sql good pratices

8/9/2019 SQL Good Pratices

1/77

Unit 1

Module 1: Processing and Complexity of SQL

sentences

Phases in the processing of an SQL sentence

Every time racle is re!uested to execute an SQL sentence" it internally performs a series of

actions" #hich are mainly grouped into $ %loc&s: Parse 'analysis(" Execute and )etch 'data

collection(*

PARSE

Existence of the sentence in the

cache" syntactic and semantic chec&*

+t is chec&ed that the SQL

sentence exists in the cache" it

is syntactically chec&ed 'i*e* it

is #ritten correctly( and it is

semantically chec&ed 'correctstructure" fields and

permissions(*

,ransformation

Some !ueries 'e*g* those that

include vie#s" su%!ueries(

have to %e transformed to the

most suita%le ne# sentence*

Some sentences can %e

replaced #ith others that are

more optimal or simpler*

RBO '-ule.

%ased

ptimi/er(*

+t decides

on the plan

in

accordance

#ith rules

CBO 'Cost.

%ased

ptimi/er(*

+t decides

on the plan

#ith

respect to

the

statistics of

the o%0ects

involved

Calculation

of the costof the

o%0ect and

cardinality

,he costs of access to the

o%0ects and their cardinality

'estimation of the num%er of

ro#s involved( are calculated*

Costs #ith

different 0oin

orders

ifferent execution plans are

evaluated %y changing the 0oin

order %et#een the ta%les '2 to

2(" choosing the least costly

order*

Creation of

structures

for

execution

,he structures necessary for

the execution 'cursors" etc*(are created*

EXECUTE

,he memory necessary for the

%ind varia%les is ta&en and

they are assigned and

executed in accordance #ith

the plan selected during the

parse*

FETCH

,he !ueried data %loc&s are

fetched and" if applica%le"

orderings are performed*


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Unit 1

Module 1: Processing and Complexity of SQL

sentences

31* Complexity of SQL sentences

+t is not considered suita%le to use extremely long and complex sentences" as they are more

costly to maintain and more difficult for people to understand* Moreover" these complex

sentences usually result in unsuita%le execution plans*

+t is advisa%le to %rea& these sentences do#n into various SQL calls even if this means more

code in our applications* +n the long run they are maintained %etter and give %etter response

times*


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Unit 1

Module 2: ,he ptimi/er

4hat is the optimi/er5

Every time a sentence is executed against the data%ase" one of the things that racle must

decide is the optimal execution plan 'determine ho# to access each o%0ect" and in #hich order

the 0oins to the various ta%les involved are performed(*

,he optimi/er is responsi%le for doing this* Currently there are t#o types of optimi/er for racle:

Rule-based: the criteria for deciding on the plan are in accordance #ith a series of fixed rules"

regardless of the volume or distri%ution of data 'for example" the existence of indexes and types

of indexes(* Maintenance of the rule.%ased optimi/er #as discontinued in racle version 6 and

it #ill no longer %e supported from version 17*

Cost-based: racle ta&es into account the volume and distri%ution of data in accordance #ith

previously gathered statistics* epending on the volume" it decides on the 0oin orders or the use

or not of indexes" for example*

+t is clear that for the cost.%ased optimi/er it is essential to have relia%le statistics" so they must

%e periodically updated*


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Unit 1

Module 2: ,he ptimi/er

4hich optimi/er shall + use5

f the t#o optimi/ers 'rule.%ased and cost.%ased( it is al#ays advisa%le to use the cost.%ased

optimi/er* 'Maintenance of the rule.%ased optimi/er #as discontinued in racle version 6 and it

#ill no longer %e supported from version 17*(

Propose the use of one type of opt!"er

Sesson

8L,E- SESS+9 SE, P,+M+E-;38L<

)rom racle =i:

8L,E- SESS+9 SE, P,+M+E-;ME<

#atabase optimi/er;mode<

Senten$eUsing >ints

?@A -ULE @?" ?@A )+-S,;-4S @?" ?@A 8LL;-4S @?

Possble %alues:

-ULE -ule.%ased*

)+-S,;-4S

Cost.%ased" optimi/ing return of the first ro#s*

)rom racle =i:)+-S,;-4S"

)+-S,;-4S;B1171771777D

8LL;-4S Cost.%ased" optimi/ing return of all the ro#s*

C>SEracle chooses if there are statistics in the

o%0ects" it tries cost.%ased '8LL;-4S(

,he use of the follo#ing racle characteristics #ill force the use of the cost.%ased optimi/er

rather than the rule.%ase optimi/er:

,a%le #ith fixed P8-8LLEL omain +ndexes '+nter Media(

Partitioning Parallel Create ,a%le 8s Select

+, '+ndex rgani/ed ,a%les( )unction.%ased indexes

+nverted indexes Query -e#rite activated

8ny hint" other than -ULE


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6/77

Unit 1

Module $: Execution Plan

Execution plan: ho# to consult it and interpret it

)or the optimi/ation of SQL sentences it is essential to understand the access plan 'execution

plan( that racle uses in the execution of the sentence*

,here are various methods to understand the execution plan:

1* )rom SQL@Plus executing EJPL8+9 PL89 )-*

2* )rom SQL@Plus executing SE, 8U,,-8CE*

$* 3etting the trace of the session and using t&prof*

E&e$uton plan 'th EXP(A)* P(A*

+t is necessary for there to %e a ta%le in the username called PL89;,8KLE" #here the

execution plan is stored* +f this ta%le does not exist in the username executing the sentence"

and given that the structure of the PL89;,8KLE varies %et#een the different versions of

racle" as& the ata%ase 8dministration team to generate the ta%le in your username*

)rom SQL@Plus" execute for example:

