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Transcript of 6/15/20151 PSU’s CS 587 CS 587/410, Spring 2008 Relational Database Systems /Database Internals...
04/18/23 1PSU’s CS 587
CS 587/410, Spring 2008
Relational Database Systems /Database Internals
Len Shapiro, len at pdx.edu
Syllabus: www.cs.pdx.edu/~len/587
This work, and all other files in this series, are licensed under the Creative Commons Attribution License. To view a copy of this license, visit http://creativecommons.org/licenses or send a letter to Creative Commons, 171 2nd Street, Suite 300, San Francisco, California, 94105, USA.
These slides have benefitted from the content of slides developed by Raghu Ramakrishnan and Johannes Gehrke, whom I thank.
These slides are best viewed with “slide show”. To print the slides, consider using "File/Print/Properties/Pages per sheet", 4-up, instead of Power Point's "handouts" mode.
Slides will be online by 6PM each Monday evening. If not, email me!
04/18/23 2PSU’s CS 587
Plan for today 3x5 cards: Name, email, prerequisite
Review Syllabus
Personal Introductions What is your background in Database? What would you like to get out of the course?
Minibase Intro, Buffer Manager assignment
Lecture on Chapter 8
Lecture on part of Chapter 9 needed for homework
Quit 10 minutes early to form teams What is your background in Java?
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8.1 Database Architecture (cf. pg. 20)
Plan Executor
Files & AccessMethods
Buffer Manager
Disk Space Manager
Parser
Operator EvaluatorOptimizer
LockManager
TransactionManager
RecoveryManager
Index Files
Data Files
System Catalog
Web Forms SQL interfaceApplic. Front ends
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Learning Objectives for Chapter 8
Identify the input, output and function of each component in the architecture diagram
Be able to describe the relationships between different classifications of indexes Alternatives, clustering, uniqueness, denseness, primary
For each cell in the table on the “cost of operations” slide , explain the underlying algorithm and justify the cost.
Definitions: Index only plans, composite keys
Given a workload, choose an optimal index configuration.
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Table of Contents: Chapter 8 (*review)
8.1 Data on external storage* Database architecture
8.2 File organizations and indexing* Indexes Alternatives for k* data entries Index classification: clustered, dense, unique, primary
8.3 Index search data structures* Hash-based indexes Tree indexes
8.4 Comparing File Organizations
8.5 Indexes and performance tuning*
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8.2 Alternative File Organizations
Many alternatives exist, each ideal for some situations, and not so good in others: Heap (random order) files: Suitable when typical access is a
file scan retrieving all records. Sorted Files: Best if records must be retrieved in some order,
or only a `range’ of records is needed. Hashed files: Best for quick equality access.
RDBMSs typically use heapfiles plus indexes
A tree index using alternative 1 (see below) is similar to a sorted file, a hash index using alternative 1 is the same as a hash file.
8.Storage
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How the Records Fall
16
9
22
41
62
15
2
11
2
9
11
15
16
22
41
62
16
9
41
2
22
62
11
15
Heap Sorted HashedBucket
0
1
2
3
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Indexes An index on a file speeds up selections on the search
key fields for the index. Any subset of the fields, or a prefix of a field, of a relation
can be the search key for an index on the relation. Search key is not the same as key (minimal set of fields that
uniquely identify a record in a relation).
An index contains a collection of data entries, and supports efficient retrieval of all data entries k* with a given key value k. Given data entry k*, we can find record with key k in at most
one disk I/O. (Details soon …)
8.Storage
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Typical index architecture
Used by modern DBMSs
What are typical search structures in CS?
(Data file)
Data entries
Data Records
(Index File)k* k* k*
Search structure: given a key, it finds the data entry for that key
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Alternatives for Data Entry k* in Index
In a data entry k* we can store:1. Data record with key value k, or
• In this case the previous page is not accurate
2. <k, rid of data record with search key value k>, or
3. <k, list of rids of data records with search key k>
Choice of alternative for data entries is orthogonal to the indexing technique used to locate data entries with a given key value k. Examples of indexing techniques: B+ trees, hash-based
structures Typically, index contains auxiliary information that directs
searches to the desired data entries
8.Storage
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Alternative 1: k* holds the data
42 Red
24 Green
30 Blue
10 Blue
14 Red
21 Blue INDEX
.
.
.
