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http://www.microsoft.com/billgates/speeches/SIGMOD98.asp. Homework V. V.1 Exercise 5.3: 2, 3, 6, 7, 8, 9, 10. V.2. Exercise 5.6: 9. (Extra Credit) Write a JDBC based program for querying some database. Reading Exercise: QBE (required)

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Homework v
Homework V

  • V.1 Exercise 5.3: 2, 3, 6, 7, 8, 9, 10.

  • V.2. Exercise 5.6: 9.

  • (Extra Credit) Write a JDBC based program for querying some database.

  • Reading Exercise:

    • QBE (required)

    • S. Chakravarthy. Architectures and monitoring techniques for active databases: an evaluation. Data and Knowledge Enginnering 16(1):1--16, 1995.


Homework v1
Homework V

  • V.4 Find out parameters of a high quality disk.

  • V.5. Make a table of RAID configuration with D disks versus effective reading/writing bandwidth, and effective space utilization, and state any idiosyncracy. State any assumptions.

  • Reading Exercise: What are Reed-Solomon Codes?


Memory mgmt
Memory Mgmt

Your memory is a monster: you forget - it doesn’t. It simply files things away. It keeps things for you, or hides things from you and summons them to your recall with a will of its own. You think you have a memory; but it has you!

From “A prayer for owen meany” by John Irving.


Disk space management
Disk Space Management

  • Lowest layer of DBMS software manages space on disk.

  • Higher levels call upon this layer to:

    • allocate/de-allocate a page

    • read/write a page

  • Request for a sequence of 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 is managed.


Buffer management in a dbms

DB

Buffer Management in a DBMS

Page Requests from Higher Levels

  • Data must be in RAM for DBMS to operate on it.

  • \\Table of <frame#, pageid> pairs is maintained.

BUFFER POOL

disk page

free frame

MAIN MEMORY

DISK

choice of frame dictated

by replacement policy


When a page is requested
When a Page is Requested ...

  • If requested page is not in 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.

  • If requests can be predicted (e.g., sequential scans)

  • pages can be pre-fetchedseveral pages at a time!


More on buffer management
More on Buffer Management

  • Requestor of page must unpin it, and indicate whether page has been modified:

    • dirtybit is used for this.

  • Page in pool may be requested many times,

    • a pin count is used. A page is a candidate for replacement iff pin count = 0.

  • CC & recovery may entail additional I/O when a frame is chosen for replacement. (Write-Ahead Log protocol; more later.)


Buffer replacement policy
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.

  • Sequential flooding: Nasty situation caused by LRU + repeated sequential scans.

    • # buffer frames < # pages in file means each page request causes an I/O. MRU much better in this situation (but not in all situations, of course).


Dbms vs os file system
DBMS vs. OS File System

OS does disk space & buffer mgmt: why not let OS manage these tasks?

  • Differences in OS support: portability issues

  • Some limitations, e.g., files can’t span disks.

  • Buffer management in DBMS requires ability to:

    • pin a page in buffer pool, force a page to disk (important for implementing CC & recovery),

    • adjust replacement policy, and pre-fetch pages based on access patterns in typical DB operations.


Record formats fixed length
Record Formats: Fixed Length

  • Information about field types same for all records in a file; stored in systemcatalogs.

  • Finding i’th field requires scan of record.

F1

F2

F3

F4

L1

L2

L3

L4

Base address (B)

Address = B+L1+L2


Record formats variable length

4

$

$

$

$

Record Formats: Variable Length

  • Two alternative formats (# fields is fixed):

F1 F2 F3 F4

Fields Delimited by Special Symbols

Field

Count

F1 F2 F3 F4

Array of Field Offsets

  • Second offers direct access to i’th field, efficient storage

  • of nulls(special don’t know value); small directory overhead.


Page formats fixed length records
Page Formats: Fixed Length Records

Slot 1

Slot 1

Slot 2

Slot 2

  • Record id = <page id, slot #>. In first alternative, moving records for free space management changes rid; may not be acceptable.

Free

Space

. . .

. . .

Slot N

Slot N

Slot M

N

. . .