,he execution plan #ill %e stored in the PL89;,8KLE*

All %ersons of Ora$le:

Fou can !uery it %y performing a select* Fou can use the follo#ing select to visually interpret the

execution plan a little %etter:

,he result sho#s the execution plan:


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Fro! Ora$le +:

,he execution plan can %e o%tained using this sentence:

,he result sho#s the execution plan:

E&e$uton plan 'th SET AUTOTRACE

+t is necessary for there to %e a ta%le in the username called PL89;,8KLE" #here the

execution plan is stored* +f this ta%le does not exist in the username executing the sentence"

and given that the structure of the PL89;,8KLE varies %et#een the different versions of

racle" as& the ata%ase 8dministration team to generate the ta%le in your username*

)rom SQL@Plus" execute for example:


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+t #ill directly o%tain the execution plan in the follo#ing format:

E&e$uton plan 'th T,PROF

racle ena%les the registration in a trace file of all the sentences that are executed in a session"

and %ased on this file" using t&prof" you can !uery the execution plans* 8n example is provided

%elo#* ,o find out in #hich directory" for the data%ase you are using" the trace files are created"

contact the ata%ase 8dministration team*

4ith the sentence 8L,E- SESS+9 SE, SQL;,-8CE< you can activate or deactivate

dumping in the trace file* Locate the trace file you have generated 'tec71;ora;6N*trc(:

%tain the execution plans in file tec71;ora;6N*t&p" using t&prof:

,he format of the t&prof command is:


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t&prof Oinput file Ooutput file sys


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12/77

Unit 1


Example of the interpretation of a plan

,o interpret the execution plan o%tained %y any of these three methods" it is necessary to read

from inside to outside to understand ho# access to the data is achieved* ,his execution plan

sho#s:

Step $ ')ULL of emp( is executed" returning record %y record to 2 '9ES,E LP(*

)or each record returned %y $" steps and are executed 'search in PR;EP, index 'uni!ue

scan( and search for this record in the EP, ta%le accessing %y ro#id(*

Each one of the ro#s fetched %y $" and ." is collected in the 9ES,E LP in a single

record '+9(" %eing step 2*

,he step ')ULL of the S8L3-8E ta%le( is executed*

,hen step 1 ')+L,E-(" #hich actually implements the 9, EJ+S,S of the select" is executed*

8ccordingly" each record that 2 returns" and that is not in " #ill %e included in the result*

+t can %e graphically represented as follo#s:


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,he execution order of this tree is read from %ottom to top and left to right* +f you have anydou%ts #hen interpreting execution plans" you can consult the ata%ase 8dministration team*


14/77

Unit 1


,ypes of oin

4hen in a SELEC, sentence more than one ta%le is indicated in the )-M clause" racle"

#hen it o%tains the data" must 0oin the data of each ta%le" in accordance #ith the conditions of

the 4>E-E* ,his union is called a 0oin*

racle" regardless of the num%er of ta%les in the )-M" al#ays performs ta%le 0oins t#o %y t#o"

applying the results of 0oining t#o ta%les to the next ta%le and so on until it has gone through all

the ta%les of the )-M*

,here are currently three different types of 0oin:

1* 9ested Loop

2* Sort.merge oin

$* >ash 0oin

,he %ehaviour of the optimi/er can %e changed so that it chooses another 0oin method" using

the hints USE;9L" USE;ME-3E" USE;>8S>*

*ested (oop on

,he optimi/er chooses a ta%le as the master or outer* )or each record of the outer racle

searches in the inner for all the records that lin& to the records of the outer* +n the example

%elo# the dept ta%le is the master" and for each record that is found in dept" matches are

searched for in emp*

Sort-!er.e on

8 sort.merge 0oin can only %e performed #ith e!ui0oin '


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nce the t#o groups of data have %een ordered" racle 0oins one to the other 'ME-3E +9(*

Hash on

+t can only %e used #ith e!ui0oin '


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17/77

Unit 1


4hich 0oin to use5

+f the ro#s to %e returned %y the 0oin are not many 'fe#er than 17"777 ro#s approx*(:

racle tends to use 9ested Loop*

+f the ro#s to %e returned %y the 0oin are many 'more than 17"777 ro#s approx*(:

+t is advisa%le to use the >ash oin* 'Provided that the cost.%ased optimi/er is used* +f not" there

is no other option %ut to use the sort.merge 0oin*(

,he sort.merge 0oin is not advisa%le 'for many ro#s(" due to the ordering costs" so it is

prefera%le to use the hash 0oin #ith cost.%ased optimi/er*

8ctually" the choice racle ma&es is much more complex and does not exclusively depend on

the num%er of records" ta&ing into account the costs of the E?S operations" the ordering and

hash memory areas" the existing statistics" the data model" the access indexes" etc* >ere only

an overvie# of the choice of one plan or another is provided" although it is the optimi/er that has

more information to decide on the optimal 0oin plan 'and even so it does not al#ays ma&e the

%est decision(*


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Unit 2

Module 1: +n the SELEC,

CU9,'@( versus CU9,'1(

+n versions prior to 6i" the use of count'@(" count'1( or count'c1( could have difference response

times #hen using indexes or not*

From Oracle 8, the use of count(*) or count(1) is the same as they can use accesses by index

(fast index scan), obtaining the same response times.


19/77

Unit 2


rder of the fields in the SELEC,

,he order of the fields in the SELEC, clause does not affect the performance of the sentences

at all*


20/77

Unit 2


S1* )ields necessary in the select

It is adisable to aoid the use of !"#"$% *, restricting the fields search in the &select& to the

fields that are really necessary. %his reduces the olume of information accessed and

transported from the serer to the client. It also facilitates 'uic access to indexes, compared to

access to indextable (+hen there is an index that allo+s it).

