You have fallen into the "I need a buffer" trap.  You want a distance test:

SELECT AsText(geom) FROM point_table 
WHERE
  geom && Expand(GeometryFromText('POINT(10000 20000)',2769),900) 
AND
  Distance(geom,GeometryFromText('POINT(10000 20000)',2769) < 900

The first clause forces an index scan, followed by the exact test for falling
within the desired range.  In specific answer to your question, Expand() creates
an expanded bounding box, while Buffer() creates an expanded feature.  Expand()
is great for creating expanded bounding boxes for index filters as shown above,
but not useful for much else.

P

PS - You don't generally need a buffer unless you are going to actually do
something with the buffer geometry later (like intersect it with something else
to make a new geometry).

Quoting CYW <cyw@dls.net>:

> I need to find points that is within a certain range of a given point.
> Initially I tried a query below:
> 
> SELECT AsText(geom) from point_table where geom &&
>         Buffer(GeometryFromText('POINT(10000 20000 )', 2769),900));
> 
> Then I noticed that I should also be able to use another function Expand
> instead of Buffer.
> 
> Reading the manual, I can't tell which one is better here.
> 
> Additionally, is there a function that can give me the closest point or how
> should I formulate a query for that?
> 
> Any suggestions?
> 
> Thanks,
> _cyw_
> 
> 
> 
> Manual excerpts below.
> ------------------------------------
> Buffer(geometry, double, [integer])
> 
>     Returns a geometry that represents all points whose distance from this
> Geometry is less than or equal to distance. Calculations are in the Spatial
> Reference System of this Geometry. The optional third parameter sets the
> number of segment used to approximate a quarter circle (defaults to 8).
> 
> expand(geometry, float)
> 
>     This function returns a bounding box expanded in all directions from the
> bounding box of the input geometry, by an amount specified in the second
> argument. Very useful for distance() queries, to add an index filter to the
> query.
> 
> 
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