Oracle Index Structure and Management

Indexes: Structure, Splits and Free
Space Management Internals
OakTable World, 29 September 2014
Christian Antognini
BASEL BERN BRUGG LAUSANNE ZUERICH DUESSELDORF FRANKFURT A.M. FREIBURG I.BR. HAMBURG MUNICH STUTTGART VIENNA
2014 © Trivadis
Indexes: Structure, Splits and Free Space Management Internals
29 September 2014
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2014 © Trivadis
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@ChrisAntognini
 Senior principal consultant, trainer and partner
at Trivadis in Zurich (CH)
 christian.antognini@trivadis.com
 http://antognini.ch
 Focus: get the most out of Oracle Database
 Logical and physical database design
 Query optimizer
 Application performance management
 Author of Troubleshooting Oracle Performance
Apress, 2008/2014
 OakTable Network, Oracle ACE Director

AGENDA
1. Concepts
2. Index Keys
3. Splits
4. Free Space
5. Reorganizations
6. Bitmap
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Concepts
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Structure of a B-Tree
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5
24 51
12 29 42 86
2 ROWID
8 ROWID
10 ROWID
11 ROWID
12 ROWID
18 ROWID
20 ROWID
24 ROWID
26 ROWID
29 ROWID
34 ROWID
38 ROWID
42 ROWID 51 ROWID
55 ROWID
73 ROWID
branch
blocks
86 ROWID
99 ROWID
left<24
right≥24
leaf blocks
root block

2014 © Trivadis
Nulls
 NULL values are stored in the index only when at least one of the
indexed columns is not NULL
 Exception: bitmap indexes always store indexed data
 For unique indexes the uniqueness is guaranteed only if at least one of
the indexed columns is not NULL
 NULL values are stored after not NULL values in the index
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2014 © Trivadis
Myths
 Oracle B-tree indexes can become “unbalanced” over time and need to
be rebuilt
 Deleted space in an index is “deadwood” and over time requires the
index to be rebuilt
 If an index reaches “x” number of levels, it becomes inefficient and
requires the index to be rebuilt
 If an index has a poor clustering factor, the index needs to be rebuilt
 To improve performance, indexes need to be regularly rebuilt
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Source: Oracle B-Tree Index Internals: Rebuilding The Truth, Richard Foote

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Index Keys
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Internal Key
 Internally a unique key is needed to sort the indexed columns
 For unique indexes the internal key is the same as the index key
 For non-unique indexes the ROWID of the indexed row is added to the
index key to make it unique
 For global indexes an extended ROWID is used, otherwise a restricted
ROWID is enough
2014 © Trivadis
 Therefore
 The index entries are sorted in a consistent way
 Even for non-unique indexed the order of the index entries changes only
when the indexed table is reorganized (i.e. not if the index is rebuilt!)
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Branch Block Key
 In branch blocks the internal key is truncated after the first byte that
differs from the last key of the left leaf block
 All keys are preceded by a length field
 1 byte for data up to 127 bytes
 Otherwise 2 bytes
- The high bit of the first byte is set (0x80)
- The remaining bits of the first byte and the second byte contain the length
 Partially stored keys are terminated by 0xFE
 Nulls have a column length of 0xFF, trailing nulls are stored
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RDBA
Len
Key 1
Data
Key 1
Len
Key X
Data
Key X

Leaf Block Key
 All keys are preceded by a length field
 1 byte for data up to 127 bytes
 Otherwise 2 bytes
- The high bit of the first byte is set (0x80)
- The remaining bits of the first byte and the second byte contain the length
 Nulls have a column length of 0xFF, trailing nulls are stored
 For global indexes an extended ROWID is used, otherwise a restricted
ROWID is enough
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Flag Lock
Len
Key 1
Data
Key 1
Len
ROWID
ROWID
(optional)

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Splits
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Splits
 Two split methods are implemented
 50:50 split
 The keys are evenly distributed over two blocks
 Based on size, not number of keys
 99:1 split (a.k.a. 90:10)
 The new key is the right-most of the index
 A new leaf block containing only the new key is added
 Useful for increasing values like sequences and timestamps
 A change in the structure of an index (like a split) is not rollbacked if the
transaction that caused it is rollbacked
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Splits
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#1

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Splits
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#1
#2 #3

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Splits
#2 #4 #3
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#1
#2 #3

2014 © Trivadis
Splits
#1
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#1
#2 #4 #3 #5
#2 #4 #3

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Splits
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Free Space
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Reminder
 PCTFREE is only used when the index is created (or rebuilt)
 An update is implemented as a delete followed by an insert
 The free space in leaf blocks is strongly dependent on the type of split (50:50
2014 © Trivadis
or 99:1)
 PCTUSED cannot be used for indexes
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Reorganizations
 If the free space is not automatically reused, it should be manually
reclaimed
 This is unusual, therefore such an operation should only be performed if it is
really necessary
 To manually reclaim free space two possibilities exists
 Rebuild: the index is recreated
 Coalesce: free space in contiguous blocks is merged and, therefore, some
blocks are unchained
 Unnecessary rebuild/coalesce may lead to sub-optimal data density in
leaf blocks
 Too much free space, i.e. too low density
 Too much block splits, i.e. too high density
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Rebuild vs. Coalesce
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Pro Cons
Rebuild
 Doesn’t lock table
(online rebuilds only)
 Reclaim all the free
space
 Locks table during rebuild
(offline rebuilds only)
 Doubles space usage
temporarily
 The index is completely
recreated (to do so large
sorts may be performed)
Coalesce
 Doesn’t lock table
 Current index structure is
reused
 Doesn’t reclaim all the free
space
 Can generate lot of redo

2014 © Trivadis
Bitmap
 A bitmap index is a regular B-tree, only the internal key differs from the
non-bitmap index!
 The key is composed by
 Indexed column(s)
 Start/End ROWID
 Bitmap representing the rows that have the key
- To save space it is “compressed”
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Summary
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 B-tree indexes cannot become
“unbalanced”
 Deleted space in an index can
be reused
 Indexes do not need to be
regularly rebuilt
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Questions and answers ...
Christian Antognini
Senior Principal Consultant
christian.antognini@trivadis.com
BASEL BERN BRUGG LAUSANNE ZUERICH DUESSELDORF FRANKFURT A.M. FREIBURG I.BR. HAMBURG MUNICH STUTTGART VIENNA
2014 © Trivadis
29 September 2014
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Oracle Index Structure and Management

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