+f it is not necessary to use all the fields of the ta%le" not using the SELEC, @ can also %e

%eneficial to give more clarity #hen reading SQL code and avoid errors in the cursors #hen

assigning values to varia%les*


21/77

Unit $

Module 1: +n the )-M

9um%er of ta%les in the )-M

,he cost.%ased optimi/er must choose the order in the 0oins %et#een the ta%les must %e

performed '0oins are al#ays performed in pairs of ta%les(* ,his choice is made during the

P8-SE of the sentence" trying all possi%le 0oin com%inations and choosing the one #ith the

least cost*

4hen the num%er of ta%les in the )-M 'num%er of 0oins( is very high" the optimi/er decides

not to test all the possi%le com%inations as it #ould ta&e a long time" only testing some of the

com%inations" so there is a greater pro%a%ility of not choosing the %est execution plan*

4hen the total nu!ber of tables n the FRO/ s !ore than e.ht" racle can no longerperform all the necessary chec&s and non.optimal plans are chosen*

+t is advisa%le to try to avoid putting more than 6 ta%les in the )-M" performing the !uery in

various separate SELEC,s* +f it is completely impossi%le" as a last resort" the I-E-EI hint

could %e used" ordering the ta%les in the )-M in accordance #ith the order most suita%le for

performing the +9s*


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Unit $


rder of the ta%les in the )-M

)or the cost.%ased optimi/er '#hich is the one that should %e used(" the order of the tables n

the FRO/ does not affe$t the execution plans nor the performance of the sentence*

,he only exception is #hen the I-E-EI hint is used*


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Unit $


istri%uted sentences

racle ena%les" transparently for the user" !ueries of o%0ects that are in remote data%ases*

>o#ever" this transparency is not maintained in the execution plans or in the performance" so

special care must %e ta&en #ith sentences over distri%uted data%ases*

Ho' dstrbuted senten$es are pro$essed

1* -emote SQL: #hen all the ta%les in the )-M %elong to remote ta%les in a single data%ase"

the sentence is sent as is to the remote data%ase" and the execution plan is o%tained and

executed as if it #ere in local* ,he resulting data must go from the remote data%ase to the local

data%ase*

2* istri%uted SQL: #hen in the )-M there are local and remote ta%les" or remote ta%les in

various data%ases" racle must %rea& do#n the sentence" to execute the part that corresponds

to each data%ase and also to execute the local part in the local data%ase*

,he local data%ase I%ecomes the masterI 'if not specified to the contrary #ith a >int(" so it is the

data%ase that receives all the data and is responsi%le for the 0oins" groupings and orderings*

Re$o!!endatons for dstrbuted senten$es

•

8l#ays use the cost.%ased optimi/er*

• 8lthough distri%uted access is transparent for the application" it is important to &no# that

the execution plans are strongly modified" and it is a good idea to analyse ho# the

division of the !uery is performed in the various data%ases*

• +t is not true that #hen moving from a non.distri%uted data%ase to a distri%uted one #e

should not #orry* -ather the opposite is true" all mixed plans 'local?remote( must %e

revie#ed*

• Special care must %e ta&en #ith the data volume to %e ImovedI over the net#or&" as all

the data is fetched to the ImasterI server*

• ,his data volume is not the final volume the !uery returns" %ut the volume re!uired to

perform the 0oins* '8lthough the select returns a single record" it can ma&e thousands of

records move to ma&e the 0oins*(


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26/77

Unit

Module 1: +n the ata Model

-edundant indexes

8n index is considered to %e redundant and should %e eliminated #hen there is another index

that has the same columns on the left* ,he follo#ing indexes #ould %e redundant:

Ths nde& s redundant be$ause

t s $ontaned n the one on the

r.ht0

)nde& that should re!an

c1 c1" c2

c1" c2 c1" c2" c$


27/77

Unit


+ndexes and )oreign Reys

+f in the IparentI ta%le update or delete operations are to %e performed" an index must %e

defined in the IchildI ta%le %y the same fields of the foreign &ey" for t#o reasons:

1* Performance" improving" #hen performing update or delete in the IparentI" the search in the

child that assures that the update or delete can %e performed*

2* Ky %loc&s %et#een the IparentI and IchildI ta%les*

Operaton

perfor!ed on

the table

1thout nde& n F, felds

of the 2$hld2 table

1th nde& n F, felds of

the 2$hld2 table

2parent2 Chld 2table2 blo$3s Chld 2table2 blo$3s

#E(ETE

UP#ATE

Up to Ora$le 4:

,he entire ta%le" to avoid

inconsistent changes* 9o

insert" update or delete can

%e performed in the child

ta%le*

Fro! Ora$le +:

nly the records affected %y

the change of the parent ta%le

are %loc&ed" not allo#ing the

fields of the foreign &ey to %e

changed*

5ersons 67 47 4 and +:

,he affected records*


28/77

Unit


Use of composite indexes

8 composite index is an index that is formed %y various fields* +t is important to remem%er that

for the composite index to %e used at least the first field of the field of the index must appear in

the 4>E-E*

)or example" + have an index formed %y fields c1" c2 and c$*

)n the 1HERE ) as3

for:

Could ) use the

nde&8

c1" c2" c$

c1" c2

c1

c2

c$

c2" c$

c1" c$

Exceptions:

1* +ndex )ast )ull Scan: #hen there is an index that has all the fields specified in the

SELEC," so instead of performing a )ull ,a%le Scan" it performs a complete scan of the

index '+ndex )ast )ull Scan(*

2* 8n -E- KF is re!uested and the fields %elong to an index that" moreover" does not

allo# null*

$* )rom racle =i" there is the option of using the index %y means of an I+9EJ SR+P

