t1 (age int, color char(10), year char(4), c1 int, c2 int)

organize by dimensions (age, color, year)

• Three dimension block indexes (one each for age, color and year).

• A composite block index is also created which includes (age,color, year).

• Traditional “RID” indexes can also be created on an MDC

• Can logical AND/OR between BID and RID indexes

Color

Year

Age

2002 2003 2004

1

2

3

Red

Blue

Green

2004, Red, 1

2004, Blue, 1

2004, Green, 1

2003, Red, 1

2003, Blue, 1

2003, Green, 1

2002, Red, 1

2002, Blue, 1

2002, Green, 1

Select Processing in MDC (ex #1)

Select …From TableWhere Age = ‘1’

Color

Year

Age

2002 2003 2004

1

2

3

Red

Blue

Green

2002, Red, 1 2003, Red, 1 2004, Red, 1

Select …From TableWhere color = ‘Red’


Color

Year

Age

2002 2003 2004

1

2

3

Red

Blue

Green

2002, Red, 1


Select …

From Table

Where color = ‘Red’

And Age = 1

And Year = ‘2002’

Insert Processing in MDC

• Probe composite block index to see if this is a new combination of dimensions (new logical cell)

• If existing, search list of BIDs to look for space to insert row

• If new logical cell or all blocks full for an existing cell, then create a new block

Delete Processing in MDC

• If the record being deleted is not the last record in block, UDB just deletes the record and removes its RID from any record based indexes

• If deleting last record in block, UDB frees the block by changing its IN_USE status bit and removing the BID from all block indexes and also remove RID from record based indexes

Update Processing in MDC

• Updates on non-dimension values are done in place just as with regular tableso No need to update block indexes unless no

space is found and a new block needs to be added to cell

• Updates of dimension values are treated as delete/inserto Block indexes will need to be updated

MDC Benefits• Can cluster in multiple dimensions• Clustering is automatically and dynamically

maintained over time.• Reorg not necessary for re-clustering• Block indexes are much smaller and have much

less overhead for maintenance and logging

Design Guidelines for MDC

• MDC is great tool• But, used incorrectly, can make things worse just

as much as it can make things better• Requires knowledge of data and data useage by

users

MDC Design• Most important design criteria for MDC is to select

proper dimension columns and appropriate exent size

• Columns that are used in queries as equality or range predicates

• Low cardinality• Desire high density – blocks are mostly full• Generally no more than 3 or 4 dimensions

MDC Size Considerations

• At least one extent will be allocated for every unique combination of dimensions in the data

• Evaluate dimension volumetrics and row size to establish tablespace extent sizeo Select dimcol1, dimcol2, dimcol3, count(*)

from table• Example: 8k page size * 32 page extent size

gives 256k extent size• If you have 1 million unique dimension

combinations – minimum table size of 256 GB!!

What happens if you choose wrong??

• A high cardinality column(s) will explode the size of your table and destroy performance!!!

• Remember that a block is physically allocated for each unique combination of dimension key values

• NEVER use a high cardinality column or a unique column for an MDC dimension

Down Right StupidStupid

Stupid

Stupid

Stupid

Choosing a unique column as a dimension is just:

Using column expressions with MDC• What if a column is a good dimension candidate,

but cardinality is way too high (ex: timestamp column)

• Create table t1 (c1 timestamp, c2 int, c3 int generated always as year(c1)) organize by dimensions (c2, c3)

• Monotonic – generated column increases/decreases the same as base column

• A non-monotonic column will only allow equality or IN predicates on the base column to use the block index

MDC tables and database partitioning

• DB2 LUW DPF partitioning is just a way to spread the data across partitions (not range partitioning like DB2/ZOS

• The reason for partitioning a table is independent of whether the table is an MDC table or a regular table

• Can partition on a dimension column or a non-dimension columno However, partitioning on a dimension column means

that all rows for a particular dimension value exist on only 1 partition

• If partitioning, remember that logical cells can spread across partitionso Important for sizing of extents

Block Index Considerations

• Composite block index columns are ordered based upon “organize by dimensions” clause

• Create table t1 (c1 int, c2 int, c3 int, c4 int) organize by dimensions (c1, c4, (c3,c1), c2)o Composite index will be (c1,c4,c3,c2)

• Create table t1 (c1 int, c2 int, c3 int, c4 int) organize by dimensions (c1, c2, (c3,c1), c4)o Composite index will be (c1,c2,c3,c4)

The Customer is always right

EverythingElse

Customer

To make a long story short

Questions???

A Primer on Multidimensional Clustering for UDB LUW

Documents

db2 optimizer

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lines of code