Which proximity function provides fastest and make use of spatial index in postGIS?

Question

I am new to PostgreSQL / PostGIS. I am evaluating it to solve a simple algorithm : Try to find all points in a radius (meters). Here is my table :

=> \d+ theuser;
                         Table "public.theuser"
  Column  |          Type          | Modifiers | Storage  | Description 
----------+------------------------+-----------+----------+-------------
 id       | bigint                 | not null  | plain    | 
 point    | geometry               |           | main     | 
Indexes:
    "theuser_pkey" PRIMARY KEY, btree (id)
    "point_index" gist (point)
Referenced by:
    ...
Has OIDs: no

I add a gist index to the point column , I don't know if it is the correct design. All 'points' inserted are with SRID=4326.

It seems there're 2 ways to get nearby points:

ST_Distance , ST_Distance_Sphere . Take 2 for example :

select * from theuser where
ST_distance_sphere(point , ST_GeomFromText('POINT(120.9982 24.788)',4326)) < 100;

I wonder which algorithm make use of the "point_index" ? If there are millions of points , can both execute very fast ?

Another question , how can I query the SRID of a cell (I searched by found no answer) ? All I can do is by hibernate-spatial-postgis , getting "com.vividsolutions.jts.geom.Point" , and get the SRID from the returned point. How do I query it in SQL ? Thanks.

Environment :

=> select version();
                                                  version                                                  
-----------------------------------------------------------------------------------------------------------
 PostgreSQL 8.4.9 on i486-pc-linux-gnu, compiled by GCC gcc-4.4.real (Ubuntu 4.4.3-4ubuntu5) 4.4.3, 32-bit

=> SELECT postgis_lib_version();
 postgis_lib_version 
---------------------
 1.4.0

---- updated ----

Thanks @filiprem , I tried this :

=> explain select * from theuser where
ST_distance_sphere(point , ST_GeomFromText('POINT(120.9982 24.788)',4326)) < 100;
                                                          QUERY PLAN                                                           
-------------------------------------------------------------------------------------------------------------------------------
 Seq Scan on theuser  (cost=0.00..1.15 rows=3 width=2644)
   Filter: (st_distance_sphere(point, '0101000020E610000080B74082E23F5E407D3F355EBAC93840'::geometry) < 100::double precision)
(2 rows)

How do I know if it makes use of the "point_index" gist (point) ? Will it survive under high data volume searching ?

Answer 1

I heard once that ST_DWithin is the fastest, actually in the documentation they say that in newer versions ST_DWithin has been tunned up.

Prior to 1.3, ST_Expand was commonly used in conjunction with && and ST_Distance to achieve the same effect and in pre-1.3.4 this function was basically short-hand for that construct. From 1.3.4, ST_DWithin uses a more short-circuit distance function which should make it more efficient than prior versions for larger buffer regions.

Also it uses bounding box comparitions and indexes:

This function call will automatically include a bounding box comparison that will make use of any indexes that are available on the geometries.