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CREATE CAST

Name

CREATE CAST—deﬁneanewcast

Synopsis

CREATE CAST (

source_type

target_type

)

WITH FUNCTION

function_name

(

argument_type

[, ...])

[ AS ASSIGNMENT | AS IMPLICIT ]

CREATE CAST (

source_type

target_type

)

WITHOUT FUNCTION

[ AS ASSIGNMENT | AS IMPLICIT ]

CREATE CAST (

source_type

target_type

)

WITH INOUT

[ AS ASSIGNMENT | AS IMPLICIT ]

Description

CREATE CAST

deﬁnes anewcast.Acastspeciﬁes howtoperformaconversionbetweentwodata

types.Forexample,

SELECT CAST(42 AS float8);

convertstheintegerconstant42totype

float8

byinvokingapreviouslyspeciﬁedfunction,inthis

case

float8(int4)

.(Ifnosuitablecasthasbeendeﬁned,theconversionfails.)

Twotypescanbebinarycoercible,whichmeansthattheconversioncanbeperformed“forfree”with-

outinvokinganyfunction.Thisrequiresthatcorrespondingvaluesusethesameinternalrepresenta-

tion.Forinstance,thetypes

text

and

varchar

arebinarycoerciblebothways.Binarycoercibilityis

notnecessarilyasymmetricrelationship.Forexample,thecastfrom

xml

text

canbeperformed

forfreeinthepresentimplementation,butthereversedirectionrequires afunctionthatperforms at

leasta syntaxcheck.(Twotypesthat arebinarycoerciblebothways arealsoreferredto asbinary

compatible.)

YoucandeﬁneacastasanI/Oconversioncastbyusingthe

WITH INOUT

syntax.AnI/Oconversion

castisperformedbyinvokingtheoutputfunctionofthesourcedatatype,andpassingtheresulting

stringtothe input functionofthe targetdatatype.Inmanycommoncases,this featureavoids the

need to write a separate cast function for conversion. AnI/Oconversion cast acts the same as a

regularfunction-basedcast;onlytheimplementationisdifferent.

By default,a castcan be invoked onlyby anexplicit castrequest,that is an explicit

CAST(

typename

)

x::typename

construct.

Ifthecastismarked

AS ASSIGNMENT

thenitcanbeinvokedimplicitlywhenassigningavaluetoa

columnofthetargetdatatype.Forexample,supposingthat

foo.f1

isacolumnoftype

text

,then:

INSERT INTO foo (f1) VALUES (42);

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CREATECAST

willbeallowedifthecastfromtype

integer

totype

text

ismarked

AS ASSIGNMENT

,otherwise

not.(Wegenerallyusethetermassignmentcasttodescribethiskindofcast.)

Ifthecastismarked

AS IMPLICIT

thenitcanbeinvokedimplicitlyinanycontext,whetherassign-

mentorinternallyinanexpression.(Wegenerallyusethetermimplicitcasttodescribethiskindof

cast.)Forexample,considerthisquery:

SELECT 2 + 4.0;

Theparserinitiallymarkstheconstantsasbeingoftype

integer

and

numeric

respectively.Thereis

integer+numeric

operatorinthesystemcatalogs,butthereisa

numeric+numeric

operator.

Thequerywillthereforesucceedifacastfrom

integer

numeric

isavailableandismarked

IMPLICIT

—whichinfactitis.Theparserwillapplytheimplicitcastandresolvethequeryasifit

hadbeenwritten

SELECT CAST ( 2 AS numeric ) + 4.0;

Now, the catalogs also provide a cast from

numeric

integer

. If that cast were marked

IMPLICIT

—whichitisnot—thentheparserwouldbefacedwithchoosingbetweentheaboveinter-

pretationandthealternativeofcastingthe

numeric

constantto

integer

andapplyingthe

integer

+integer

operator.Lackinganyknowledgeofwhichchoicetoprefer,itwouldgiveupanddeclare

thequeryambiguous.Thefactthatonlyoneofthetwocastsisimplicitisthewayinwhichweteach

theparsertopreferresolutionofamixed

numeric

-and-

integer

expressionas

numeric

;thereisno

built-inknowledgeaboutthat.