1

1

1

M

0

M ... 3 2 1

number

of records

number

of slots

PACKED

UNPACKED, BITMAP


Page formats variable length records
Page Formats: Variable Length Records

Rid = (i,N)

Page i

  • Can move records on page without changing rid; so, attractive for fixed-length records too.

Rid = (i,2)

Rid = (i,1)

N

Pointer

to start

of free

space

20

16

24

N . . . 2 1

# slots

SLOT DIRECTORY


Files of records
Files of Records

  • Page or block is OK when doing I/O, but higher levels of DBMS operate on records, and files of records.

  • FILE: A collection of pages, each containing a collection of records. Must support:

    • insert/delete/modify record

    • read a particular record (specified using record id)

    • scan all records (possibly with some conditions on the records to be retrieved)


Unordered heap files
Unordered (Heap) Files

  • Simplest file structure contains records in no particular order.

  • As file grows and shrinks, disk pages are allocated and de-allocated.

  • To support record level operations, we must:

    • keep track of the pages in a file

    • keep track of free space on pages

    • keep track of the records on a page

  • There are many alternatives for keeping track of this.


Heap file implemented as a list
Heap File Implemented as a List

  • The header page id and Heap file name must be stored someplace.

  • Each page contains 2 `pointers’ plus data.

Data

Page

Data

Page

Data

Page

Full Pages

Header

Page

Data

Page

Data

Page

Data

Page

Pages with

Free Space


Heap file using a page directory

Data

Page 1

Header

Page

Data

Page 2

Data

Page N

DIRECTORY

Heap File Using a Page Directory

  • The entry for a page can include the number of free bytes on the page.

  • The directory is a collection of pages; linked list implementation is just one alternative.

    • Much smaller than linked list of all HF pages!


Indexes
Indexes

  • A Heap file allows us to retrieve records:

    • by specifying the rid, or

    • by scanning all records sequentially

  • Sometimes, we want to retrieve records by specifying the values in one or more fields, e.g.,

    • Find all students in the “CS” department

    • Find all students with a gpa > 3

  • Indexes are file structures that enable us to answer such value-based queries efficiently.


System catalogs
System Catalogs

  • For each index:

    • structure (e.g., B+ tree) and search key fields

  • For each relation:

    • name, file name, file structure (e.g., Heap file)

    • attribute name and type, for each attribute

    • index name, for each index

    • integrity constraints

  • For each view:

    • view name and definition

  • Plus statistics, authorization, buffer pool size, etc.

  • Catalogs are themselves stored as relations!



Summary
Summary

  • Disks provide cheap, non-volatile storage.

    • Random access, but cost depends on location of page on disk; important to arrange data sequentially to minimize seek and rotation delays.

  • Buffer manager brings pages into RAM.

    • Page stays in RAM until released by requestor.

    • Written to disk when frame chosen for replacement (which is sometime after requestor releases the page).

    • Choice of frame to replace based on replacement policy.

    • Tries to pre-fetch several pages at a time.


Summary contd
Summary (Contd.)

  • DBMS vs. OS File Support

    • DBMS needs features not found in many OS’s, e.g., forcing a page to disk, controlling the order of page writes to disk, files spanning disks, ability to control pre-fetching and page replacement policy based on predictable access patterns, etc.

  • Variable length record format with field offset directory offers support for direct access to i’th field and null values.

  • Slotted page format supports variable length records and allows records to move on page.


Summary contd1
Summary (Contd.)

  • File layer keeps track of pages in a file, and supports abstraction of a collection of records.

    • Pages with free space identified using linked list or directory structure (similar to how pages in file are kept track of).

  • Indexes support efficient retrieval of records based on the values in some fields.

  • Catalog relations store information about relations, indexes and views. (Information that is common to all records in a given collection.)


Alternative file organizations
Alternative File Organizations

Many alternatives exist, each ideal for some situation , and not so good in others:

  • Heap 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:Good for equality selections.

    • File is a collection of buckets. Bucket = primary page plus zero or moreoverflow pages.

    • Hashing functionh: h(r) = bucket in which record r belongs. h looks at only some of the fields of r, called the search fields.