SC89I" #hich in some cases can improve performance #ith respect to I)ULL ,8KLE

SC89I* +n any case" the use of I+9EJ SR+P SC89I is not considered to %e very

effective* +t is advisa%le to continue defining the right order in the fields of the index 9a

ne' feature of Ora$le +*


29/77

Unit


Effective indexes

+ndexes are important to ma&e data searches !uic&er* +t is important to al#ays create the most

effective indexes possi%le" in accordance #ith the follo#ing rules:

1* ,hey must %e created in accordance #ith the fields used in the 4>E-E*

2* ,hey #ill %e useful #hen a small set of data is searched for" #ithin the total volume of

the ta%le* '+f a lot of data is searched for" a I)ull ScanI of the ta%le may %e more

effective*(

$* +t is advisa%le not to a%use the num%er of indexes over ta%les* 'IExcess of indexesI*(

* +t must not %e redundant #ith respect to another existing index* 'I-edundant indexesI*(

* )or composite indexes" the fields #ith the greatest selectivity must %e attempted to %e

used as the first fields of the index* ,he selectivity of a field 'and therefore of the index(

is measured in accordance #ith the repetition or not of this field #ithin the ta%le* )or

example" in a ta%le of employees" the selectivity of the sex" to#n and + fields #ill %e as

follo#s:

Feld Sele$t%ty

+ Very

selective

to#n Less selective

sex 9ot very

selective

,a&e into account IUse of composite indexesI*


30/77

Unit


%solete data types

,he follo#ing data types are o%solete for racle 'updated for version =*7*2(:

Obsolete data

type

)t !ust be

repla$ed 'th

the data type

/o%e to

obsolete n

%erson

Support of the obsolete data type

L93 'char up

to 23%(

CLK 'char up

to 3%(6*7*

=*7*2* Supported %y compati%ility* +n

future versions it could disappear*

-84 '%inary upto 2777 %ytes(

KLK '%inaryup to 3%(

6*7* =*7*2* Supported %y compati%ility* +nfuture versions it could disappear*

L93 -84

'%inary up to

23%(

KLK '%inary

up to 3%(6*7*

=*7*2* Supported %y compati%ility* +n

future versions it could disappear*

)or data types that are still supported %y compati%ility" their migration to the ne# data types is

very important to assure the operation of applications in future versions of racle and to %e a%le

to use the improvements that the ne# data types provide*


31/77

Unit


My index is not used

Some of the reasons #hy my index is not used are covered %elo#*

1* oes the index exist5

2* 8re the statistics up to date5

$* 8t least the first header of the index must appear in the 4>E-E '-ule IM* Use of

composite indexesI(* Exceptions:

. +ndex )ast )ull Scan: #hen there is an index that has all the fields specified in the

SELEC," so instead of performing a )ull ,a%le Scan" it performs a complete scan of the

index '+ndex )ast )ull Scan(*

. 8n -E- KF is re!uested and the fields %elong to an index that" moreover" does not

allo# nulls*

. )rom racle =i" there is the option of using the index %y means of an I+9EJ SR+P

SC89I" #hich in some cases can improve performance #ith respect to I)ULL ,8KLE

SC89I* +n any case" the use of I+9EJ SR+P SC89I is not considered to %e veryeffective* +t is advisa%le to continue defining the right order in the fields of the index 'a

ne# feature of racle =i(*

* 8re the fields of the index used for the 0oin5 +f so" the use of the index depends on:

. ,he type of +9 that is performed* '9ES,E LP is the only one that ena%les the

use of an index*(

. ,he order in #hich the +9s are performed* Specifically" #ith 9ES,E LP"

depending on #hich ta%le is the incoming one and #hich is the outgoing" the index is

used or not*

* +s a function %eing applied over the indexed field5 '-ule I4* Comparers and indexesI(

8 function of the SUKS,- type 'field1"1"2( prevents the index from %eing used" unless

an index %ased on a function is used 'the index is created applying the SUKS,-

function(*

* +s an implicit conversion of types %eing performed5 '-ule I4* +mplicit conversionsI(

+mplicit conversions can disa%le the indexes* o not use implicit conversions*


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N* +t uses an index" %ut not the one + #ant*

,he use of one index compared to another is chosen %y the cost.%ased optimi/er: it

chooses the one #ith the lesser cost* +t is extremely difficult for t#o indexes to have the

same cost 'same si/e" same num%er of sheets" depth" %loc&s" etc*(* +f this happens" the

cost.%ased optimi/er chooses in alpha%etical order* ,here is al#ays the option of >ints

to ensure the use of the index that #e #ant* '-ule I>1* Considerations a%out >intsI(

6* My index is not good*

+t may %e that my index is not very good 'not very restrictive(* '-ule IM* Effective

indexesI(

+t may %e that the data is not uniformly distri%uted" and as there are no histograms" the

optimi/er dra#s the #rong conclusions* 'See Ivervie# of SQL. ,he ptimi/er .

