Query with multiples operators BETWEEN - Mailing list pgsql-general

This qwery takes about 2 seconds :

SELECT e.name
FROM Observation o, Exposure_EPIC e
WHERE o.numObs = e.obs
  AND e.instrPN IS NOT NULL
  AND o.RA BETWEEN 3 AND 5;

This one about 5 seconds :

SELECT e.name
FROM Observation o, Exposure_EPIC e
WHERE o.numObs = e.obs
  AND e.instrPN IS NOT NULL
  AND o.DE BETWEEN 2 AND 7;

And this takes a lot of time!!!! (I can't say how much because I 've
always given it up before its end,
                                                             at least
more than 10 minutes)

SELECT e.name
FROM Observation o, Exposure_EPIC e
WHERE o.numObs = e.obs
  AND e.instrPN IS NOT NULL
  AND o.RA BETWEEN 3 AND 5
  AND o.DE BETWEEN 2 AND 7;

Could someone help or explain me?

Thanks in advance and excuse my bad English
Renaud THONNART