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Alternative 2: One pointer per k*
10*
10*
21*
24*
30*
42*
10 Blue
42 Red
10 Green
30 Blue
21 Red
24 Blue
INDEX
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Alternative 3: 1 or more pointers per k*
10*
18*
21*
24*
30*
10 Blue
18 Red
10 Green
30 Blue
21 Red
24 Blue
INDEX
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Alternatives for Data Entries (Contd.)
Alternative 1: If this is used, index structure is a file organization for data
records (instead of a Heap file or sorted file). At most one single-attribute index on a given collection of data
records can use Alternative 1. (Otherwise, data records are duplicated, leading to redundant storage and potential inconsistency.)
Alternatives 2 and 3: Alternative 3 more compact than Alternative 2, but leads to
variable sized data entries even if search keys are of fixed length.
8.Storage
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8.2.1 Clustered vs. Unclustered Index Suppose that Alternative (2) is used for data entries, and that
the data records are stored in a Heap file. To build clustered index, first sort the Heap file (with some free space
on each page for future inserts). Overflow pages may be needed for inserts. (Thus, order of data recs is
`close to’, but not identical to, the sort order.)
Index entries
Data entries
direct search for
(Index File)
Data Records
data entriesCLUSTERED
(Data file)
Data entries
Data Records
UNCLUSTERED
8.Storage
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8.2.2 Index Classification Primary vs. secondary: If search key contains primary key, then
called primary index. In practice, primary means alternative 1
Unique index: Search key contains a candidate key.
Clustered vs. unclustered: If order of data records is the same as, or `close to’, order of data entries, then called clustered index. Alternative 1 implies clustered. A file can be clustered on at most one single-attributesearch key. Cost of retrieving data records through index varies greatly based on
whether index is clustered or not!
8.Storage
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Index Classification (Cont.)• Dense vs. Sparse: If
there is at least one data entry per search key value (in some data record), then it’s dense.
– Alternative 1 always leads to dense index.
– Every sparse index is clustered!
– Sparse indexes are smaller; however, some useful optimizations are based on dense indexes.
Ashby, 25, 3000
Smith, 44, 3000
Ashby
Cass
Smith
22
25
30
40
44
44
50
Sparse Indexon
Name Data File
Dense Indexon
Age
33
Bristow, 30, 2007
Basu, 33, 4003
Cass, 50, 5004
Tracy, 44, 5004
Daniels, 22, 6003
Jones, 40, 6003
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Index ClassificationClustered, sparse indexes are smaller; they work well for range searches
and sorting.
But…data blocks are difficult to keep sorted and some useful optimizations are based on dense indexes.
Note: one file can have at most one clustered index - all of the additional indices must be unclustered.
sparse dense
clustered YES YES
unclusteredNO! YES
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8.3.1 Hash-based Index Examples
hage
Smith,44,3000
Jones,40,6003
Tracy,44,5004
Ashby,25,3000
Basu,33,4003
Bristow,29,2007
Cass,50,5004
Daniels,22,6003
h(age)=00
h(age)=01
h(age)=10
3000
3000
5004
5004
4003
2007
6003
6003
h sal
h(sal)=00
h(sal)=11
Employees file hashed on ageIndex on salary
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8.3.1 Hash-Based Indexes
Good for equality selections.
Index is a collection of buckets. Bucket = primary page plus zero or more overflow pages.
Buckets contain data entries.
Hashing function h: h(r) = bucket in which (data entry for) record r belongs. h looks at the search key fields of r. No need for “index entries” in this scheme.
8.Storage
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8.3.2 B+ Tree Indexes
Leaf pages contain data entries, and are chained (prev & next) Non-leaf pages have index entries; only used to direct searches:
P0 K 1 P 1 K 2 P 2 K m P m
index entry
Non-leaf
Pages
Pages (Sorted by search key)
Leaf
8.Storage
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Example B+ Tree
Find 28*? 29*? All > 15* and < 30*
Insert/delete: Find data entry in leaf, then change it. Need to adjust parent sometimes. And change sometimes bubbles up the tree
2* 3*
Root
17
30
14* 16* 33* 34* 38* 39*
135
7*5* 8* 22* 24*
27
27* 29*
Entries <= 17 Entries > 17
Note how data entriesin leaf level are sorted
8.Storage
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Assume an employee file is organized by the search key <age, salary>.
We will consider various file organizations, and operations like scan, equality, range, insert, delete.
For each file organization, we will compute the cost of each operation.
