Why my postgis not use index on geometry field? - postgresql

Question

postgresql 9.5 + postgis 2.2 on windows.
I firstly create a table:
CREATE TABLE points (
id SERIAL,
ad CHAR(40),
name VARCHAR(200)
);
then, add a geometry field 'geom':
select addgeometrycolumn('points', 'geom', 4326, 'POINT', 2);
and create gist index on it:
CREATE INDEX points_index_geom ON points USING GIST (geom);
then, I insert about 1,000,000 points into the table.
I want to query all points that within given distance from given point.
this is my sql code:
SELECT st_astext(geom) as location FROM points
WHERE st_distance_sphere(
st_geomfromtext('POINT(121.33 31.55)', 4326),
geom) < 6000;
the result is what I want, but it is too slow.
when I explain analyze verbose on this code, I found it dose not use points_index_geom (explain shows seq scan and no index).
So i want to know why it dose not use index, and how should i improve?

Answer 1

You cannot expect ST_Distance_Sphere() to use an index on this query. You are doing a calculation on the contents of the geom field and then you are doing a comparison on the calculation result. Databases may not use an index in such a scenario unless you have a function index that does pretty much the same calculation as in your query.
The correct way to find locations with in a given distance from some point is to use ST_DWithin
ST_DWithin — Returns true if the geometries are within the specified
distance of one another. For geometry units are in those of spatial
reference and For geography units are in meters and measurement is
defaulted to use_spheroid=true (measure around spheroid), for faster
check, use_spheroid=false to measure along sphere.
and
This function call will automatically include a bounding box
comparison that will make use of any indexes that are available on the
geometries.