# 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:

- geometry a is covered by geometry b -> intersection is geometry a
- geometry b is covered by geometry a -> intersection is geometry b
- 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) );
```