Cost model for our analysis
Cost Model for Our Analysis

We ignore CPU costs, for simplicity:

  • B: The number of data pages

  • R: Number of records per page

  • D: (Average) time to read or write disk page

  • Measuring number of page I/O’s ignores gains of pre-fetching blocks 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!


Assumptions in our analysis
Assumptions in Our Analysis

  • Single record insert and delete.

  • Heap Files:

    • Equality selection on key; exactly one match.

    • Insert always at end of file.

  • Sorted Files:

    • Files compacted after deletions.

    • Selections on sort field(s).

  • Hashed Files:

    • No overflow buckets, 80% page occupancy.


Cost of operations
Cost of Operations

  • Several assumptions underlie these (rough) estimates!


Cost of operations1
Cost of Operations

  • Several assumptions underlie these (rough) estimates!


Indexes1
Indexes

  • An index on a file speeds up selections on the search key fields for the index.

    • Any subset of the fields 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.


Alternatives for data entry k in index
Alternatives for Data Entry k* in Index

  • Three alternatives:

    • Data record with key value k

    • <k, rid of data record with search key value k>

    • <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


Alternatives for data entries contd
Alternatives for Data Entries (Contd.)

  • Alternative 1:

    • If this is used, index structure is a file organization for data records (like Heap files or sorted files).

    • At most one index on a given collection of data records can use Alternative 1. (Otherwise, data records duplicated, leading to redundant storage and potential inconsistency.)

    • If data records very large, # of pages containing data entries is high. Implies size of auxiliary information in the index is also large, typically.


Alternatives for data entries contd1
Alternatives for Data Entries (Contd.)

  • Alternatives 2 and 3:

    • Data entries typically much smaller than data records. So, better than Alternative 1 with large data records, especially if search keys are small. (Portion of index structure used to direct search is much smaller than with Alternative 1.)

    • If more than one index is required on a given file, at most one index can use Alternative 1; rest must use Alternatives 2 or 3.

    • Alternative 3 more compact than Alternative 2, but leads to variable sized data entries even if search keys are of fixed length.


Index classification
Index Classification

  • Primary vs. secondary: If search key contains primary key, then called primary index.

    • 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, but not vice-versa.

    • A file can be clustered on at most one search key.

    • Cost of retrieving data records through index varies greatly based on whether index is clustered or not!


Clustered vs unclustered index
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

UNCLUSTERED

CLUSTERED

direct search for

data entries

Data entries

Data entries

(Index File)

(Data file)

Data Records

Data Records


Index classification contd
Index Classification (Contd.)

  • Dense vs. Sparse: If there is at least one data entry per search key value (in some data record), then 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

22

Basu, 33, 4003

25

Bristow, 30, 2007

30

Ashby

33

Cass, 50, 5004

Cass

Smith

Daniels, 22, 6003

40

Jones, 40, 6003

44

44

Smith, 44, 3000

50

Tracy, 44, 5004

Sparse Index

Dense Index

on

on

Data File

Name

Age


Index classification contd1
Index Classification (Contd.)

Examples of composite key

indexes using lexicographic order.

  • 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.

    • Lexicographic order

11,80

11

12

12,10

name

age

sal

12,20

12

13,75

bob

12

10

13

<age, sal>

cal

11

80

<age>

joe

12

20

10,12

sue

13

75

10

20

20,12

Data records

sorted by name

75,13

75

80,11

80

<sal, age>

<sal>

Data entries in index

sorted by <sal,age>

Data entries

sorted by <sal>


Summary1
Summary

  • Many alternative file organizations exist, each appropriate in some situation.

  • If selection queries are frequent, sorting the file or building an index is important.

    • Hash-based indexes only good for equality search.

    • Sorted files and tree-based indexes best for range search; also good for equality search. (Files rarely kept sorted in practice; B+ tree index is better.)

  • Index is a collection of data entries plus a way to quickly find entries with given key values.


Summary contd2
Summary (Contd.)

  • Data entries can be actual data records, <key, rid> pairs, or <key, rid-list> pairs.

    • Choice orthogonal to indexing technique used to locate data entries with a given key value.

  • Can have several indexes on a given file of data records, each with a different search key.

  • Indexes can be classified as clustered vs. unclustered, primary vs. secondary, and dense vs. sparse. Differences have important consequences for utility/performance.


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