>istogramsI*(

=* 8re you as&ing a%out null values5

9ull values are not stored in the index 'for composite indexes if all the values are null"

they are not stored in the index(* 8n index cannot %e used #hen as&ing a%out null

values '-ule I4* Comparers over indexed fields*I(

17* 8 remote ta%le is %eing used*

,he use of remote ta%les 'distri%uted !ueries( is complicated for the optimi/er* 4e #ill

pu%lish recommendations for distri%uted !ueries at a future date*

11* W+s Parallel Query %eing used5

Ma&e sure that Parallel Query is not %eing used and chec& #hether the ta%les have a

degree of parallelism activated* ,he use of parallel !uery causes a tendency to perform

I)ull ,a%le Scan in parallelI rather than access %y indexes*

12* W8re %ind varia%les %eing used5

+n some circumstance the use of %ind varia%les causes the non.use of indexes" as the

optimi/er does not &no# the value of the data used and performs fixed pre.calculations

that might %e erroneous*


33/77

Unit


Kloc&s in %itmap indexes

Kitmap indexes are a special type of racle index" #hich might %e more %eneficial than the

K@,rees" #hen the num%er of different values that the field can have is very small*

,hese indexes" for each one of the different values" #ill internally store a 1 or a 7 per record"

depending on #hether this record has this value or not* ,hey might occupy much more space"

%ut they might %e much !uic&er*

>o#ever" they have a s.nf$ant dsad%anta.e for environments in #hich modifications or

insertions are made concurrently %y various users*

8ny ML 'ata Manipulation Language( operation" such as insert" update or delete" #ill cause

an exclusive %loc& in IpartI of the index" #hich might impede modifications of that ta%le"

provided it affects the fields that form part of the %itmap index*

9ormally" they are used in a very %eneficial manner in decision.ma&ing environments 'data

#arehouse( and in a not very %eneficial manner in online transactional environments 'L,P(*


34/77

Unit

Module 1: +n the

4>E-E

Use of Kind Varia%les

,he execution of an SQL sentence re!uires some preliminary IcompilationI steps 'parse( that

analyse its syntax" chec& the existence of o%0ects" verify the permissions and decide its

execution plan*

Since version N of racle" a cache of SQL sentences has %een included" housed #ithin the

Shared Pool" #hich is responsi%le for storing all the latest SQL sentences that have %een

executed in the data%ase* ,he purpose of this cache is to improve response times #hen

executing an SQL sentence* +f this sentence is in the cache" the preliminary chec&ing and

execution plan steps can %e avoided" moving directly on to executing the SQL sentence" %ased

on the information stored in the cache*

Hard Parse

4hen a sentence is not in the Shared Pool cache" it must carry out a full or hard parse" #hich

means o%taining memory for the sentence" chec&ing the syntax" the o%0ects" the data types" and

o%taining its execution plan* ,his process is a complex process" re!uiring significant use of the

CPU*

Soft Parse

+f the sentence is already in the Shared Pool and is Isharea%leI" racle only needs to perform a

Isoft parseI" #hich is much less costly*

Shareable senten$es n the Shared Pool0

,#o sentences are considered to %e sharea%le in the Shared Pool" ena%ling !uic&er execution

using the information of the cache" #hen the follo#ing conditions are met:

. ,hey are #ritten exactly the same on the 8SC++ level 'including letters" %lan& spaces" upper

case letters" etc*(*

. ,he o%0ects referenced in %oth sentences are the same 'same user(*

. +f %ind varia%les are used" they are of the same type and si/e for %oth sentences*


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!uantity of L+RE conditions generated and the impossi%ility of optimi/ing searches in

accordance #ith fixed rules* ur advice:

. o not use these fixed sentences and ma&e them dynamically depending on the search

criteria*

. +n addition to %eing dynamic" try to divide the search on various screens grouping and limiting

the search conditions 'for example" if you search %y name" force the entry of the surname if you

search %y address" force the entry of the city or post code" etc*(" forcing the use of compulsory

fields in the search* ',hese compulsory fields #ill %e candidates for inclusion in the indexes of

the data%ase*(

. )inally" the least advisa%le recommendation: the use of pseudo.dynamic sentences* ,hese

sentences usually use a %inary field that serves as a flag to &no# if + am as&ing for a specific

field or not* 8n example:

,his sentence uses the %ind varia%les Ivsurname1I" Ivsurname2I and IvnameI correctly to

identify the search values %y surnames and name*

,he %ind varia%les Iflagsurname2I and IflagnameI are used to determine #hether #e are

searching or not %y second surname and %y name* +f these flags have a value of 7" it means

they do not search %y this field" and than&s to the - they impede the search in the rest of the

condition 'surname2" customer;name(* +f they have a value of 1" they search %y the condition

after the -*

,hese flags partially improve the performance of these sentences" provided that these flags are

not used #ith %ind varia%les* +f #e search %y name of the customer" for example" the sentence

should %e executed as follo#s:

- ;ueres by ran.es0 +n some !ueries that as& for ranges '" O" %et#een(" or #hen L+RE is

used" for example" the use of %ind varia%les can result in #orse plans* ,his is %ecause the

optimi/er does not &no# the exact value of the varia%le" and it cannot calculate ho# %ig the


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range to %e !ueried is* ,he optimi/er assumes fixed estimated rules and it can result in #orse

plans*

8s a general rule" it is advisa%le to use %ind varia%les due to their significant impact on the

performance of the data%ase* +f partitioning" histograms or pseudo.dynamic sentences areused" they should %e used in fields not affected %y the partitioning" histogram or flag"

respectively* +f in dou%t" you can consult the ata%ase 8dministration team*


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Unit

Module 1: +n the 4>E-E

Long +9L+S,s

Long +9L+S,s are considered to %e +9 clauses in #hich a long list of values is detailed* +t is

advisa%le to replace long +9L+S,s #ith search ta%les 'performing a 0oin #ith a ta%le that contains

values to %e searched for(*


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Unit


perators over indexed fields

,he use in the 4>E-E of operators over indexed fields 'operators to the left of the comparer(

may ma&e the use of indexes impossi%le* >ere are some examples of good and %ad practices:

Bad pra$t$e n 1HERE E-E amount A 777 < advance

payment

4>E-E amount < advance payment .

