Issue with POSTGIS

Hello ,

My question is related to some functions in PostGIS.

Problem:

We are trying to find x points from a table that are within 20
nautical miles from a given point. Would like to use the ST_Dwithin
function as it would use the GIST index. I could use the
ST_Distance_Spheroid function but that would scan the entire table.

(1) So here's the query I would like to use:

select * from table_A where st_dwithin(shape, GeomFromText('POINT(-90
45)',4269), 20.0/60);

The shape field in table_A is also stored in the coordinate system for
4269. Since a nautical mile translates into 1 arcminute of latitude,
the distance is divided by 60.

(2) I am expecting to get 11 records and confirmed it by using the
st_distance_spheroid function as show below:

select * from table_A where
ST_Distance_Spheroid(shape ,ST_GeomFromText('POINT(-90 45)',4269),
'SPHEROID["GRS 1980",6378137,298.257222101,AUTHORITY["EPSG","7019"]]')
<=1852*20;

(3) One another test I did was to use the following and got 11 records
as expected. Here what I did was to convert to planar coordinates.

Select * from table_A where st_dwithin( transform(shape,
2163),transform(GeomFromText('POINT(-90 45)',4269),2163), 20*1852);

The query shown in (1) gives me 3 records. What am I missing? Please
help.

Thanks & Regards,
Itishree