float8 comparison failure

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                        POSTGRESQL BUG REPORT TEMPLATE
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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:
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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: 
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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