8.Storage
8.4 Comparing File Organizations
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Comparing File Organizations
Heap files (random order; insert at eof)
Sorted files, sorted on <age, sal>
Clustered B+ tree file, Alternative (1), search key <age, sal>
Heap file with unclustered B + tree index on search key <age, sal>
Heap file with unclustered hash index on search key <age, sal>
8.Storage
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Operations to Compare
Scan: Fetch all records from disk
Equality search
Range selection
Insert a record
Delete a record
8.Storage
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Cost Model for Our Analysis
We ignore CPU costs, for simplicity: B: The number of data pages R: Number of data records per page F: Fanout, number of index records per page D: (Average) time to read or write disk page Measuring number of page I/O’s ignores gains of pre-fetching
a sequence of pages; thus, even I/O cost is only approximated.
Average-case analysis; based on several simplistic assumptions.
Good enough to show the overall trends!
8.Storage
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Assumptions in Our Analysis Heap Files:
Equality selection on key; exactly one match.
Sorted Files: Files compacted after deletions.
Indexes: Alt (2), (3): data entry size = 10% size of record Hash: No overflow buckets.
• 80% page occupancy => File size = 1.25 data size Tree: 67% occupancy (this is typical).
• Implies file size = 1.5 data size
8.Storage
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Assumptions (contd.)
Scans: Leaf levels of a tree-index are chained. Index data-entries plus actual file scanned for unclustered
indexes.
Range searches: We use tree indexes to restrict the set of data records
fetched, but ignore hash indexes.
8.Storage
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Cost of Operations (a) Scan (b) Equality (c ) Range (d) Insert (e) Delete
(1) Heap BD 0.5BD BD 2D Search +D
(2) Sorted BD Dlog 2B D(log 2 B + # pgs with match recs)
Search + BD
Search +BD
(3) Clustered
1.5BD Dlog F 1.5B D(log F 1.5B + # pgs w. match recs)
Search + D
Search +D
(4) Unclust. Tree index
BD(R+0.15) D(1 + log F 0.15B)
D(log F 0.15B + # match recs)
Search + 2D
Search + 2D
(5) Unclust. Hash index
BD(R+0.125) 2D BD Search + 2D
Search + 2D
Several assumptions underlie these (rough) estimates!
8.Storage
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8.5 Indexes and Performance Tuning
One of a DBA’s most important duties is to deal with performance problems
One of the most effective methods for improving performance is to choose the best indexes for the given workload
Why not index everything? What are the two costs of an index?
Part of a DBA’s job: choose indexes so the database will “run fast”. What information does the DBA need, to do this?
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8.5.1 Understanding the Workload
For each query in the workload: Which relations does it access? Which attributes are retrieved? Which attributes are involved in selection/join conditions? How
selective are these conditions likely to be?
For each update in the workload: Which attributes are involved in selection/join conditions? How
selective are these conditions likely to be? The type of update (INSERT/DELETE/UPDATE), and the attributes that
are affected.
8.Storage
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Choice of Indexes What indexes should we create?
Which relations should have indexes? What field(s) should be the search key? Should we build several indexes?
What kinds of indexes should we create? Clustered? Hash/tree?
Before creating an index, must also consider the impact on updates in the workload! Trade-off: Indexes can make queries go faster, updates slower.
Require disk space, too.
Also consider dropping indexes
8.Storage
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Choice of Indexes (Contd.)
One approach: Consider the most important queries in turn. Consider the best plan using the current indexes, and see if a better plan is possible with an additional index. If so, create it. Obviously, this implies that we must understand how a DBMS evaluates
queries and creates query evaluation plans!
Second approach: Use an Index Tuning Wizard [163]. Inputs: A workload (it can also capture a workload) and an amount of
disk storage for indexes Output: an index configuration
8.Storage
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Index Selection Guidelines Attributes in WHERE clause are candidates for index keys.
Exact match condition suggests hash index. Range query suggests tree index.
• Clustering is especially useful for range queries; can also help on equality queries if there are many duplicates.
Try to choose indexes that benefit as many queries as possible. Since only one index can be clustered per relation, choose it based on important queries that would benefit the most from clustering.
Multi-attribute search keys should be considered when a WHERE clause contains several conditions. Order of attributes is important for range queries. Such indexes can sometimes enable index-only strategies for
important queries.
8.Storage
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Index-Only Plans
A number of queries can be answered without retrieving any tuples from one or more of the relations involved if a suitable index is available.
For index-only strategies, clustering is not important!
