Transaction Internals

8/18/2019 Transaction Internals

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TransactionInternals

Julian Dyke

Independent Consultant

Web Version

© 2007 Julian Dyke


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© 2007 Julian Dyke

Agenda

Transactions

edo

!ndo

ollback

ead Consistency

!ndo"based #eatures

$A%$W&C'

#las(back


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© 2007 Julian Dyke

)*a+ples All e*a+ples in t(is presentation are based on cricket

T)A, VAC-A2./0!'& '!,)

WIC)T& '!,)

&C$)

T(e table (as no inde*es

T(e 3ollo4ing table (as been used in all e*a+ples in t(ispresentation


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© 2007 Julian Dyke

Transactions A transaction is a set o3 D,5 state+ents e*ecuted

se6uentially by a session

&tarts 4it( t(e 3irst o3 t(e 3ollo4ing state+ents e*ecuted byt(e session

I'&)T !8DAT)

D)5)T)

,)9)

&)5)CT #$ !8DAT) 5$C TA5)

)nds 4it( eit(er a C$,,IT or $55AC


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© 2007 Julian Dyke

Transactions ACID properties

Ato+icity " all c(anges +ade by t(e transaction are eit(erco++itted or rolled back

Consistency " t(e database is trans3or+ed 3ro+ one :alidstate to anot(er

Isolation " results o3 t(e transaction are in:isible to ot(er

transactions until t(e transaction is co+plete Durability " once t(e transaction co+pletes; t(e results o3

t(e transaction are per+anent

In $racle transactions can also be

recursi:e

audit

autono+ous


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© 2007 Julian Dyke

edo All database c(anges generate redo

ecords c(anges +ade toData and inde* seg+ents

!ndo seg+ents

Data dictionary

Control 3iles .indirectly

edo is used

During reco:ery o3 database

Instance reco:ery

,edia reco:ery


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© 2007 Julian Dyke

!ndo )nsures ACID properties are +aintained 3or eac( transaction

Contains c(anges re6uired to re:erse redo including

c(anges to data and inde* blocks

c(anges to transaction lists

c(anges to undo blocks

All undo operations generate redo

'ot all redo operations generate undo

I+ple+ented using undo seg+ents ,anually"+anaged .rollback seg+ents

&yste+"+anaged .$racle


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!ndo !sed to rollback unco++itted transactions

y session issuing $55AC state+ent y 8,$' on be(al3 o3 3ailed session

During instance reco:ery

During +edia reco:ery

!sed to i+ple+ent read"consistency

!nco++itted c(anges cannot be seen by ot(er sessions

!sed to i+ple+ent 3las(back

$racle


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!ndo &eg+ent -eader !ndo seg+ents are allocated at instance startup

!ndo seg+ents can be added dyna+ically

)ac( undo seg+ent (eader contains

8ool o3 3ree undo e*tents

&et o3 undo slots

$ne undo slot is allocated to eac( transaction

!ndo slot contains list o3 undo e*tents

)*tents can +igrate 3ro+ one undo seg+ent to anot(er

!ndo slots are used cyclically re+ain in (eader as long as possible

reduces probability o3 $A"01??? &naps(ot too old



!ndo &eg+ent -eader &tructure

lock -eader

)*tent Control -eader

)*tent ,ap

etention Table

lock Tail

#ree lock 8ool

Transaction Table

T! &,! -)AD) 5$C



Transaction ID .EID ):ery transaction (as a uni6ue ID based on

!ndo seg+ent nu+ber !ndo seg+ent slot nu+ber

!ndo seg+ent se6uence nu+ber .4rap

A transaction ID .EID is allocated to eac( transaction during

t(e 3irst D,5 state+ent= #or e*a+ple 0002=02B=00000DA

Details about transaction can be 3ound in VFTA'&ACTI$'

EID!&' &eg+ent nu+ber EID&5$T &lot nu+ber

EID&G' &e6uence nu+ber



Transaction ID .EID

!ndo&eg+ent

1

!ndo&eg+ent

2

!ndo&eg+ent

/

!ndo&eg+ent

&ession1

!8DAT) bo4ling&)T o:ers >

W-)) bo4ler > 7

!8DAT) batting&)T runs > 2?

W-)) bats+an > B

!8DAT) score&)T runs > B0

W-)) tea+ > @A!&@

!ndo &eg+ent /

&lot &tatus WrapH &C' DA

01 10 D0 1/ B00ea

02 10 C# 1/7 B00e10/ < D0 1/B B00e3

0 < CB 127 B003b

0? < C# 12B B003d?

