PostgreSQL聪明地使用ST_Within

Question

maybe my question can seem stupid, but I'd like to ask if somebody can tell me how can I improve this query:

UPDATE twitter
   SET poi=poi.type_name FROM poi
 WHERE ST_Within (ST_SetSRID(ST_MakePoint(x_coords,y_coords),4326),
 ST_SetSRID(ST_Buffer(poi.wkb_geometry, 0.0005),4326));

I have 2 tables with points: " twitter " and " poi ", I need to create a buffer of 50m around " poi " and find all points of " twitter " which are within this buffer and then copy " type_name " attribute of " poi " to " twitter " point. The problem is that the query lasts too long and seems it'll never finish, from time to time I try to check if some attributues were copied, but " twitter.poi " column remains always empty.

I tried to limit the query by adding in WHERE " username " to check if the query is right and it works.

UPDATE twitter
       SET poi=poi.type_name FROM poi
     WHERE uname='some_username' AND ST_Within (ST_SetSRID(ST_MakePoint(x_coords,y_coords),4326),
     ST_SetSRID(ST_Buffer(poi.wkb_geometry, 0.0005),4326));

The only one thing that I can't understand is how to improve the query for entire " twitter " table.

Answer 1

You didn't provide a table definition, but you seem to have two columns for the coordinates in table twitter : x_coords and y_coords . Either replace those with a geometry column and create a simple GiST index or create a functional GiST index like:

CREATE INDEX idx_twitter_point_4326
ON twitter  USING gist (ST_SetSRID(ST_MakePoint(x_coords,y_coords), 4326));

And another one on poi :

CREATE INDEX idx_poi_wkb_geometry_4326
ON poi USING gist (ST_SetSRID(p.wkb_geometry, 4326));

This should be used to speed up ST_Within() :

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

The function ST_distance() might serve you better:

UPDATE twitter t
SET    poi = p.type_name
FROM   poi p
WHERE  ST_Distance(ST_SetSRID(ST_MakePoint(t.x_coords, t.y_coords), 4326)
                  ,ST_SetSRID(p.wkb_geometry, 4326)) < 0.0005
AND    t.poi IS DISTINCT FROM p.type_name;

This is assuming there can at most one poi in the vicinity of a twitter entry. Else you should use a subquery that picks the closest poi for each twitter .

The additional WHERE clause t.poi IS DISTINCT FROM p.type_name avoid empty updates.

Not using PostGis myself, so untested.