Itis wise tobeconservativeaboutmarkingcasts asimplicit.Anoverabundance ofimplicitcasting

paths cancause PostgreSQL tochoose surprising interpretations ofcommands, ortobe unable to

resolvecommandsatallbecausetherearemultiplepossibleinterpretations.Agoodruleofthumbis

tomakeacastimplicitlyinvokableonlyforinformation-preservingtransformationsbetweentypesin

thesamegeneraltypecategory.Forexample,thecastfrom

int2

int4

canreasonablybeimplicit,

butthecastfrom

float8

int4

shouldprobablybeassignment-only.Cross-type-categorycasts,

suchas

text

int4

,arebestmadeexplicit-only.

Note:Sometimesitisnecessaryforusabilityorstandards-compliancereasonstoprovidemultiple

implicitcastsamongasetoftypes,resultinginambiguitythatcannotbeavoidedasabove.The

parser has afallback heuristicbased on typecategories andpreferredtypes thatcan helpto

providedesiredbehaviorinsuchcases.SeeCREATETYPEformoreinformation.

Tobeabletocreateacast,youmustownthesourceorthetargetdatatypeandhave

USAGE

privilege

ontheothertype.Tocreateabinary-coerciblecast,youmustbesuperuser.(Thisrestrictionismade

becauseanerroneousbinary-coerciblecastconversioncaneasilycrashtheserver.)

Parameters

source_type

Thenameofthesourcedatatypeofthecast.

target_type

Thenameofthetargetdatatypeofthecast.

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CREATECAST

function_name

(

argument_type

[,...])

Thefunctionusedtoperformthecast.Thefunctionnamecanbeschema-qualiﬁed.Ifitisnot,

thefunctionwillbelookedupintheschemasearchpath.Thefunction’sresultdatatypemust

matchthetargettypeofthecast.Itsargumentsarediscussedbelow.

WITHOUT FUNCTION

Indicatesthatthesourcetypeisbinary-coercibletothetargettype,sonofunctionisrequiredto

performthecast.

WITH INOUT

Indicatesthatthecastis anI/Oconversioncast,performedbyinvokingtheoutputfunctionof

thesourcedatatype,andpassingtheresultingstringtotheinputfunctionofthetargetdatatype.

AS ASSIGNMENT

Indicatesthatthecastcanbeinvokedimplicitlyinassignmentcontexts.

AS IMPLICIT

Indicatesthatthecastcanbeinvokedimplicitlyinanycontext.

Cast implementationfunctions can have one to three arguments.Theﬁrst argument type must be

identicaltoorbinary-coerciblefromthecast’ssourcetype.Thesecondargument,ifpresent,mustbe

type

integer

;itreceivesthetypemodiﬁerassociatedwiththedestinationtype,or

-1

ifthereisnone.

Thethirdargument,ifpresent,mustbetype

boolean

;itreceives

true

ifthecastisanexplicitcast,

false

otherwise.(Bizarrely,theSQLstandarddemandsdifferentbehaviorsforexplicitandimplicit

castsinsomecases.Thisargumentissuppliedforfunctionsthatmustimplementsuchcasts.Itisnot

recommendedthatyoudesignyourowndatatypessothatthismatters.)

Thereturntypeofacastfunctionmustbeidenticaltoorbinary-coercibletothecast’stargettype.

Ordinarilyacastmusthavedifferentsourceandtargetdatatypes.However,itisallowedtodeclare

acastwithidenticalsourceandtargettypes ifithas acastimplementationfunctionwithmorethan

oneargument.Thisisusedtorepresenttype-speciﬁclengthcoercionfunctionsinthesystemcatalogs.

Thenamedfunctionisusedtocoerceavalueofthetypetothetypemodiﬁervaluegivenbyitssecond

argument.

Whenacasthasdifferentsourceandtargettypesandafunctionthattakesmorethanoneargument,

itsupportsconvertingfromonetypetoanotherandapplyingalengthcoercioninasinglestep.When

nosuchentryisavailable,coerciontoatypethatusesatypemodiﬁerinvolvestwocaststeps,oneto

convertbetweendatatypesandasecondtoapplythemodiﬁer.

