query optimizer dont treat correctly OR

Hello folks

See the command bellow. I use some thing simmilar about an decade on
Oracle,
Sybase, MSSQL, DB2, etc. But with Postgresql , he generate an FULL TABLE
SCAN,
and consequenyly it take about 10 minutes to run (Very big table..)

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SELECT A.SR_RECNO , A.CDEMP, A.NRPED,A.SR_RECNO
FROM FTB01 A
WHERE ( A.CONTROLE <= ' ' AND A.CDEMP = '75' AND A.NRPED < '0000261' )
OR ( A.CONTROLE = ' ' AND A.CDEMP < '75' )
OR ( A.CONTROLE < ' ' )
ORDER BY A.CDEMP DESC, A.NRPED DESC, A.SR_RECNO DESC
LIMIT 170

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Otherwise, is i write the query on the form of an little more "dummy" and
eliminating the
"OR" and changing by UNION, the time of execution drops to less menos
of two seconds

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

SELECT TMP1.* FROM (
SELECT A.SR_RECNO, A.CDEMP, A.NRPED,A.SR_RECNO
FROM FTB01 A WHERE ( A.CONTROLE <= ' ' AND A.CDEMP = '75' AND A.NRPED <
'0000261' )
ORDER BY A.CDEMP DESC, A.NRPED DESC, A.SR_RECNO DESC LIMIT 170 ) TMP1
UNION
SELECT TMP2.* FROM (
SELECT A.SR_RECNO, A.CDEMP, A.NRPED,A.SR_RECNO
FROM FTB01 A WHERE ( A.CONTROLE = ' ' AND A.CDEMP < '75' )
ORDER BY A.CDEMP DESC, A.NRPED DESC, A.SR_RECNO DESC LIMIT 170 ) TMP2
UNION
SELECT TMP3.* FROM (
SELECT A.SR_RECNO, A.CDEMP, A.NRPED,A.SR_RECNO
FROM FTB01 A WHERE OR ( A.CONTROLE < ' ' )
ORDER BY A.CDEMP DESC, A.NRPED DESC, A.SR_RECNO DESC LIMIT 170 ) TMP3
ORDER BY CDEMP DESC, NRPED DESC, SR_RECNO DESC
LIMIT 170

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

The comand above works (even being 10 x slower then other Databases
) with our generate the full scan.

Why Post do this wrong julgment with the initial command?
Exist some thing that i can configure to to make postgres works correctly ?

Obs.:

* Tested on versions 7.3.2 e 7.4.1
* Obvialy the vacuumm full analyse was executed

Thanks
Luiz