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LISTEN
Name
LISTEN—listenforanotification
Synopsis
LISTEN
channel
Description
LISTEN
registersthecurrentsessionasalisteneronthenotificationchannelnamed
channel
.Ifthe
currentsessionisalreadyregisteredasalistenerforthisnotificationchannel,nothingisdone.
Wheneverthecommand
NOTIFY
channel
isinvoked,eitherbythissessionoranotheroneconnected
tothesamedatabase,allthesessionscurrentlylisteningonthatnotificationchannelarenotified,and
eachwillinturnnotifyitsconnectedclientapplication.
Asessioncanbeunregisteredforagivennotificationchannelwiththe
UNLISTEN
command.Ases-
sion’slistenregistrationsareautomaticallyclearedwhenthesessionends.
ThemethodaclientapplicationmustusetodetectnotificationeventsdependsonwhichPostgreSQL
applicationprogramminginterfaceituses.Withthelibpqlibrary,theapplicationissues
LISTEN
as
an ordinarySQL command,and thenmust periodicallycallthe function
PQnotifies
to findout
whetheranynotificationeventshavebeenreceived.Otherinterfacessuchaslibpgtclprovidehigher-
levelmethodsforhandlingnotifyevents;indeed,withlibpgtcltheapplicationprogrammershouldnot
evenissue
LISTEN
or
UNLISTEN
directly.Seethedocumentationfortheinterfaceyouareusingfor
moredetails.
NOTIFYcontainsamoreextensivediscussionoftheuseof
LISTEN
and
NOTIFY
.
Parameters
channel
Nameofanotificationchannel(anyidentifier).
Notes
LISTEN
takeseffectattransactioncommit.If
LISTEN
or
UNLISTEN
isexecutedwithinatransaction
thatlaterrollsback,thesetofnotificationchannelsbeinglistenedtoisunchanged.
Atransactionthathasexecuted
LISTEN
cannotbepreparedfortwo-phasecommit.
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LISTEN
Examples
Configureandexecutealisten/notifysequencefrompsql:
LISTEN virtual;
NOTIFY virtual;
Asynchronous notification "virtual" received from server process with PID 8448.
Compatibility
Thereisno
LISTEN
statementintheSQLstandard.
See Also
NOTIFY,UNLISTEN
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LOAD
Name
LOAD—loadasharedlibraryfile
Synopsis
LOAD ’
filename
Description
ThiscommandloadsasharedlibraryfileintothePostgreSQLserver’saddressspace.Ifthefilehas
beenloaded already,thecommand does nothing. Shared library files that contain C functions are
automaticallyloadedwheneveroneoftheirfunctionsiscalled.Therefore,anexplicit
LOAD
isusually
onlyneededtoloadalibrarythatmodifiestheserver’sbehaviorthrough“hooks”ratherthanproviding
asetoffunctions.
ThefilenameisspecifiedinthesamewayasforsharedlibrarynamesinCREATEFUNCTION;in
particular,onecanrelyonasearchpathandautomaticadditionofthesystem’sstandardsharedlibrary
filenameextension.SeeSection35.9formoreinformationonthistopic.
Non-superuserscanonlyapply
LOAD
tolibraryfileslocatedin
$libdir/plugins/
—thespecified
filename
mustbeginwith exactly thatstring.(Itis the database administrator’s responsibility to
ensurethatonly“safe”librariesareinstalledthere.)
Compatibility
LOAD
isaPostgreSQLextension.
See Also
CREATEFUNCTION
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LOCK
Name
LOCK—lockatable
Synopsis
LOCK [ TABLE ] [ ONLY ]
name
[
*
] [, ...] [ IN
lockmode
MODE ] [ NOWAIT ]
where
lockmode
is one of:
ACCESS SHARE | ROW SHARE | ROW EXCLUSIVE | SHARE UPDATE EXCLUSIVE
| SHARE | SHARE ROW EXCLUSIVE | EXCLUSIVE | ACCESS EXCLUSIVE
Description
LOCK TABLE
obtainsatable-levellock,waitingifnecessaryforanyconflictinglockstobereleased.If
NOWAIT
isspecified,
LOCK TABLE
doesnotwaittoacquirethedesiredlock:ifitcannotbeacquired
immediately,the commandis aborted andan erroris emitted.Once obtained,the lockis held for
theremainderofthe currenttransaction.(Thereisno
UNLOCK TABLE
command;locks arealways
releasedattransactionend.)
When acquiring locks automatically forcommands that reference tables,PostgreSQL always uses
theleastrestrictivelockmodepossible.