0 < C) 12< B003cB

07 < C# 1/0 B003d0

0B < C# 1/1 B003d1

0 10 D0 1/< B0030

EID 000/=00=00000d0 EID 000=007=00000



!ndo )*tents )ac( undo e*tent contains contiguous set o3 undo blocks

)ac( undo block can only be allocated to one transaction

!ndo blocks contain

!ndo block (eader

!ndo records



!ndo lock &tructure

lock -eader

!ndo lock -eader

!ndo ecord Inde*

!nused &pace

!ndo ecords

lock Tail

T! !'D$ 5$C



!ndo lock !ndo lock -eader contains

Transaction ID .EID 3or current last transaction to useblock

&e6uence nu+ber o3 undo block

'u+ber o3 undo records in undo block

'ot necessarily in current transaction

!ndo records are c(ained toget(er

Allo4 transaction to be rolled back

!ndo records are also used cyclically re+ain in block 3or as long as possible

reduces probability o3 $A"01??? &naps(ot too old


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!ndo C(ange Vectors " Data locks #or data blocks

$8 ?=1 .11=1Delete o4 8iece " D8

&lot

$8 11=2Insert o4 8iece " I8

&lot

c0 @A!&@c1 100c2

$8 ?=1 .11=1!pdate o4 8iece " !8

&lot

c1 100c2

$8 11=?

!pdate o4 8iece " !8

&lot

c1 10c2 ?

$8 ?=1 .11=1Insert o4 8iece " I8

&lot

c0 @A!&@c1 10c2 ?

$8 11=/Delete o4 8iece " D8

&lot

I'&)T I'T$ score.tea+; runs; 4icketsVA5!)&.@A!&@;100;

I'&)T!8DAT) score&)T

runs > 10;4ickets > ?

W-)) tea+ > @A!&@

!8DAT)D)5)T) #$, scoreW-)) tea+ > @A!&@

D)5)T)

!ndo

edo

&T$8


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!ndo C(ange Vectors " Inde* locks Assu+e uni6ue inde* on &C$) .T)A,

$8 ?=1 .10=22

8urge 5ea3 o4key @A!&@

$8 10=2Insert 5ea3 o4key @A!&@data 01 00 20 7 00 00

$8 ?=1 .10=22

estore lea3 ro4key @A!&@data 01 00 20 7 00 00

$8 10=Delete lea3 ro4slot 0

I'&)T I'T$ score.tea+; runs; 4icketsVA5!)&.@A!&@;100;

I'&)T!8DAT) score&)T tea+ > @)'9@W-)) tea+ > @A!&@

!8DAT)D)5)T) #$, scoreW-)) tea+ > @)'9@

D)5)T)

!ndo

edo

$8 ?=1 .10=228urge lea3 ro4key @)'9@

$8 10=2Insert 5ea3 o4key @)'9@data 01 00 20 7 00 00

$8 ?=1 .10=22

estore lea3 ro4key @)'9@data 01 00 20 7 00 00

$8 10=Delete lea3 ro4slot 0

&T$8


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&)5)CT #$ !8DAT) edo and !ndo 9eneration

$8 ?=1 .11=15ock o4 " 5

&lot

$8 11=5ock o4 " 5

&lot

&)5)CT runs; 4ickets#$, scoreW-)) tea+ > @A!&@#$ !8DAT)

!ndo

edo

&T$8


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&)5)CT #$ !8DAT) &)5)CT #$ !8DAT) is bad 3or so +any reasons=====

o4s are locked pessi+istically,ore c(ance o3 contention

o4s could be locked opti+istically by any subse6uent

!8DAT) state+entApplication logic +ay need +odi3ication

&)5)CT #$ !8DAT) generates!ndo " +ore space in bu33er cac(e; $A01??? etc

edo " increased p(ysical I$

&)5)CT #$ !8DAT) state+ents cannot be batc(ed)ac( re6uires a separate pair o3 c(ange :ectors


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!8DAT) &tate+ents edo and !ndo 9eneration

$8 ?=1 .11=15ock o4 " 5

&lot

$8 11=5ock o4 " 5

&lot

&)5)CT runs; 4ickets#$, scoreW-)) tea+ > b1#$ !8DAT)

!ndo

edo

C)AT) $ )85AC) 8$C)D!) update%runs.p%tea+ VAC-A2;p%runs '!,)

I&l%runs '!,)l%4ickets '!,)

)9I'

&)5)CT runs; 4icketsI'T$ l%runs; l%4ickets#$, scoreW-)) tea+ > p%tea+#$ !8DAT)

