Can function results be used in WHERE?

I have a function, the results of which seem to apply to ORDER BY and
HAVING, but not to WHERE.  Is this expected?



-- Return distance in some mystery units (TODO: convert to miles or
kilometers)
CREATE FUNCTION calculate_distance(double precision, double precision,
double precision, double precision) RETURNS double precision   AS '
BEGIN   RETURN (3963 * acos( sin($1/57.2958) * sin($3/57.2958) +
cos($1/57.2958) * cos($3/57.2958) * cos($4/57.2958 - $2/57.2958) ));
END;
'   LANGUAGE plpgsql;



demo=# select
pod_code,lat,lon,calculate_distance(lat,lon,37.789629,-122.422082) from
eg_pod where 4 > 5::double precision order by 4 limit 10;pod_code |    lat    |     lon     | calculate_distance
----------+-----------+-------------+--------------------      44 |         0 |           0 |                  0
45|         0 |           0 |                  0      69 | 37.789629 | -122.422082 |                  0      51 |
37.788166| -122.421488 |  0.106273303754946      71 | 37.794228 | -122.421382 |  0.320393524437476      73 | 37.787878
|-122.411644 |  0.583267064983836      37 | 37.791736 | -122.411604 |  0.590977027054446      46 | 37.784929 |
-122.412782|  0.603416307249032      50 | 37.780329 | -122.418482 |  0.672685350683496      30 | 37.780419 |
-122.417764|  0.679355355047995
 
(10 rows)

sdemo=# select
pod_code,lat,lon,calculate_distance(lat,lon,37.789629,-122.422082) from
eg_pod having calculate_distance(lat,lon,37.789629,-122.422082) > 5
order by 4;pod_code |    lat    |     lon     | calculate_distance
----------+-----------+-------------+--------------------      21 | 37.710581 | -122.468864 |   6.03655070159813
77| 37.805427 |  -122.29528 |   7.01595024232628      29 | 37.802684 | -122.275976 |    8.0364304687727      12 |
37.806133| -122.273827 |   8.18282157050301      23 | 37.797327 |  -122.26598 |   8.54878571904839      57 | 37.829592
|-122.266347 |   8.94791199923289      35 | 37.809327 |  -122.25448 |   9.26077996779577      47 | 37.851957 |
-122.270376|   9.34292370436932
 



demo=# select version();                                                 version
-----------------------------------------------------------------------------------------------------------PostgreSQL
7.4.12on i686-pc-linux-gnu, compiled by GCC gcc (GCC) 3.2.3
 
20030502 (Red Hat Linux 3.2.3-20)
(1 row)


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