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Commit43a899f

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Fix corner-case loss of precision in numeric ln().
When deciding on the local rscale to use for the Taylor seriesexpansion, ln_var() neglected to account for the fact that the resultis subsequently multiplied by a factor of 2^(nsqrt+1), where nsqrt isthe number of square root operations performed in the range reductionstep, which can be as high as 22 for very large inputs. This couldresult in a loss of precision, particularly when combined with largerscale values, for which a large number of Taylor series terms isrequired (up to around 400).Fix by computing a few extra digits in the Taylor series, based on theweight of the multiplicative factor log10(2^(nsqrt+1)). It remains tobe proven whether or not the other 8 extra digits used for the Taylorseries is appropriate, but this at least deals with the obviousoversight of failing to account for the effects of the finalmultiplication.Per report from Justin AnyhowStep. Reviewed by Tom Lane.Discussion:https://postgr.es/m/16280-279f299d9c06e56f@postgresql.org
1 parentdba9153 commit43a899f

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3 files changed

+42
-1
lines changed

3 files changed

+42
-1
lines changed

‎src/backend/utils/adt/numeric.c‎

Lines changed: 9 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -8500,6 +8500,7 @@ ln_var(const NumericVar *arg, NumericVar *result, int rscale)
85008500
NumericVarni;
85018501
NumericVarelem;
85028502
NumericVarfact;
8503+
intnsqrt;
85038504
intlocal_rscale;
85048505
intcmp;
85058506

@@ -8530,19 +8531,22 @@ ln_var(const NumericVar *arg, NumericVar *result, int rscale)
85308531
* rscale as we work so that we keep this many significant digits at each
85318532
* step (plus a few more for good measure).
85328533
*/
8534+
nsqrt=0;
85338535
while (cmp_var(&x,&const_zero_point_nine) <=0)
85348536
{
85358537
local_rscale=rscale-x.weight*DEC_DIGITS /2+8;
85368538
local_rscale=Max(local_rscale,NUMERIC_MIN_DISPLAY_SCALE);
85378539
sqrt_var(&x,&x,local_rscale);
85388540
mul_var(&fact,&const_two,&fact,0);
8541+
nsqrt++;
85398542
}
85408543
while (cmp_var(&x,&const_one_point_one) >=0)
85418544
{
85428545
local_rscale=rscale-x.weight*DEC_DIGITS /2+8;
85438546
local_rscale=Max(local_rscale,NUMERIC_MIN_DISPLAY_SCALE);
85448547
sqrt_var(&x,&x,local_rscale);
85458548
mul_var(&fact,&const_two,&fact,0);
8549+
nsqrt++;
85468550
}
85478551

85488552
/*
@@ -8555,8 +8559,12 @@ ln_var(const NumericVar *arg, NumericVar *result, int rscale)
85558559
*
85568560
* The convergence of this is not as fast as one would like, but is
85578561
* tolerable given that z is small.
8562+
*
8563+
* The Taylor series result will be multiplied by 2^(nsqrt+1), which has a
8564+
* decimal weight of (nsqrt+1) * log10(2), so work with this many extra
8565+
* digits of precision (plus a few more for good measure).
85588566
*/
8559-
local_rscale=rscale+8;
8567+
local_rscale=rscale+(int) ((nsqrt+1)*0.301029995663981)+8;
85608568

85618569
sub_var(&x,&const_one,result);
85628570
add_var(&x,&const_one,&elem);

‎src/test/regress/expected/numeric_big.out‎

Lines changed: 19 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -1612,6 +1612,25 @@ SELECT x, bc_result, ln(x::numeric), ln(x::numeric)-bc_result AS diff FROM t;
16121612
1.0e1000 | 2302.5850929940457 | 2302.5850929940457 | 0.0000000000000
16131613
(10 rows)
16141614

