[ODI] chapter1 When Update statement is executed, How does oracle undo work?

Oracle Deep Internal report no. 001

When Update statement is executed,

How does oracle undo work?

Research & Contents Team

When Update statement is executed,

How does oracle undo work?

01

Lee Kun Oh (이근오) [email protected]

Kwon Kon Woo (권건우) [email protected]

2011, Oracle Core Essential Internals for DBAs and Developers, Jonathan Lewis

"Undo" mechanism as explained in Jonathan Lewis's book

It's so difficult to understand only through

the book.I'd like to check internal actual situation of oracle

with my own eyes

Simple flow chart in oracle when processing Update statement

2011, Oracle Core Essential Internals for DBAs and Developers, Jonathan Lewis

UPDATE Table A SET col1 = ‘B’

AA

Redolog

undo

DataFile

AB

A

B

SGA

Undo SegmentData Block

Shared Pool Buffer Cache

1. Initial state when a value of "A" is entered (The first stage before update)

v$transaction

Undo Headerindex state cflags wrap# uel scn dba parent-xid nub -------------------------------------------------------------------------------------------

0x00 9 0x00 0x04ec 0x0001 0x0000.003d6d72 0x00000000 0x01 9 0x00 0x04ec 0x0002 0x0000.003d6d72 0x00000000 0x02 9 0x00 0x04ec 0x0003 0x0000.003d6d72 0x00000000

Undo Block

ITLItl Xid Uba Flag Lck Scn/Fsc

0x01 0x000a.000.00000534 0x018000d2.01e6.01 --U- 1 fsc 0x0000.003dbe3a0x02 0x0000.000.00000000 0x00000000.0000.00 ---- 0 fsc 0x0000.00000000

Rowsblock_row_dump:tab 0, row 0, @0x1f88tl: 16 fb: --H-FL-- lb: 0x1 cc: 2col 0: [10] 31 20 20 20 20 20 20 20 20 20col 1: [ 1] 41

ASCII HEXA code 41=> Value ‘A’

* Rec #0x1

BH

SGA



1_1. State in which generating CU Block into Buffer Cache by selecting one from DB block(The preparation stage before update)

v$transaction



Undo Block




* Rec #0x1

BH

ASCII HEXA code 41=> Value ‘A’

BH (0x643fa438) file#: 5 rdba: 0x0140dcad (5/56493) class: 1 ba: 0x643ba000st: XCURRENT md: EXCL fpin: 'kdswh11: kdst_fetch' tch: 1Itl Xid Uba Flag Lck Scn/Fsc

0x01 0x000a.000.00000534 0x018000d2.01e6.01 --U- 1 fsc 0x0000.003dbe3a0x02 0x0000.000.00000000 0x00000000.0000.00 ---- 0 fsc 0x0000.00000000tl: 16 fb: --H-FL-- lb: 0x1 cc: 2col 0: [10] 31 20 20 20 20 20 20 20 20 20col 1: [ 1] 41

UPDATE Table A SET col1 = ‘B’

SGA



BH (0x643fa1c8) file#: 5 rdba: 0x0140dcad (5/56493) class: 1 ba: 0x643b6000st: XCURRENT md: EXCL fpin: 'kdswh11: kdst_fetch' tch: 1

Itl Xid Uba Flag Lck Scn/Fsc0x01 0x000a.000.00000534 0x018000d2.01e6.01 --U- 1 fsc 0x0000.003dbe3a0x02 0x0000.000.00000000 0x00000000.0000.00 ---- 0 fsc 0x0000.00000000tl: 16 fb: --H-FL-- lb: 0x1 cc: 2col 0: [10] 31 20 20 20 20 20 20 20 20 20col 1: [ 1] 41



2.. When executing Update statement, generates new CU block in Buffer Cache and changes from CU block to CR block

v$transaction

CR




ASCII HEXAcode 41=> Value ‘A’



Undo Block

* Rec #0x1

BH

copy XCU block

change from XCU to CR

BH‘a438’XCU block is copied to BH‘a1c8’XCU blockBH‘a438’block is changed from XCU to CR

0x643fa438

0x643fa1c8

SGA





2_1. Address data is generated in v$transaction. (Allocating memory about Row Cache new tx)

CR







Undo Block

* Rec #0x1

BH

V$transactionXID_INFO UBA_INFO TXN_INFO-------------------------- --------------- ------------------------------addr : status : IDLExidusn : ubafil : start_date : 2015-12-10 10:52:46xidslot : ubablk : ses_addr : 000000006B21CC20xidsqn : ubasqn :

0000000069B01FB0



For processing TX, Address is allocated in v$transactiondata at the very first

SGA



3. Xidusn's value is allocated in v$transaction. (No. 7 usn in here)







Undo Block

* Rec #0x1

BH


0000000069B01FB07



CR



After allocating address, USN(Undo Segment Number) is registered in v$transaction data for the second time