Acasttoorfromadomaintypecurrentlyhasnoeffect.Castingtoorfromadomainusesthecasts

associatedwithitsunderlyingtype.

Notes

UseDROPCASTtoremoveuser-deﬁnedcasts.

Rememberthatifyouwanttobeabletoconverttypesbothwaysyouneedtodeclarecastsbothways

explicitly.

Itisnormallynotnecessarytocreatecastsbetweenuser-deﬁnedtypesandthestandardstringtypes

(

text

varchar

,and

char(

)

,as wellas user-deﬁned types that are deﬁnedto be in the string

category).PostgreSQLprovidesautomaticI/Oconversioncastsforthat.Theautomaticcaststostring

types aretreatedasassignmentcasts,while theautomaticcastsfromstringtypesareexplicit-only.

Youcanoverridethisbehaviorbydeclaringyourowncasttoreplaceanautomaticcast,butusually

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CREATECAST

theonlyreasontodosoisifyouwanttheconversiontobemoreeasilyinvokablethanthestandard

assignment-onlyorexplicit-onlysetting.Anotherpossiblereasonisthatyouwanttheconversionto

behavedifferentlyfromthetype’sI/Ofunction;butthatissufﬁcientlysurprisingthatyoushouldthink

twiceaboutwhetherit’sagoodidea.(Asmallnumberofthebuilt-intypesdoindeedhavedifferent

behaviorsforconversions,mostlybecauseofrequirementsoftheSQLstandard.)

While notrequired,it is recommended thatyou continueto follow this oldconventionofnaming

cast implementationfunctionsafterthe targetdatatype.Manyusers areusedtobeingable tocast

datatypesusingafunction-stylenotation,thatis

typename

(

).Thisnotationisinfactnothingmore

norlessthanacallofthecastimplementationfunction;itisnotspeciallytreatedasacast.Ifyour

conversion functions are notnamedtosupport this conventionthenyou willhavesurprisedusers.

SincePostgreSQLallowsoverloadingofthesamefunctionnamewithdifferentargumenttypes,there

is nodifﬁcultyinhavingmultiple conversionfunctionsfromdifferenttypes that all use the target

type’sname.

Note:Actuallytheprecedingparagraphisanoversimpliﬁcation:therearetwocasesinwhicha

function-callconstructwillbe treatedas acastrequestwithouthavingmatchedittoanactual

function.Ifafunctioncall

name

(

)doesnotexactlymatchany existingfunction,but

name

isthe

name of adata typeand

pg_cast

provides a binary-coerciblecastto this type from thetype

,thenthecallwillbeconstruedasabinary-coerciblecast.This exceptionis madesothat

binary-coerciblecastscanbeinvokedusingfunctionalsyntax,eventhoughtheylackanyfunction.

Likewise,ifthereisno

pg_cast

entrybutthecastwouldbetoorfromastringtype,thecallwill

beconstruedasanI/Oconversioncast.ThisexceptionallowsI/Oconversioncaststobeinvoked

usingfunctionalsyntax.

Note:Thereisalso anexceptiontotheexception:I/Oconversion casts fromcomposite types

to stringtypes cannotbeinvoked usingfunctional syntax, butmust be written in explicitcast

syntax(either

CAST

notation).Thisexceptionwasaddedbecauseaftertheintroductionof

automatically-providedI/Oconversioncasts,itwasfoundtooeasytoaccidentallyinvokesucha

castwhenafunctionorcolumnreferencewasintended.

Examples

Tocreateanassignmentcastfromtype

bigint

totype

int4

usingthefunction

int4(bigint)

CREATE CAST (bigint AS int4) WITH FUNCTION int4(bigint) AS ASSIGNMENT;

(Thiscastisalreadypredeﬁnedinthesystem.)

Compatibility

The

CREATE CAST

commandconformstotheSQLstandard,exceptthatSQLdoesnotmakeprovi-

sionsforbinary-coercibletypesorextraargumentstoimplementationfunctions.

AS IMPLICIT

isa

PostgreSQLextension,too.

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CREATECAST

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