LOCK TABLE
providesforcaseswhenyoumightneedmore
restrictive locking.Forexample,supposeanapplicationruns a transaction at theReadCommitted
isolationlevelandneeds toensurethatdatainatable remains stableforthedurationofthetrans-
action.Toachievethisyoucouldobtain
SHARE
lockmodeoverthetablebeforequerying.Thiswill
preventconcurrentdatachangesandensuresubsequentreadsofthetableseeastableviewofcom-
mitteddata,because
SHARE
lockmodeconflictswiththe
ROW EXCLUSIVE
lockacquiredbywriters,
andyour
LOCK TABLE
name
IN SHARE MODE
statementwillwaituntilanyconcurrentholdersof
ROW EXCLUSIVE
modelockscommitorrollback.Thus,onceyouobtainthelock,therearenoun-
committedwritesoutstanding;furthermorenonecanbeginuntilyoureleasethelock.
Toachieveasimilareffectwhenrunningatransactionatthe
REPEATABLE READ
or
SERIALIZABLE
isolationlevel,you have to execute the
LOCK TABLE
statement before executing any
SELECT
or
data modification statement. A
REPEATABLE READ
or
SERIALIZABLE
transaction’s view of data
willbefrozenwhenits first
SELECT
ordatamodificationstatementbegins.A
LOCK TABLE
laterin
thetransactionwillstillpreventconcurrentwrites—but it won’t ensurethat whatthe transaction
readscorrespondstothelatestcommittedvalues.
Ifatransactionofthis sortisgoingtochange thedata inthetable,thenitshoulduse
SHARE ROW
EXCLUSIVE
lockmodeinsteadof
SHARE
mode.Thisensuresthatonlyonetransactionofthistype
runsatatime.Withoutthis,adeadlockispossible:twotransactionsmightbothacquire
SHARE
mode,
andthenbe unabletoalsoacquire
ROW EXCLUSIVE
mode toactuallyperformtheirupdates.(Note
thata transaction’s own locks neverconflict,soa transactioncanacquire
ROW EXCLUSIVE
mode
whenitholds
SHARE
mode—butnotifanyoneelseholds
SHARE
mode.)Toavoiddeadlocks,make
surealltransactionsacquirelocksonthesameobjectsinthesameorder,andifmultiplelockmodes
areinvolvedfora single object,then transactions should always acquirethemostrestrictive mode
first.
MoreinformationaboutthelockmodesandlockingstrategiescanbefoundinSection13.3.
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LOCK
Parameters
name
Thename(optionallyschema-qualified)ofanexistingtabletolock.If
ONLY
isspecifiedbefore
thetablename,onlythattableislocked.If
ONLY
isnotspecified,thetableandallitsdescendant
tables(ifany)arelocked.Optionally,
*
canbespecifiedafterthetablenametoexplicitlyindicate
thatdescendanttablesareincluded.
Thecommand
LOCK TABLE a, b;
is equivalentto
LOCK TABLE a; LOCK TABLE b;
.The
tablesarelockedone-by-oneintheorderspecifiedinthe
LOCK TABLE
command.
lockmode
Thelockmodespecifieswhichlocksthislockconflictswith.LockmodesaredescribedinSec-
tion13.3.
Ifnolockmodeisspecified,then
ACCESS EXCLUSIVE
,themostrestrictivemode,isused.
NOWAIT
Specifiesthat
LOCK TABLE
shouldnotwaitforanyconflictinglockstobereleased:ifthespeci-
fiedlock(s)cannotbeacquiredimmediatelywithoutwaiting,thetransactionisaborted.
Notes
LOCK TABLE ... IN ACCESS SHARE MODE
requires
SELECT
privileges on the target table.All
otherformsof
LOCK
requiretable-level
UPDATE
,
DELETE
,or
TRUNCATE
privileges.
LOCK TABLE
isuselessoutsideatransactionblock:thelockwouldremainheldonlytothecompletion
ofthestatement.ThereforePostgreSQLreportsanerrorif
LOCK
isusedoutsideatransactionblock.
UseBEGINandCOMMIT(orROLLBACK)todefineatransactionblock.
LOCK TABLE
onlydealswith table-level locks,andsothemodenamesinvolving
ROW
are allmis-
nomers.Thesemodenamesshouldgenerallybereadasindicatingtheintentionoftheusertoacquire
row-levellockswithinthelockedtable.Also,
ROW EXCLUSIVE
modeisashareabletablelock.Keep
inmindthat all thelockmodes haveidentical semantics sofaras
LOCK TABLE
isconcerned,dif-
feringonlyintherules aboutwhichmodes conflictwithwhich.Forinformationonhowtoacquire
an actualrow-levellock,see Section13.3.2andtheThe LockingClause inthe
SELECT
reference
documentation.
