Re: Exponantial Function (exp) with bigger argument ? - Mailing list pgsql-novice

Thanks for the answer Tom. As you suggested, I converted the integer to a numeric type and found out, that the
correspondingexp function can handle numbers up to 6999.0. But this is still not enough. Currently, I'm trying to
extendPostgres with a custom C-Function. But, I am running into troubles there, too. 
This could also be due to my nescience concerning C and Linux.  My code looks as follows:
/*
 ** Filename: exp_c.c
 */
 #include "postgres.h"
 #include "fmgr.h"
 // Magic block
 #ifdef PG_MODULE_MAGIC
 PG_MODULE_MAGIC;
 #endif
 #include "mpfr.h"
 // We're using the version-1 calling method
 PG_FUNCTION_INFO_V1(exp_c);

 Datum exp_c(PG_FUNCTION_ARGS){
      if (PG_ARGISNULL(0) || PG_ARGISNULL(1)){
        PG_RETURN_NULL();
      }
    float8 exponent = PG_GETARG_FLOAT8(0);
    int32 precision = PG_GETARG_INT32(0);
      // The result
    mpfr_t x;
      // Initialize variable with given precision
    mpfr_init2(x, precision);
    // Set to given exponent
    mpfr_set_ld(x, exponent, MPFR_RNDN);
    // Calculate
    mpfr_exp(x,x, MPFR_RNDN);
    // Get return value
    long double rs = mpfr_get_ld(x,MPFR_RNDN);
      // Clear x
    mpfr_clear(x);
      // Return
      PG_RETURN_FLOAT8(rs);
 }

I compiled the program with "gcc -c -fpic exp_c.c -lmpfr -lgmp" and created a shared library with this one "gcc -shared
-oexp_c.so exp_c.o". 
This worked fine. Btw. we are using Ubuntu 10.10 Server and postgres 8.4 (version() function output: "PostgreSQL 8.4.8
onx86_64-pc-linux-gnu, compiled by GCC gcc-4.4.real (Ubuntu/Linaro 4.4.4-14ubuntu5) 4.4.5, 64-bit"). 

I copied the exp_c.so tot he $libdir directory. In postgres  the following commands were executed:

CREATE OR REPLACE FUNCTION exp(exponent double precision, prec integer) RETURNS double precision AS
'$libdir/exp_c','exp_c'LANGUAGE c; 

SELECT exp(1::double precision, 2);

Unfortunately this does not work. The output is (Translated into english):

FAILURE: Library »/usr/lib/postgresql/8.4/lib/exp_c.so« could not be loaded: /usr/lib/postgresql/8.4/lib/exp_c.so:
undefinedsymbol: mpfr_get_ld 

********** FAILURE **********

FAILURE: Library »/usr/lib/postgresql/8.4/lib/exp_c.so« could not be loaded: /usr/lib/postgresql/8.4/lib/exp_c.so:
undefinedsymbol: mpfr_get_ld 
SQL Status:58P01

I think this has something to do with the binder/linker and the mpfr header file, which is not available during
linking.Am I correct ? How can I overcome this ? 

Kind regards,
Gregor

-----Ursprüngliche Nachricht-----
Von: Tom Lane [mailto:tgl@sss.pgh.pa.us]
Gesendet: Dienstag, 3. Mai 2011 20:01
An: Gregor GT. Trefs
Cc: pgsql-novice@postgresql.org
Betreff: Re: [NOVICE] Exponantial Function (exp) with bigger argument ?

"Gregor GT. Trefs" <gregor.trefs@delphit.com> writes:
> Is there any possibility to overcome the restriction to not use a
> higher number than appr. 100 as an argument for the exponential
> function in postgres ?

What restriction?


regression=# select exp(1000.0);
                                                          exp

-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

197007112172228764285408431116443651047300981094802008755853985828350443654498948052236544085754035611119684706809582853182955874910502440327439392806215139501177952207939837125875818468684009900351395991300432715505235653014871192637790968152087501139107799418773107714238073871162818125586289629983090134192022542188366296411429692950563401460360277794420292466575635467954804734395417151344955106563797211636884598324669752528955502.2
(1 row)

Mind you, it would be unwise to assume that that answer is *exact* ... exactly what is the "business logic" that
requiressuch large exponentials, and are you sure you're getting non-garbage results on any other platform? 

            regards, tom lane