777

4>E-E ,-U9C'date( <

,-U9C'sysdate(

4>E-E date KE,4EE9

,-U9C'sysdate( 89 ,-U9C'sysdate(A

7*=====4>E-E account;name < 9VL'

:acc;name" account;name(

4>E-E account;name L+RE 9VL'

:acc;name"XHX(


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Unit


+mplicit conversions

4hen in a 4>E-E clause the data type of the column does not match the data type of the

value as&ed a%out" racle can not generate an error" and internally perform a conversion

'implicit conversions(*

,hese implicit conversions in many cases cause the non.use of the indexes*

Letting racle perform the implicit conversion is considered to %e a %ad practice* 4e must try to

ma&e the field types of the varia%les and the fields match" or expressly perform the conversion*

)or example" in the EP, ta%le there is an index #ith the field IdescriptionI 'varchar2(* ,his is#hat happens:

Condton #oes t use the nde&8

4>E-E description < X1X

4>E-E description <

,;C>8-'1(

4>E-E description < 1


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Unit


>aving and 4here

+t is important to correctly distinguish and use the >8V+93 and the 4>E-E*

WHERE is used to filter the records to be returned.

HAVING is applied after the WHERE, with grouping functions (GR!" #$% and is used to filter groups.

It is good to al+ays filter +ith the -"" the greatest number of records so that the grouping

functions can manage fe+er records, rather than not filtering +ith -"" and oerloading the

grouping.

Bad pra$t$e 'th HA5)*<


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Unit


-49UM

,he -49UM clause in a 4>E-E is used to return only a num%er of records of the total of

records that meet the rest of the 4>E-E condition*

,he -49UM clause can only %e used #ith the operators IOI and IO


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1* racle performs I,8KLE 8CCESS )ULLI on the entire VUELS ta%le" going through a

total of N"N11 records 'not limited %y -49UM(* ,hen %y it goes through the primary

&ey of the PL88S ta%le" called PL8;PR" %ut in this case it does limit the num%er of

-49UM records '=(* 8 >ash oin is performed on these t#o data %loc&s" returning a

total of = records*

2* 8 I,8KLE 8CCESS )ULLI is performed on the entire CMP89+8S ta%le" #ithout %eing

limited %y -49UM" finding a total of 6 records* ,hen it performs a >ash oin on the 6CMP89+8S records and the = records returned in step 1* ,he hash 0oin returns =

records*

$* ,he ICU9, S,PREFI is performed" #hich is really counting up to the = records that

must %e returned to respect the -49UMO17 that appears in the 4>E-E clause*

8ctually this CU9, in this execution does not filter" %ecause in the previous step only

the = desired records #ere returned*

8s o%served in this plan" the I)ULL ,8KLE SC89I operations have not %een limited %y the

-49UM" #hereas the operation I+9EJ ')8S, )ULL SC89(I has limited the analysed

records" and returning = records" has managed to return #hat #as re!uested in the -49UM*

Se$ond e&a!ple


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LetXs see the same sentence" %ut in this case #e have added a pair of >ints to achieve a much

#orse execution plan" %ut it does allo# us to sho# ho# the plan is not completely limited %y the

-49UM condition*

,his is its execution plan #ith its estimation of cost and of records:

8nd this is the actual execution 'of records(" o%tained #ith t&prof:

,his execution plan sho#s that to return the nine records re!uested %y -49UMO17" a total of $"$1$"=$ records have %een gone through 'in various ta%les(*


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Unit


8nti0oin

Ant-on O%er%e'

8n anti0oin is a !uery that returns records from a ta%le that do not correspond to records from

another ta%le* +t is in some #ay the opposite operation to a 0oin*

+n racle there are the follo#ing #ays of performing an 8nti.oin:

• 4ith 9, +9

• 4ith 9, EJ+S,S

• 4ith M+9US

• 4ith U,E-.+9

Use of *OT EX)STS %ersus *OT )*

Use 9, EJ+S,S rather than 9, +9 #hen there is an index in the 0oin field of the ta%le of the

su%select*

4e have the follo#ing sentence #ith 9, +9 #ith its corresponding execution plan:

racle first performs the su%select 'SELEC, deptno )-M emp(" so as it does not have any

4>E-E it goes through the entire emp ta%le* ,hen it goes through the dept ta%le" loo&ing for

the departments that do not have any employee associated*

>o#ever" reconstructing the sentence #ith 9, EJ+S,S o%tains the follo#ing result:


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racle no# performs a 9ES,E LP" so for each department 'EP,( it performs a search

KF +9EJ in the employees ta%le 'EMP(* So #e have avoided performing the )ULL SC89 of

emp" accessing no# %y the index of the 0oin field*

,a&e into account that #e assume that the volume of the EMP ta%le is %ig enough to ma&e it

#orth using an index rather than a full scan of the entire ta%le*


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Unit

Module 1: rderings?3roupings

vervie# of ordering

rderings are costly operations for the data%ase" and they may %e one of the ma0or reasons

#hy our sentences are not as !uic& as #e #ould li&e*

8s a general rule they must %e avoided #hen not essential*

,he operations that re!uire an ordering are:

• Creation of an index

• Use of the grouping clauses 3-UP KF" +S,+9C,

• Use of the clause -E- KF

• oin using S-,.ME-3E

• Use of the set operators U9+9" +9,E-SEC, and M+9US


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Unit


-E- KF

/se the O0" 2 clause only +hen it is important for the application to obtain ordered data *

-E- KF is fre!uently used #ithout ordering %eing necessary*


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Unit


U9+9 8LL versus U9+9

,he U9+9 set operators aggregate the results of various SELEC,s* U9+9 versus U9+9

8LL performs an additional ordering to eliminate repeated records*

/se /3IO3 4## ersus /3IO3 +hen +e +ant to receie repeated records or +e are sure that

+e +ill not receie any repeated records.