Examples
Obtaina
SHARE
lockonaprimarykeytablewhengoingtoperforminsertsintoaforeignkeytable:
BEGIN WORK;
LOCK TABLE films IN SHARE MODE;
SELECT id FROM films
WHERE name = ’Star Wars: Episode I - The Phantom Menace’;
-- Do ROLLBACK if record was not returned
INSERT INTO films_user_comments VALUES
(_id_, ’GREAT! I was waiting for it for so long!’);
COMMIT WORK;
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LOCK
Takea
SHARE ROW EXCLUSIVE
lockonaprimarykeytablewhengoingtoperformadeleteopera-
tion:
BEGIN WORK;
LOCK TABLE films IN SHARE ROW EXCLUSIVE MODE;
DELETE FROM films_user_comments WHERE id IN
(SELECT id FROM films WHERE rating < 5);
DELETE FROM films WHERE rating < 5;
COMMIT WORK;
Compatibility
There is no
LOCK TABLE
intheSQL standard,whichinstead uses
SET TRANSACTION
to specify
concurrencylevelsontransactions.PostgreSQLsupportsthattoo;seeSETTRANSACTIONforde-
tails.
Exceptfor
ACCESS SHARE
,
ACCESS EXCLUSIVE
,and
SHARE UPDATE EXCLUSIVE
lockmodes,the
PostgreSQLlockmodesandthe
LOCK TABLE
syntaxarecompatiblewiththosepresentinOracle.
1543
MOVE
Name
MOVE—positionacursor
Synopsis
MOVE [
direction
[ FROM | IN ] ]
cursor_name
where
direction
can be empty or one of:
NEXT
PRIOR
FIRST
LAST
ABSOLUTE
count
RELATIVE
count
count
ALL
FORWARD
FORWARD
count
FORWARD ALL
BACKWARD
BACKWARD
count
BACKWARD ALL
Description
MOVE
repositionsacursorwithoutretrievinganydata.
MOVE
worksexactlylikethe
FETCH
command,
exceptitonlypositionsthecursoranddoesnotreturnrows.
Theparametersforthe
MOVE
commandareidenticaltothoseofthe
FETCH
command;refertoFETCH
fordetailsonsyntaxandusage.
Outputs
Onsuccessfulcompletion,a
MOVE
commandreturnsacommandtagoftheform
MOVE
count
The
count
is the number of rows that a
FETCH
command with the same parameters wouldhave
returned(possiblyzero).
Examples
BEGIN WORK;
DECLARE liahona CURSOR FOR SELECT
*
FROM films;
-- Skip the first 5 rows:
1544
MOVE
MOVE FORWARD 5 IN liahona;
MOVE 5
-- Fetch the 6th row from the cursor liahona:
FETCH 1 FROM liahona;
code
| title
| did | date_prod
|
kind
|
len
-------+--------+-----+------------+--------+-------
P_303 | 48 Hrs | 103 | 1982-10-22 | Action | 01:37
(1 row)
-- Close the cursor liahona and end the transaction:
CLOSE liahona;
COMMIT WORK;
Compatibility
Thereisno
MOVE
statementintheSQLstandard.
See Also
CLOSE,DECLARE,FETCH
1545
NOTIFY
Name
NOTIFY—generateanotification
Synopsis
NOTIFY
channel
[ ,
payload
]
Description
The
NOTIFY
commandsendsanotificationeventtogetherwithanoptional“payload”stringtoeach
clientapplicationthathas previouslyexecuted
LISTEN
channel
forthespecifiedchannelnamein
thecurrentdatabase.Notificationsarevisibletoallusers.
NOTIFY
providesasimpleinterprocesscommunicationmechanismforacollectionofprocessesac-
cessingthesamePostgreSQLdatabase.Apayloadstringcanbesentalongwiththenotification,and
higher-levelmechanismsforpassingstructureddatacanbebuiltbyusingtablesinthe databaseto
passadditionaldatafromnotifiertolistener(s).
Theinformationpassedtotheclientforanotificationeventincludesthenotificationchannelname,
thenotifyingsession’sserverprocessPID,andthepayloadstring,whichisanemptystringifithas
notbeenspecified.
Itisuptothedatabasedesignertodefinethechannelnamesthatwillbeusedinagivendatabaseand
whateach onemeans.Commonly,thechannelnameis thesameas thename ofsome tableinthe
database,andthenotifyeventessentiallymeans,“Ichangedthistable,takealookatittoseewhat’s
new”.Butnosuchassociationisenforcedbythe
NOTIFY
and
LISTEN
commands.Forexample,a
databasedesignercoulduseseveraldifferentchannelnamestosignaldifferentsortsofchangestoa
singletable.Alternatively,thepayloadstringcouldbeusedtodifferentiatevariouscases.