1615+
-- inputs with 1000 decimal places
1616+
--
1617+
-- bc(1) results computed with a scale of 2000 and rounded to 1000 decimal
1618+
-- places
1619+
WITH t(x, bc_result) AS (VALUES
1620+
(484990182159328900690402236933516249572671683638747490717351807610531884491845416923860371219625151551889257298200816555016472471293780254009492949585031653913930735918829139712249577547959394351523545901788627247613322896296041868431769047433229466634098452564756860190085118463828382895145244362033728480588969626012192733802377468089120757046364393407262957242230928854711898925295251902007136232994524624903257456111389508582206404271734668422903183500589303866613158037169610592539145461637447957948521714058034772237111009429638870236361143304703683377693378577075353794118557951847394763531830696578809001981568860219578880229402696449243344235099860421846016326538272155937175661905904288335499593232232926636205909086901191153907183842087577811871344870731324067822883041265129394268082883745408414994.8967939438561591657171240282983703914075472645212002662497023142663831371447287624846942598424990784971781730103682951722370983277124599054059027055336437808366784501932987082321905202623642371063626378290734289114618092750984153422293450048717769065428713836637664433167768445609659527458911187829232316677137895259433038764404970599325009178297626038331436654541552998098529141205301472138026818453893127265938030066392881979113522757891639646670670272542401773230506961559808927249585675430838495658225557294666522469887436551840596777627408780618586500922973500018513068499587683746133637919751545157547095670767246977244726331271787622126889459658539988980096764323712767863722912919120929339399753431689512753214200090670880647731689804555417871258907716687575767185444541243606329768784843125926070743277339790277626515824924290352180761378846035233155198504033292692893297993698953705472933411199778880561376633444249703838589180474329586470353212010427945060694794274109764269805332803290229,
1621+
1864.3702986939570026328504202935192533137907736189919154633800554877738455118081651650863235106905871352085850240570561347180517240105510505203972860921397909573687877993477806728098306202020229409548306695695574102950949468160529713610952021974630774784174851619325758380143625473386495586347322798415543385655090746985183329114860118551572428921774322172798724455202876781611633419444058398798142214904998877857425038669920064728855823072107227506485770367799671977282350083029452784747350395161797215115525867416898416360638482342253129160308632504217096916335590470843180746834864303790913372081974355613359678634194879425862536147988835528973291020680020540866655622823550861337486588647231688134992810403147262346312159819432914207194632564009749236609081399504118359354620598232725290537215007867979331582119891661859015726276335168158288396939655310210558566592649049602925182137256134162660116182293851038854455437841571331011002023088829768308520393956515509475418031437505751407687618234418262),
1622+
(87190145885430429849953615409019208993240447426362428988181639909267773304254748257120061524000254226856815085523676417146197197996896030672521334101413071112068202429835905642444187493717977611730127126387257253646790849384975208460867137315507010888782632024640844766297185244443116696943912406389670302370461137850160539373600494054874979342373255280815156048999900951842673141766630630919020492255966628630634124452614590400422133958133100159154995520080124736657520969784129924799670552560034302960877087853678350801769339861812435411200669026902417951572668727488315537985378304242438181615160041688723201917323705450185975141141262578884689500612295576288125956289035673242989906973367691922065122033180281670221390667818909912035903387888639331486823729897326624516015340.0330856710565117793999512551468220085711713631167607285185762046751452975325645379302403715842570486302993296501788672462090620871511446272026693318239212657949496275318383141403236705902077406660768573015707706831878445598837931116223956945944726162551477136715847593742032488181481888084716920605114101902724395659898621880016853548602514706686907951229872573180602614761229992106144727082722940736406782659562775289407005631298246624198606031298081220736931229256511054595028182057216042683060059115371651410352645266000330509331097811566633211452233019461903115970558624057877018778178814946285827512359903934291318219271464841957435711594154280905473802599888081783098187210283997106131616471807951265003903143099667366508222327805543948921694362089860577380749774036318574113007382111997454202845559941557812813566442364810680529092880773126707073967537693927177460459341763934709686530005721141046645111784404932103241501569571235364365556796422998363930810983452790309019295181282099408260156,
1623+
1793.5767085750017553306932533574391150814202249805881581227430032600579405884415934520704053351781361105595296647510475380766428668443641914861849764330704062323054023252886955844207807229267936432730818329225450152491146839618683772020068682795388746108876393249306737841247788224204701299467519965182171772253974884845661168860422489046657965359832930382114760565628765599962013955588754803194908990025689040598990346417563277021386852342928910383706995866844541160576254266641602065102228267316550706943783591722246885978355472097314691737807509436806788803362444745551013400341861820755594413819894154786253014501454443272120342005711761286524843010157182464200556865694401941794983935172457481497909987740544409272349152397774548604845897687504977786762391359552407068124283290504752932824699865504970420939586707791994870941813718246825616335675307740641350673558328821461530563823677144691877374809441673507467507447891562257806191361453045937798278733402269265623588493124129181374135958668436774),
1624+
(93936642222690597390233191619858485419795942047468396309991947772747208870873993801669373075421461116465960407843923269693395211616591453397070258466704654943689268224479477016161636938138334729982904232438440955361656138189836032891825113139184685132178764873033678116450665758561650355252211196676137179184043639278410827092182700922151290703747496962700158844772453483316974221113826173404445159281421213715669245417896170368554410830320000019029956317336703559699859949692222685614036912057150632902650913831404804982509990655560731349634628713944739168096272097122388116038119844786988276635032016787352796502360718569977397214936366251320294621522016.6483354941025384161536675750898007896744690911429670830432784905421638721478353275821072200938900938046264210604940707974410950770029535636602548377806284157951164875821446035013896786653932045182167021839184824627082391478016195098055107001433336586881395912782883663046617432598969149948351689103230162742769845955320418573803127107923535948653168889411316007796459064267436246637115946581149511513369842911210359447262641996566147462977170742544980481275049898092152042927981394239266559286915303786701737610786594006685748456635797125029722684151298695274097006242412384086302106763844070230264910503179385988626477852818174114043927841085089058972074427820150462261941575665882880501074676800316585217150509780489224388148722603385921057007086785238310735038314861960410473809826927329368597558806004392175746233568789445929554890241140656324160187253042639339549705859147930476532359840809944163908006480881926041259363654863689570520534301207043189181147254153307163555433328278834311658232337,
1625+
1510.4332713542154696529645934345554302578243896764921637693542962119938599884313210100957753316832762996428481801312323020427109678979117469716796746760060470871840325255146954580681101106876674367471955788143763250819168311353856748872452260808797135108102729064040463343792765872545182299889360257515315869180266759715933989413256377582681707188367254513700731642913479683031478361835565783219287780434673712341147656477670848734998849030451414278832848680301511646182446524915091598080243532068451726548537866633622180283865668708517173065893429240665300584705585310049892047293928733753369421499719516009692095913169665213597158441636480707309244604139865130782756488091268094213446272360006907802989573582755585110277620911226015342778471352130366770729972784317323917141031824334355639769512749560550167491709646539950725523461943580211843652293561678342656010571108219244870234329176123205423872844099992204896411752620881541000940129833754169391528449211839693800724450201835161044717173715867437))
1626+
SELECT trim_scale(ln(x::numeric)-bc_result) AS diff FROM t;
1627+
diff
1628+
------
1629+
0
1630+
0
1631+
0
1632+
(3 rows)
1633+
16151634
--
16161635
-- Tests for LOG() (base 10)
16171636
--