Undo Segment Number

SGA





4. In Undo segment, Slot value is changed from 9 to 10

Undo Block

* Rec #0x1 slt: 0x00 objn: 70670 (0x0001140e) objd: Undo type: Regular undo Begin trans Last buffer split: Noncol: 2 nnew: 1 size: 0Vector content:col 1: [ 1] 41

BH (0x643e4b50) file#: 6 rdba: 0x018000a2 (6/162) class: 30 ba: 0x64184000st: XCURRENT md: EXCL fpin: 'kcbwh2: kcbchg1' tch: 0

Before image value ‘A’





Undo Block Address

10 0x80 0x04ed 0x0000 0x0000.003dbe4d 0x018000a2

Undo slot active


0000000069B01FB07



CR



Undo Segment Number

SGA





5. Update value A to B of XCU block in Buffer Cache







Undo Block




Undo Block Address

10 0x80 0x04ed 0x0000 0x0000.003dbe4d 0x018000a2

0x01 0x000a.000.00000534 0x018000d2.01e6.01 C--- 0 scn 0x0000.003dbe3a0x02 0x0007.000.000004ed 0x018000a2.01d5.01 ---- 1 fsc 0x0000.00000000tl: 16 fb: --H-FL-- lb: 0x2 cc: 2

col 1: [ 1] 42

Undo slot active


0000000069B01FB07



CR

Update ‘A’ to ‘B’ of CU block

CleanOut : Itl No.1 change (--U- C---)Txn data change: Itl No.2 change (Xid,Uba)Lock Byte change: 0x1 0x2

Undo Segment Number

SGA



V$transactionXID_INFO UBA_INFO TXN_INFO-------------------------- --------------- ------------------------------addr : 0000000069B01FB0 status : IDLExidusn : 7 ubafil : start_date : 2015-12-10 10:52:46xidslot : ubablk : ses_addr : 000000006B21CC20xidsqn : ubasqn :



CR

6. Allocate into other column except xid and uba in v$transaction







Undo Block



0x018000a2

70000000069B01FB0

11261

162

ACTIVE

Undo Block Address


Undo slot active

6

469

10 0x80 0x04ed 0x0000 0x0000.003dbe4d




col 1: [ 1] 42

Registered with the order as below in v$transaction.* addr->xidusn-> xidslot, xidsqn,

ubablk

SGA






col 1: [ 1] 42



10 0x80 0x04ed 0x0000 0x0000.003dbe4d



CR

7. Commit execute (Allocating equal scn to Itl slot and undo header slot)





Undo Block




Undo Block Address

0x018000a20x0000.003dbe4e

--U- 1 fsc 0x0000.003dbe4e

Undo slot active

Allocating equal scn to Itl slot and undo header slot


After commit, TX data is deleted in v$transaction

Commit;

SGA



8. Executing Check point (CU block reflects disk block)






Undo Block




Undo Block Address

0x018000a2

tl: 16 fb: --H-FL-- lb: 0x2 cc: 2col 0: [10] 31 20 20 20 20 20 20 20 20 20col 1: [ 1] 42

Value 'B' reflects Data block

0x01 0x000a.000.00000534 0x018000d2.01e6.01 C--- 0 scn 0x0000.003dbe3a0x02 0x0007.000.000004ed 0x018000a2.01d5.01 --U- 1 fsc 0x0000.003dbe4e

Undo slot active



CR



0x01 0x000a.000.00000534 0x018000d2.01e6.01 C--- 0 scn 0x0000.003dbe3a0x02 0x0007.000.000004ed 0x018000a2.01d5.01 --U- 1 fsc 0x0000.003dbe4etl: 16 fb: --H-FL-- lb: 0x2 cc: 2

col 1: [ 1] 42




col 1: [ 1] 42

10 0x80 0x04ed 0x0000 0x0000.003dbe4d0x0000.003dbe4e


Check Point;

SGA






tl: 16 fb: --H-FL-- lb: 0x2 cc: 2col 0: [10] 31 20 20 20 20 20 20 20 20 20col 1: [ 1] 42

0x01 0x000a.000.00000534 0x018000d2.01e6.01 C--- 0 scn 0x0000.003dbe3a0x02 0x0007.000.000004ed 0x018000a2.01d5.01 --U- 1 fsc 0x0000.003dbe4e

Summary : DML Operation in a Data Block



Undo Block




Undo Block Address

0x018000a2

Undo slot active



CR




col 1: [ 1] 42

10 0x80 0x04ed 0x0000 0x0000.003dbe4d0x0000.003dbe4e

Value 'B' reflects Data block


Oracle Deep Internal

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[ODI] chapter1 When Update statement is executed, How does oracle undo work?