When
NOTIFY
isusedtosignaltheoccurrenceofchangestoaparticulartable,ausefulprogramming
techniqueistoputthe
NOTIFY
inarulethatistriggeredbytableupdates.Inthis way,notification
happensautomaticallywhenthetableischanged,andtheapplicationprogrammercannotaccidentally
forgettodoit.
NOTIFY
interacts withSQL transactions insome important ways.Firstly,if a
NOTIFY
is executed
insideatransaction,thenotifyeventsarenotdelivereduntilandunlessthetransactioniscommitted.
This is appropriate,sinceifthe transactionis aborted,allthecommandswithinit have had noef-
fect,including
NOTIFY
.Butitcanbedisconcertingifoneisexpectingthenotificationeventstobe
deliveredimmediately.Secondly,ifalisteningsessionreceivesanotificationsignalwhileitiswithin
atransaction,the notificationeventwillnot be deliveredtoits connectedclientuntiljustafterthe
transactionis completed(eithercommittedoraborted).Again,thereasoningis thatifanotification
weredeliveredwithinatransactionthatwaslateraborted,onewouldwantthenotificationtobeun-
done somehow—butthe servercannot“take back”a notificationonce ithas sent it totheclient.
Sonotificationeventsareonlydeliveredbetweentransactions.Theupshotofthisisthatapplications
using
NOTIFY
forreal-timesignalingshouldtrytokeeptheirtransactionsshort.
Ifthesamechannelnameissignaledmultiple times fromthesame transactionwithidenticalpay-
loadstrings,thedatabaseservercandecidetodeliverasinglenotificationonly.Ontheotherhand,
notificationswithdistinctpayloadstringswillalwaysbedeliveredasdistinctnotifications.Similarly,
1546
NOTIFY
notificationsfromdifferenttransactionswillnevergetfoldedintoonenotification.Exceptfordrop-
ping laterinstancesofduplicate notifications,
NOTIFY
guaranteesthatnotificationsfromthe same
transactiongetdeliveredintheordertheyweresent.Itisalsoguaranteedthatmessagesfromdiffer-
enttransactionsaredeliveredintheorderinwhichthetransactionscommitted.
Itiscommonforaclientthatexecutes
NOTIFY
tobelisteningonthesamenotificationchannelitself.
Inthatcaseitwillgetbackanotificationevent,justlikealltheotherlisteningsessions.Dependingon
theapplicationlogic,thiscouldresultinuselesswork,forexample,readingadatabasetabletofind
thesameupdatesthatthatsessionjustwroteout.Itispossibletoavoidsuchextraworkbynoticing
whetherthenotifyingsession’sserverprocessPID(suppliedinthenotificationeventmessage)isthe
sameas one’s ownsession’s PID(available from libpq).When theyarethesame,the notification
eventisone’sownworkbouncingback,andcanbeignored.
Parameters
channel
Nameofthenotificationchanneltobesignaled(anyidentifier).
payload
The“payload”stringtobecommunicatedalongwiththenotification.Thismustbespecifiedas
asimplestringliteral.Inthedefaultconfigurationitmustbeshorterthan8000bytes.(Ifbinary
dataorlargeamountsofinformationneedtobecommunicated,it’sbesttoputitinadatabase
tableandsendthekeyoftherecord.)
Notes
There isa queuethat holds notificationsthathavebeensentbutnot yet processed byalllistening
sessions.Ifthisqueuebecomesfull,transactionscalling
NOTIFY
willfailatcommit.Thequeueis
quitelarge(8GBinastandardinstallation)andshouldbesufficientlysizedforalmosteveryusecase.
However,nocleanupcantakeplaceifasessionexecutes
LISTEN
andthenentersatransactionfora
verylongtime.Oncethequeueishalffullyouwillseewarnings inthelogfilepointingyoutothe
sessionthatispreventingcleanup.Inthiscaseyoushouldmakesurethatthissessionendsitscurrent
transactionsothatcleanupcanproceed.
Atransactionthathasexecuted
NOTIFY
cannotbepreparedfortwo-phasecommit.
pg_notify
Tosendanotificationyoucanalsousethefunction
pg_notify
(
text
,
text
)
.Thefunctiontakesthe
channelnameasthefirstargumentandthepayloadasthesecond.Thefunctionismucheasiertouse
thanthe
NOTIFY
commandifyouneedtoworkwithnon-constantchannelnamesandpayloads.
Examples
Configureandexecutealisten/notifysequencefrompsql:
LISTEN virtual;
NOTIFY virtual;
Asynchronous notification "virtual" received from server process with PID 8448.
1547
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