‎src/test/regress/sql/numeric_big.sql‎

Lines changed: 14 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -1193,6 +1193,20 @@ WITH t(x, bc_result) AS (VALUES
11931193
('1.0e1000',2302.5850929940457))
11941194
SELECT x, bc_result, ln(x::numeric), ln(x::numeric)-bc_resultAS diffFROM t;
11951195

1196+
-- inputs with 1000 decimal places
1197+
--
1198+
-- bc(1) results computed with a scale of 2000 and rounded to 1000 decimal
1199+
-- places
1200+
1201+
WITH t(x, bc_result)AS (VALUES
1202+
(484990182159328900690402236933516249572671683638747490717351807610531884491845416923860371219625151551889257298200816555016472471293780254009492949585031653913930735918829139712249577547959394351523545901788627247613322896296041868431769047433229466634098452564756860190085118463828382895145244362033728480588969626012192733802377468089120757046364393407262957242230928854711898925295251902007136232994524624903257456111389508582206404271734668422903183500589303866613158037169610592539145461637447957948521714058034772237111009429638870236361143304703683377693378577075353794118557951847394763531830696578809001981568860219578880229402696449243344235099860421846016326538272155937175661905904288335499593232232926636205909086901191153907183842087577811871344870731324067822883041265129394268082883745408414994.8967939438561591657171240282983703914075472645212002662497023142663831371447287624846942598424990784971781730103682951722370983277124599054059027055336437808366784501932987082321905202623642371063626378290734289114618092750984153422293450048717769065428713836637664433167768445609659527458911187829232316677137895259433038764404970599325009178297626038331436654541552998098529141205301472138026818453893127265938030066392881979113522757891639646670670272542401773230506961559808927249585675430838495658225557294666522469887436551840596777627408780618586500922973500018513068499587683746133637919751545157547095670767246977244726331271787622126889459658539988980096764323712767863722912919120929339399753431689512753214200090670880647731689804555417871258907716687575767185444541243606329768784843125926070743277339790277626515824924290352180761378846035233155198504033292692893297993698953705472933411199778880561376633444249703838589180474329586470353212010427945060694794274109764269805332803290229,
1203+
1864.3702986939570026328504202935192533137907736189919154633800554877738455118081651650863235106905871352085850240570561347180517240105510505203972860921397909573687877993477806728098306202020229409548306695695574102950949468160529713610952021974630774784174851619325758380143625473386495586347322798415543385655090746985183329114860118551572428921774322172798724455202876781611633419444058398798142214904998877857425038669920064728855823072107227506485770367799671977282350083029452784747350395161797215115525867416898416360638482342253129160308632504217096916335590470843180746834864303790913372081974355613359678634194879425862536147988835528973291020680020540866655622823550861337486588647231688134992810403147262346312159819432914207194632564009749236609081399504118359354620598232725290537215007867979331582119891661859015726276335168158288396939655310210558566592649049602925182137256134162660116182293851038854455437841571331011002023088829768308520393956515509475418031437505751407687618234418262),
1204+
(87190145885430429849953615409019208993240447426362428988181639909267773304254748257120061524000254226856815085523676417146197197996896030672521334101413071112068202429835905642444187493717977611730127126387257253646790849384975208460867137315507010888782632024640844766297185244443116696943912406389670302370461137850160539373600494054874979342373255280815156048999900951842673141766630630919020492255966628630634124452614590400422133958133100159154995520080124736657520969784129924799670552560034302960877087853678350801769339861812435411200669026902417951572668727488315537985378304242438181615160041688723201917323705450185975141141262578884689500612295576288125956289035673242989906973367691922065122033180281670221390667818909912035903387888639331486823729897326624516015340.0330856710565117793999512551468220085711713631167607285185762046751452975325645379302403715842570486302993296501788672462090620871511446272026693318239212657949496275318383141403236705902077406660768573015707706831878445598837931116223956945944726162551477136715847593742032488181481888084716920605114101902724395659898621880016853548602514706686907951229872573180602614761229992106144727082722940736406782659562775289407005631298246624198606031298081220736931229256511054595028182057216042683060059115371651410352645266000330509331097811566633211452233019461903115970558624057877018778178814946285827512359903934291318219271464841957435711594154280905473802599888081783098187210283997106131616471807951265003903143099667366508222327805543948921694362089860577380749774036318574113007382111997454202845559941557812813566442364810680529092880773126707073967537693927177460459341763934709686530005721141046645111784404932103241501569571235364365556796422998363930810983452790309019295181282099408260156,