SELECT E.dno, COUNT(*)FROM Emp EGROUP BY E.dno
SELECT E.dno, MIN(E.sal)FROM Emp EGROUP BY E.dno
SELECT AVG(E.sal)FROM Emp EWHERE E.age=25 AND E.sal BETWEEN 3000 AND 5000
<E.dno>
<E.dno,E.sal>
Tree indexMay help
<E. age,E.sal> or<E.sal, E.age>
Tree index!
8.Storage
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Examples of Clustered Indexes
B+ tree index on E.age can be used to get qualifying tuples. How selective is the condition? Is the index clustered?
Consider the GROUP BY query. If many tuples have E.age > 10, using E.age
index and sorting the retrieved tuples may be costly.
Clustered E.dno index may be better!
Equality queries and duplicates: Clustering on E.hobby helps!
SELECT E.dnoFROM Emp EWHERE E.age>40
SELECT E.dno, COUNT (*)FROM Emp EWHERE E.age>10GROUP BY E.dno
SELECT E.dnoFROM Emp EWHERE E.hobby=Stamps
8.Storage
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Indexes with Composite Search Keys
Composite Search Keys: Search on a combination of fields. Equality query: Every field value is
equal to a constant value. E.g. wrt <sal,age> index:
• age=20 and sal =75 Range query: Some field value is
not a constant. E.g.:• age =20; or age=20 and sal > 10
Data entries in index sorted by search key to support range queries.
sue 13 75
bob
cal
joe 12
10
20
8011
12
name age sal
<sal, age>
<age, sal> <age>
<sal>
12,20
12,10
11,80
13,75
20,12
10,12
75,13
80,11
11
12
12
13
10
20
75
80
Data recordssorted by name
Data entries in indexsorted by <sal,age>
Data entriessorted by <sal>
Examples of composite keyindexes using lexicographic order.
8.Storage
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Composite Search Keys
To retrieve Emp records with age=30 AND sal=4000, an index on <age,sal> would be better than an index on age or an index on sal. Choice of index key orthogonal to clustering etc.
If condition is: 20<age<30 AND 3000<sal<5000: Clustered tree index on <age,sal> or <sal,age> is best.
If condition is: age=30 AND 3000<sal<5000: Clustered <age,sal> index much better than <sal,age> index!
Composite indexes are larger, updated more often.
8.Storage
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Indexes in the real world
Types of indexes supported Oracle, SQLServer and DB2 support only B+Tree indexes.
Postgres supports hash indexes but does not recommend using them.
MySQL? Everyone uses hash indexes for hash joins, but they are
constructed on the fly.
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Clustering in the real world Oracle, SQL Server:
Declares a clustered index on any primary key. It uses alternative 1.
DB2, Postgres The user can define an index to be clustered. The DBMS uses alternative 2. At first the index will be clustered. If you also specify a percentage of free space, it will place
subsequent inserts/updates (Postgres:updates only) in sorted order so the index should stay clustered for a while.
It’s up to you to recluster the index if overflow chains get to be long.
MySQL
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Text problems
Chapter 8, problems 1,3,5,7,9,11
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Slides From Chapter 9
Relevant to Buffer Management Assignment in Minibase
9.3 Disk Space Management
9.4 Buffer Management
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9.3 Disk Space Management
The lowest layer of DBMS software manages space on disk.
Higher levels call upon this layer to: allocate/de-allocate a disk page read/write a disk page
A request for a sequence of disk pages must be satisfied by allocating the pages sequentially on disk! Higher levels don’t need to know how this is done, or how free space on the disk is managed.
9.Disks
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9.4 Buffer Management in a DBMS
Data must be in RAM for DBMS to operate on it!
Table of <frame#, pageid> pairs is maintained.
DB
MAIN MEMORY
DISK
disk page
free frame
Disk Page Requests from Higher Levels
BUFFER POOL
choice of frame dictatedby replacement policy
9.Disks
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When a Disk Page is Requested ...
If requested disk page is not in buffer pool: Choose a frame for replacement If frame is dirty, write it to disk Read requested page into chosen frame
Pin the page and return its address.
9.Disks
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More on Buffer Management
Requestor of page must unpin it, and indicate whether page has been modified: Dirty bit is used for this What if requestor does not unpin the page?
Page in pool may be requested many times, a pin count is used. A page is a candidate for replacement
iff pin count = 0.
9.Disks
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Buffer Replacement Policy
Frame is chosen for replacement by a replacement policy: Least-recently-used (LRU), Clock, MRU etc.
Policy can have big impact on # of I/O’s; depends on the access pattern.
9.Disks