!8DAT) test/&)T

runs > l%runs;4ickets > l%4icketsW-)) tea+ > p%tea+

)'D

$8 ?=1 .11=1!pdate o4 8iece " !8

&lot

c1 100c2

$8 11=?!pdate o4 8iece " !8

&lot

c1 10c2

!8DAT) score&)T

runs > b/;4ickets > b2

W-)) tea+ > b1

!8DAT)

&)5)CT#$ !8DAT)

&T$8


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!8DAT) &tate+ents !8DAT) state+ents t(at include unc(anged colu+ns

Ad:antages

educe parse o:er(ead

9ood on single instance; e:en better on AC

educe space re6uired in library cac(e

5ess c(ance cursors 4ill be aged out

Disad:antages

Increase p(ysical I$ to online redo logs

Increase nu+ber o3 undo blocks in bu33er cac(e Increase probability o3 $A"01???


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Data lock &tructure

lock -eader

Data -eader

Interested Transaction 5ist

Table Inde*

o4 Inde*

!nused &pace

Data

lock Tail


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Interested Transaction 5ist )ac( datainde* block (as an Interested Transaction 5ist

list o3 transactions currently acti:e on block stored 4it(in block (eader

)ac( datainde* ro4 (eader contains a lock byte 5ock byte records current slot in IT5

)ac( ro4 can only be associated 4it( one transaction I3 a second transaction atte+pts to update a ro4 it 4ill

e*perience a ro4 lock 4aits until 3irst transaction co++itsrolls back

Initially t4o IT5 entries are reser:ed in block (eader IT5 list can gro4 dyna+ically according to de+and IT5 list cannot s(rink again


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Interested Transaction 5ist IT5 entry includes

Transaction ID .EID !ndo byte address .!A

&yste+ C(ange 'u+ber .&C'

IT5 entry is o:er4ritten by eac( c(ange to t(e block by t(e

current transaction

8re:ious c(ange is recorded in undo block

During rollback; c(anges are restored to IT5 3ro+ undo block


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ead Consistency e6uired to +aintain ACID properties o3 transaction

Transactions +ust al4ays see consistent :ersions o3blocks +odi3ied by ot(er transactions

Can be applied at

&tate+ent le:el .de3ault

Transaction le:el

!nco++itted block updates are rolled back 4(en block isread

Consistent reads

,ore speci3ically undo is applied to return block toconsistent state

!ndo +ust still be a:ailable in undo seg+ent

I3 undo (as been o:er4ritten; 6uerying session 4illrecei:e $A"01??? &naps(ot too old


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&)T TA'&ACTI$' Deter+ines le:el at 4(ic( read"consistency is applied

Can be &)T TA'&ACTI$' )AD WIT)

establis(es state+ent"le:el read consistency

subse6uent state+ents see any c(anges co++ittedbe3ore t(at state+ent started

de3ault be(a:iour &)T TA'&ACTI$' )AD $'5K

establis(es transaction"le:el read consistency

all subse6uent state+ents only see c(anges co++itted

be3ore transaction startednot supported 3or &K& user

&)T TA'&ACTI$' state+ent +ust be 3irst state+ent intransaction


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&)T TA'&ACTI$' #or e*a+ple

&ession 1 &ession 2 &ession /

&)5)CT runs#$, scoreW-)) tea+ > @)'9@

uns127

!8DAT) tea+&)T runs > 1/1W-)) tea+ > @)'9@

C$,,IT

&)T TA'&ACTI$')AD WIT) &)T TA'&ACTI$')AD $'5K


uns1/1


uns127


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$A%$W&C' 8seudocolu+n eturns conser:ati:e upper"bound &C' 3or +ost recent

c(ange in ro4

!ses &C' stored 3or transaction in IT5

&(o4s last ti+e a ro4 in sa+e block 4as updated

,ay s(o4 +ore accurate in3or+ation 3or an indi:idual ro4

'ot supported 4it( 3las(back 6uery

To con:ert $A%$W&C' to an appro*i+ate ti+esta+p uset(e &C'%T$%TI,)&TA,8 built"in 3unction e=g=

&)5)CT $A%$W&C';&C'%T$%TI,)&TA,8 .$A%$W&C'#$, score


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$A%$W&C' 8seudocolu+n #or e*a+ple " no ro4 dependencies .de3ault