1205+
1793.5767085750017553306932533574391150814202249805881581227430032600579405884415934520704053351781361105595296647510475380766428668443641914861849764330704062323054023252886955844207807229267936432730818329225450152491146839618683772020068682795388746108876393249306737841247788224204701299467519965182171772253974884845661168860422489046657965359832930382114760565628765599962013955588754803194908990025689040598990346417563277021386852342928910383706995866844541160576254266641602065102228267316550706943783591722246885978355472097314691737807509436806788803362444745551013400341861820755594413819894154786253014501454443272120342005711761286524843010157182464200556865694401941794983935172457481497909987740544409272349152397774548604845897687504977786762391359552407068124283290504752932824699865504970420939586707791994870941813718246825616335675307740641350673558328821461530563823677144691877374809441673507467507447891562257806191361453045937798278733402269265623588493124129181374135958668436774),
1206+
(93936642222690597390233191619858485419795942047468396309991947772747208870873993801669373075421461116465960407843923269693395211616591453397070258466704654943689268224479477016161636938138334729982904232438440955361656138189836032891825113139184685132178764873033678116450665758561650355252211196676137179184043639278410827092182700922151290703747496962700158844772453483316974221113826173404445159281421213715669245417896170368554410830320000019029956317336703559699859949692222685614036912057150632902650913831404804982509990655560731349634628713944739168096272097122388116038119844786988276635032016787352796502360718569977397214936366251320294621522016.6483354941025384161536675750898007896744690911429670830432784905421638721478353275821072200938900938046264210604940707974410950770029535636602548377806284157951164875821446035013896786653932045182167021839184824627082391478016195098055107001433336586881395912782883663046617432598969149948351689103230162742769845955320418573803127107923535948653168889411316007796459064267436246637115946581149511513369842911210359447262641996566147462977170742544980481275049898092152042927981394239266559286915303786701737610786594006685748456635797125029722684151298695274097006242412384086302106763844070230264910503179385988626477852818174114043927841085089058972074427820150462261941575665882880501074676800316585217150509780489224388148722603385921057007086785238310735038314861960410473809826927329368597558806004392175746233568789445929554890241140656324160187253042639339549705859147930476532359840809944163908006480881926041259363654863689570520534301207043189181147254153307163555433328278834311658232337,
1207+
1510.4332713542154696529645934345554302578243896764921637693542962119938599884313210100957753316832762996428481801312323020427109678979117469716796746760060470871840325255146954580681101106876674367471955788143763250819168311353856748872452260808797135108102729064040463343792765872545182299889360257515315869180266759715933989413256377582681707188367254513700731642913479683031478361835565783219287780434673712341147656477670848734998849030451414278832848680301511646182446524915091598080243532068451726548537866633622180283865668708517173065893429240665300584705585310049892047293928733753369421499719516009692095913169665213597158441636480707309244604139865130782756488091268094213446272360006907802989573582755585110277620911226015342778471352130366770729972784317323917141031824334355639769512749560550167491709646539950725523461943580211843652293561678342656010571108219244870234329176123205423872844099992204896411752620881541000940129833754169391528449211839693800724450201835161044717173715867437))
1208+
SELECT trim_scale(ln(x::numeric)-bc_result)AS diffFROM t;
1209+
11961210
--
11971211
-- Tests for LOG() (base 10)
11981212
--

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