Thread

1. float8 comparison failure

   Radek Kanovsky <rk@dat.cz> — 2000-07-28T13:02:35Z

   ============================================================================
                           POSTGRESQL BUG REPORT TEMPLATE
   ============================================================================
   
   
   Your name		: Radek Kanovsky
   Your email address	: rk@dat.cz
   
   
   System Configuration
   ---------------------
     Architecture (example: Intel Pentium)  	: PC - AMD K6 3D
   
     Operating System (example: Linux 2.0.26 ELF) 	: Linux 2.2.16 (Debian)
   
     PostgreSQL version (example: PostgreSQL-6.5.3): PostgreSQL-6.5.3 
                                                     (6.5.1, 6.5.2)
   
     Compiler used (example:  gcc 2.8.0)		: gcc 2.95.2
                                                     libc6 2.1.3
   
   
   Please enter a FULL description of your problem:
   ------------------------------------------------
   
   I have one table defined as follows 
   
   CREATE TABLE misto (
     idm           VARCHAR(8) NOT NULL PRIMARY KEY,
     nazev         VARCHAR(64) NOT NULL,
     okres         VARCHAR(2) NOT NULL,
     sourad_s      FLOAT,
     sourad_d      FLOAT,
     modifikace    DATETIME NOT NULL DEFAULT now(),
     modifikoval   VARCHAR(32) NOT NULL DEFAULT getpgusername()
   ) ;
   CREATE INDEX misto_okres ON misto (okres);
   CREATE INDEX misto_nazev ON misto (nazev);
   
   
   These three SELECTs don't return same result:
   
   SELECT idm,sourad_s,sourad_d FROM misto WHERE idm = 'MHO00179';
   idm     |     sourad_s|     sourad_d
   --------+-------------+-------------   
   MHO00179|48.9247114069|17.6064515081
   (1 row)
     
   SELECT idm,sourad_s,sourad_d FROM misto WHERE sourad_s = 48.9247114069;
   idm     |     sourad_s|     sourad_d
   --------+-------------+-------------
   MHO00179|48.9247114069|17.6064515081   
   (1 row)
     
   SELECT idm,sourad_s,sourad_d FROM misto WHERE sourad_d = 17.6064515081;
   idm|sourad_s|sourad_d
   ---+--------+--------
   (0 rows)
   
   
   Please describe a way to repeat the problem.   Please try to provide a
   concise reproducible example, if at all possible: 
   ----------------------------------------------------------------------
   
   The problem appears randomly. I havent found any relation
   with anything else. I often use Python interface for INSERT,
   UPDATE, SELECT.
   
   If you know how this problem might be fixed, list the solution below:
   ---------------------------------------------------------------------
   
   This helps:
   
   BEGIN;
   UPDATE misto SET sourad_d = 17.6064515081 WHERE idm = 'MHO00179';
   END;
     
   SELECT idm,sourad_s,sourad_d FROM misto WHERE sourad_d = 17.6064515081;
   idm     |     sourad_s|     sourad_d
   --------+-------------+-------------
   MHO00179|48.9247114069|17.6064515081
   
   

2. Re: float8 comparison failure

   Tom Lane <tgl@sss.pgh.pa.us> — 2000-07-28T15:16:15Z

   Radek Kanovsky <rk@dat.cz> writes:
   > [ float8 values that appear equal are not always equal ]
   
   This is not a bug, it's an inherent consequence of the inaccuracy of
   floating-point math.  Use some other data type (numeric, maybe) if
   you expect to be able to do exact equality comparisons.
   
   			regards, tom lane