The document discusses advanced database features presented by Connor McDonald, emphasizing the importance of read consistency and data integrity in SQL operations. It highlights the complexity involved in ensuring referential integrity and the need for efficient concurrency management. Through various examples, it illustrates how the Oracle consistency model simplifies development while also posing challenges related to data validation and transactional control.

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The Best Database Feature Ever
Connor McDonald
Database Advocate
Copyright © 2019 Oracle and/or its affiliates.
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Connor McDonald
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Stuff
youtube bit.ly/youtube-connor
blog bit.ly/blog-connor
twitter bit.ly/twitter-connor
400+ posts mainly on database & development
250 technical videos, new uploads every week
rants and raves on tech and the world :-)
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etc...
facebook bit.ly/facebook-connor
linkedin bit.ly/linkedin-connor
instagram bit.ly/instagram-connor
slideshare bit.ly/slideshare-connor
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the #1 thing
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"Readers don't block writers"
"Writers don't block readers"
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Maximum concurrency…
… very few locking worries
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lack of understanding
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some revision / fundamentals
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SQL> update EMP
2 set SAL = SAL * 1.10
3 /
14 rows updated.
SQL> rollback;
Rollback complete.
how ?
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we remember stuff
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SQL> update EMP
2 set SAL = SAL * 1.10
3 where ename = 'SMITH'
undo header
smith $1,0001
block 2754
jones $9002
brown $1,5003
wells $2,0004
wilson $1,0005
block 2754, row 1,
sal $1000
uba 5 5
$1,100
uba 5.1
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SQL> rollback;
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SQL> rollback;
undo header
smith $1,0001
block 2754
jones $9002
brown $1,5003
wells $2,0004
wilson $1,0005
block 2754, row 1,
sal $1000
uba 5
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$1,100
uba 5.1
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we can use this stuff for queries!
(Oracle version 4)
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read consistency
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select * from emp
where hiredate > '01/01/2004'
update emp set ename = 'SUE'
where ename = 'JOHN';
Block 3217
"I need block 3217…
… as it was at 9:00"
9:00
9:03
9:05
session 1 session 2
so what do we do now ?
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we apply undo
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Session 1
Request
Block 3217,
SCN 4567192
Block 3217,
9:03am
"SUE"
Block 3217,
change 9:03 to 8:45
change SUE => JOHN
undo segment block(s)
No good..too new
take a copy of the block
Locate
apply undo
Block 3217,
9:03am
" SUE"
Block 3217,
9:03am
" SUE"
Block 3217,
8:45am
" JOHN"
Block 3217,
8:45am
"JOHN"
Session 2
update emp
set ename = 'SUE'
where ename = 'JOHN'
commit;
Done !
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in reality: system change number
stress: vast simplification
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results are time consistent
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“I’m a developer…
I just write SQL ...
…who cares about all that?”
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So many ways…
TROUBLE
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example: copying data
“every hour,
populate data warehouse,
copy of OLTP data”
EMP DWEMP
readcon1/readcon2
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DIY data integrity
... true story
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Manufacturer
Product
alter table add primary key
alter table add foreign key
“ensure that every
product corresponds to
a valid manufacturer”
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create or replace
trigger PROD_MAN_CHECK
before insert or update on PRODUCT
for each row
declare
l_man_id number;
begin
select man_id
into l_man_id
from manufacturer
where man_id = :new.man_id;
exception
when no_data_found then
raise_application_error(-20000,
'Manufacturer is not valid');
end;
create or replace
trigger MAN_PROD_CHECK
before delete on MANUFACTURERS
for each row
declare
l_prd_cnt number;
begin
select count(*)
into l_prd_cnt
from product
where prd_id = :old.prd_id;
if l_prd_cnt > 0 then
raise_application_error(-20001,
'Manufacturer in use by product records');
end if;
end;
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testing "works"
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SQL> insert into PRODUCT (...,MAN_ID,...)
2 values (...,100, ...);
1 row created.
SQL> insert into PRODUCT (...,MAN_ID,...)
2 values (...,101,...);
ERROR at line 1:
ORA-20000: Manufacturer is not valid
SQL> delete from MANUFACTURER
2 where man_id = 5768;
ERROR at line 1:
ORA-20001: Manufacturer in use by product records
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one month later…
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An unexpected error has
occurred. Manufacturer
record not found
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SQL> select count(*)
2 from
3 ( select man_id from PRODUCT
4 minus
5 select man_id from MANUFACTURER )
6 /
COUNT(*)
----------
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data integrity in code = no data integrity
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create PRD with MAN=7
Trigger fires
- does MAN=7 exist?
- "Yes"
- OK
delete MAN=7
Trigger fires
- any PRD with MAN=7
- "No" (not yet)
- OK
commit
commit
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data validity seems simple…
"look up X before allowing Y"
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referential integrity is complex
read consistency,
blocking / locking
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beware the module specification
"When creating a product...
validate registration date – ensure not in the future,
validate …
validate …
validate manufacturer ID – must exist in Manufacturer table"
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concurrency testing vital
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what about updates ?
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SQL> update EMP
2> set SAL = 1000
3> where ENAME = 'SMITH'
change block in current mode
locate blocks in consistent mode
ENAME = ‘SMITH’
SAL = 500
ENAME = ‘JONES’
SAL = 500
demo4a/b
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so what happened ?
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Session 1
changed 4999 rows
got stuck...
Session 2
updated
row 5000
then
committed.. tried to
resume but
the data has
changed
rollback the
transaction
start again, this
time select-for-update
to avoid needing to
rollback again
updated
row 10000 got stuck at row 10000
then
committed.. OK locked all rows
go ahead and update
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beware autonomous transactions
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beware doing external things in triggers
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summary
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Oracle consistency model is …
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very very cool… and
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very very complex !
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(DIY) inter-table checking is hard to do right
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(DIY) intra-table checking is hard to do right
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let the database do it
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Thanks for attending !!!
youtube tinyurl.com/connortube
blog connor-mcdonald.com
twitter @connor_mc_d
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