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Unit N

Module 1: +n the +9SE-,

Specify )ields in the +9SE-,

Performing +9SE-, indicating directly the V8LUES and presupposing the order of the fields in

the ta%le is considered to %e a very %ad practice* Changes in the data model 'eliminating

columns" or changes in order( can cause our programs to fail" or #orse" not to fail %ut inserting

values in the #rong columns*

+9SE-, +9, employees

V8LUES 'X3arcYaX"X8randaX" XscarX"X=N=12$X(

+9SE-, +9, employees 'surname1" surname2"

name" telephone1(

V8LUES 'X3arcYaX"X8randaX" XscarX"X=N=12$X(


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Unit 6

Module 1: >ints

3eneral considerations a%out the use of >ints

,he optimi/er does not al#ays o%tain the %est execution plans* +n some cases" &no#ledge of

the %usiness and of the information stored can help to choose the %est execution plans*

>ints are proposals made to the racle ptimi/er on ho# to perform execution plans* ,he

optimi/er considers them to %e proposals and in some cases they might not %e ta&en into

account %y the optimi/er*

,he use of >ints implies the follo#ing rs3s:

•

,heir use is proposed #ith a current volume" in #hich it is chec&ed that %enefits arecontri%uted* >o#ever" changes in volume due to the evolution of the system can cause

>ints entered at a given moment to %e not suita%le in a fe# months in accordance #ith

the volume changes*

• ,he evolution of racle Soft#are" modifying the hints or expanding functionality" can

cause ma0or changes in the optimi/er* +t may %e that in ne# versions of racle" certain

>ints #ill no longer %e used" so #e must revie# the sentences that use them*

+n cases in #hich hints are used t !ust ne%er be for.otten that:

• >ints must %e used #ith moderation" in cases #hen really essential" after discarding

other possi%ilities*

• ,hey force the maintenance of clear documentation on #here they are used" as

periodically the sentences that use them must %e revie#ed" to chec& #hether they are

still suita%le 'due to racle version changes or due to the evolution of the volume and

the data(*


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Unit 6

Module 1: >ints

4hy does my >int not #or&5

>ints are proposals made to the optimi/er and in some cases they might not %e ta&en into

account %y the optimi/er*

+n any case" if your >int does not #or&" do not forget to chec& that you are respecting the

follo#ing rules:

• ,hey must %e #ritten li&e this: =>? H)*T H)*T >=

• Using any >int 'except -ULES( means that the cost.%ased optimi/er must %e used*-emem%er that the statistics must %e executed and updated*

• ,he use of the >int -ULES 'rule.%ased optimi/er( is annulled if a functionality that

re!uires the cost.%ased optimi/er is used*

• >ints must not refer to the name of the schema* +t can %e solved %y using an alias in the

ta%le* ,he follo#ing example is incorrect:

SELEC, ?@A index'scott*emp emp1( @? ***

• +f an alias is used" the hint must use the alias and not the name of the ta%le* )or

example:

SELEC, ?@A )ULL ' myalias ( @? empno )-M emp myalias 4>E-E empno 17

• ust after the IAI of the hint there must %e a space" in PL?SQL %loc&s*

• o not use hints that do not ma&e sense in the sentence* )or example" using the hint

I)+-S,;-4SI in a sentence that has an -E- KF*

• )or the +9EJ hint:

o +ts format is: SELEC, ?@A index',8KLE;98ME +9EJ;98ME( @? col1***

o +t is compulsory to use the name of the ta%le 'the alias" if used(


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o ,he name of the index is optional* +f not specified" the optimi/er chooses the

index* +f specified" it must %e the correct name" as if not" the hint is invalidated*

• )or distri%uted !ueries" in remote ta%les" the only hints that #or& are 0oin order and 0oin

type*


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Unit 6

Module 1: >ints

>ints for the use of the optimi/er

,he hints related to the use of optimi/er are:

• 8LL;-4S

• )+-S,;-4S

• C>SE

• -ULE

A((@RO1S

+t specifies that the cost.%ased optimi/er is used" trying to achieve the %est throughput*

'Minimum consumption of resources" may%e penalising the response time*(

For!at:

F)RST@RO1S

+t specifies that the cost.%ased optimi/er is used" trying to achieve the %est response time"

perhaps penalising the consumption of resources*

,he optimi/er in )+-S,;-4S mode is more li&ely to use indexes" although it might mean an

increase in dis& accesses* '9ot al#ays advisa%le*(

For!at

CHOOSE

+t specifies that the cost.%ased optimi/er is used if a ta%le has statistics* +f not" the rule.%ased

optimi/er #ill %e used*

For!at


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RU(E

+t specifies that the rule.%ased optimi/er is used*

For!at


58/77

Unit 6

Module 1: >ints

>ints for the access method

,he hints related to the ta%le access method are:

• )ULL

• -4+

• CLUS,E-

• >8S>

• +9EJ

• +9EJ;8SC

• +9EJ;CMK+9E

• +9EJ;+9

• +9EJ;ESC

• +9EJ;))S

• 9;+9EJ

• 89;EQU8L

•

USE;C9C8,


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• 9;EJP89

• -E4-+,E

• 9-E4-+,E

FU((

+t specifies that an access is performed going through the entire ta%le ')ull ta%le scan(

For!at:

RO1)#

+t specifies that an access is performed %y means of the -o#+d

For!at

)*#EX

+t specifies that a certain index is used*

For!at


60/77

Unit 6

Module 1: >ints

>ints for the performance order of the +9

,he hints related to the performance order of the +9 are:

• -E-E

• S,8-


61/77

Unit 6

Module 1: >ints

>ints for the +9 operations

,he hints related to the +9 operations are:

• USE;9L

• USE;ME-3E

• USE;>8S>

• -+V+93;S+,E

• LE8+93

• >8S>;8

• ME-3E;8

• >8S>;S

• ME-3E;S


62/77

Unit 6

Module 1: >ints

>ints for parallel executions

,he hints related to parallel executions are:

• P8-8LLEL

• 9P8-8LLEL

• PQ;+S,-+KU,E

• 8PPE9

• 98PPE9

• P8-8LLEL;+9EJ

• 9P8-8LLEL;+9EJ


63/77

Unit 6

Module 1: >ints

ther hints

ther hints" #ith varying uses" are provided %elo#:

• C8C>E

• 9C8C>E

• ME-3E

• 9;ME-3E

• U99ES,

• 9;U99ES,

• PUS>;P-E

• 9;PUS>;P-E

• PUS>;SUKQ

• S,8-;,-89S)-M8,+9

• -E-E;P-E+C8,ES


64/77

Unit =

Module 1: Programming and SQL

Commits in loop

,he commit operation is a complex" %ut not necessarily slo#" operation for the data%ase* ,he

inclusion in a loop of the execution of a commit 'Iin each cycleI( is not suita%le as it overloads

the data%ase" #hich causes the performance to #orsen*

+t should only %e done #hen the %usiness itself needs to assure that in each cycle of the loop

the changes have %een made and are visi%le to the rest of the users* ,he use of commits after a

ma0or volume of changes is usual" %ut not for every cycle*

4>+LE 'iO num;elements( LP

***** SELEC, ***

****** +9SE-, *****

CO//)T

E9 LP

4>+LE 'iO num;elements( LP

***** SELEC, ***

****** +9SE-, *****

E9 LP

CO//)T


65/77

Unit =

Module 1: Programming and SQL

Commits and closing of cursors

+n some programming languages" such as Pro@C" performing a commit in our program can

mean a closure of the cursors that are open*

+t is advisa%le to not produce this closure of the cursors" %ecause the process of reopening the

cursors may %e costly and cause unnecessary delays*

>o# could it %e solved5

9ot performing a commit during the process and in the finalcommit the cursors are closed*

4hen the cursor processes many records" it consumes a lot of

the roll%ac& segment" #hich is not suita%le*

Parameterising our programming language so that the cursors

are not closed*

Parameterising our programming language:

Pro>C 'compilation parameters(

ME< oracle

CLSE;9;CMM+,


66/77

Unit 17

Module 1: PL?SQL

Use of ICursor )-I instead of CU-S- loop*

Up to version 6 of racle" the treatment of cursors consisted of an PE9 of the cursor" a loop

that processes the data and a CLSE that closes the cursor*

)rom version 6i of racle" there is another simpler and %etter performing #ay* +t is a special

version of the )- loop designed for cursors* ,he )- performs PE9" )E,C> and CLSE of

the cursors*


67/77


68/77

Unit 17

Module 1: PL?SQL

Unnecessary use of the U8L

Using U8L unnecessarily to o%tain function results causes un#anted accesses to the

data%ase* ,hese pro%lems are usually aggravated #hen this use is done #ithin loops*


69/77

Unit 17

Module 1: PL?SQL

+mproper use of SELEC, CU9,

Some programmers are used to performing a SELEC, CU9, to chec& #hether a cursor #ill

return data* +f data is returned" the cursor is opened #ith the data* ,his #ay of programming

forces the cursor to %e executed t#ice*

+t is prefera%le to open the cursor the first time and chec& #hether there is data in the first

)E,C>*


70/77

Unit 17

Module 1: PL?SQL

Explicit Cursors versus +mplicit Cursors

+mplicit cursors are those that are not expressly declared as CU-S- in the code* +nternally

racle is responsi%le for performing all the open" fetch and close steps* ,hey can %e used in

PL?SQL only #hen they return a single record*

Explicit cursors are those that #e declare expressly as CU-S-*

8 cursor that returns a single record could %e used in PL?SQL as explicit or implicit* )t s

ad%sable to use $ursors e&pl$tly as they have %etter performance although their syntax is a

little more complex* +mplicit cursors al#ays force the execution of t#o fetches* ,he first to fetchthe first record and the second to ma&e sure there are no more values 'as they should only

return one record(*

n the contrary" in the explicit ones + fetch the first record and close the cursor* + do not perform

t#o fetches '+ IassumeI that + am sure that the cursor does not return more than one record"

and + avoid a ne# fetch(*


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ther advantages of explicit cursors:

• ,hey can %e closed %y code" previously releasing resources" #ithout having to #ait until

the PL?SQL %loc& completely finishes*

• +f the cursor has Kind Varia%les" they can %e reopened" ta&ing on the ne# values* +t is

!uic&er to use racleXs internal structures created in the definition of the cursor*


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,he difference %et#een -89R and E9SE;-89R is that #ith E9SE;-89R IgapsI are not

allo#ed in the ran&ing values #hen the ran&ed values are repeated*


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* Using the cost.%ased optimi/er" #ould these sentences have the same execution plan5

SELEC, ?@A -E-E @? e*name" d*name " c*office;name )-M emp e" dept d" office o

4>E-E e*deptid < d*deptid 89 e*officeid < o*officeid SELEC, ?@A -E-E @?

e*name" d*name " c*office;name )-M office o" emp e" dept d 4>E-E e*deptid < d*deptid

89 e*officeid < o*officeid

Fes" as the order of the ta%les in the )-M does not affect the execution plan*

9o" as the plan al#ays varies #hen the order of the ta%les is changed in the )-M*

+t could vary as it is using the -E-E hint*

N* W4hich characteristic pertains to the cost.%ased optimi/er5

,he execution plan does not vary and is %ased on racleXs internal statistics" in accordance #ith the

data loaded in the ta%les*

+t is %ased on racleXs internal statistics" in accordance #ith the data loaded in the ta%les*

,he execution plan is exclusively %ased on the existing data model*


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sql good pratices

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