# Getting intersections the faster way

Doing an `ST_Intersection` is much slower than relation checks such as `ST_Intersects` , `ST_CoveredBy`, and , `ST_Within` . In many situations you know the intersection of 2 geometries without actually computing an intersection. In these cases, you can skip the costly `ST_Intersection` call. Cases like this:

1. geometry a is covered by geometry b -> intersection is geometry a
2. geometry b is covered by geometry a -> intersection is geometry b
3. geometry a does not intersect geometry b -> intersection is empty geometry

This kind of question comes up a lot: As discussed in stackexchange Acquiring ArcGIS speed in PostGIS

Examples

For this exercise, we’ll grab the portion of each parcel that falls in a neighborhood. Unlike stack exchange one, we’ll use `ST_CoveredBy` instead of `ST_Within` check. Both constructs are similar except that `ST_CoveredBy` allows for geometries to be wholly within the boundary of another and tends to be a bit faster to compute than `ST_Within`. This subtlety becomes more important if you are comparing linestrings such as what portion of a road falls in a county as detailed in Subtleties OGC Covers Spatial.

``````SELECT p.parcel_id, n.nei_name
, CASE
WHEN ST_CoveredBy(p.geom, n.geom)
THEN p.geom
ELSE
ST_Multi(
ST_Intersection(p.geom,n.geom)
) END As geom
FROM parcels As p
INNER JOIN neighborhoods As n
ON ST_Intersects(p.geom, n.geom);
``````

In this particular case, we’d probably want to exclude the case where a parcel just borders a neighborhood (in spatial speak `touches`). We wouldn’t really consider a bordering parcel as having any part in the neighborhood, though it intersects the neighborhood. So a slightly more complicated but more accurate statement would be to exclude from consideration the case where a parcel borders a neighborhood using `ST_Touches`.

``````SELECT p.parcel_id, n.nei_name
, CASE
WHEN ST_CoveredBy(p.geom, n.geom)
THEN p.geom
ELSE
ST_Multi(
ST_Intersection(p.geom,n.geom)
) END As geom
FROM parcels As p
INNER JOIN neighborhoods As n
ON (ST_Intersects(p.geom, n.geom)
AND Not ST_Touches(p.geom, n.geom) );
``````