&C'#&C 0000=00/?BBbaIT52

EID 000 W-)) tea+ > @)'9@

C$,,IT

&)5)CT $A%$W&C'; tea+s; runs; 4ickets #$, score

$A%$W&C' Tea+s uns Wickets/?0B1/ )'9 0/?0B1/ A!& 0 0

o4 0 lb 2

o4 1 lb 0

0*/?BBbd >/?0B1/

#lag C""" 5ck 0

0*/?BBba >/?0B10


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$A%$W&C' 8seudocolu+n #or e*a+ple .ro4 dependencies

IT51

#lag "!""" 5ck 2

IT52

&C'#&C 0000=00/?BBba#lag ""!" 5ck 1

&C'#&C 0000=00/?Bc30

dscn 0000=00/?Bced

o4 1 lb 1

col 0 )'9

col 1 0

col 2 0

col 0 A!&

col 1 0

col 2 0

&C'#&C 0000=00/?Bced

o4 0 lb 1

dscn 0000=00/?Bced

C)AT) TA5) score.tea+ '!,); runs '!,); 4ickets '!,)$WD)8)'D)'CI)&

&)5)CT ora%ro4scn; tea+s; runs; 4ickets #$, score

$A%$W&C' Tea+s uns Wickets/?0


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#las(back Guery )*a+ple

&ession 1 &ession 2

!8DAT) tea+&)T runs > 11W-)) tea+ > @)'9@

C$,,IT


uns1/7

&)5)CT db+s%3las(back=get%syste+%c(ange%nu+ber #$, dual

&C'/


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#las(back Guery Can speci3y A& $# clause

eturns single"ro4

&ynta* is

&)5)CT tea+; runs; 4ickets#$, score A& $# &C' /?0/1W-)) tea+ > @)'9@

#or e*a+ple

A& $# L &C' MscnN O TI,)&TA,8 Mti+esta+pN P


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#las(back Guery Can also speci3y V)&I$'& clause

eturns +ultiple ro4s

&ynta* is

&)5)CT tea+; runs; 4ickets#$, score V)&I$'& )TW))' &C' /?0/?11 A'D /?0/?2

W-)) tea+ > @)'9@

V)&I$'& )TW))' &C' L MscnN O ,I'VA5!) PA'D L MscnN O ,AEVA5!)

V)&I$'& )TW))' TI,)&TA,8 L Mti+esta+pN O ,I'VA5!) PA'D L Mti+esta+pN O ,AEVA5!)

#or e*a+ple


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Version Guery 8seudocolu+ns Valid only 3or #las(back Version Guery= Values can be

V)&I$'&%&TATTI,) ti+esta+p o3 3irst :ersion o3 ro4s returned by 6uery

V)&I$'&%)'DTI,) ti+esta+p o3 last :ersion o3 ro4s returned by 6uery

V)&I$'&%&TAT&C'

&C' o3 3irst :ersion o3 ro4s returned by 6uery V)&I$'&%)'D&C'

&C' o3 last :ersion o3 ro4s returned by 6uery V)&I$'&%EID

#or eac( ro4 returns transaction ID o3 transaction

creating t(at ro4 :ersion V)&I$'&%$8)ATI$'

#or eac( ro4 returns operation creating t(at ro4:ersion= Can be I.nsert !.pdate or D.elete


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Version Guery 8seudocolu+ns )*a+ple

&ession 1 &ession 2

!8DAT) tea+&)T runs > 1?W-)) tea+ > @)'9@

C$,,IT


uns11

!8DAT) tea+&)T runs > 1?1W-)) tea+ > @)'9@

C$,,IT

!8DAT) tea+&)T runs > 1?/W-)) tea+ > @)'9@

C$,,IT


&C'/?0/1/


&C'/?0/1/


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Version Guery 8seudocolu+ns )*a+ple .continued

&ession 1 &ession 2

&)5)CTV)&I$'&%&TAT&C' Q&tartQ;V)&I$'&%)'D&C' Q)ndQ;V)&I$'&%EID QEIDQ;V)&I$'&%$8)ATI$' Q$pQ;score=tea+ QTea+Q;score=runs QunsQ;

score=4ickets QWicketsQ#$, score V)&I$'& )TW))' &C' /?0/1/ A'D /?0/1/W-)) tea+ > @)'9@

&tart )nd EID $p Tea+ uns Wickets/?0/12 0B000A00#C00000 ! )'9 1?/ 1/?0/1/< /?0/12 07001A00#00000 ! )'9 1?1 1

/?0/1/ /?0/1/< 0002C00)A00000 ! )'9 1? 1/?0/1/ )'9 11 1

EID > 00=02C=00000)A.Arc(itecture > EB

Can be I.nsert; !.pdateor D.elete


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