PostGIS: Query z and m dimensions (linestringzm) - postgresql

Question

Question
I have a system with multiple linestringzm, where the data is structured the following way: [x, y, speed:int, time:int]. The data is structured this way to be able to use ST_SimplifyVW on the x, y and z dimensions, but I still want to be able to query the linestring based on the m dimension e.g. get all linestrings between a time interval.
Is this possible with PostGIS or am I structuring the data incorrectly for my use case?
Example
z = speed e.g. km/h
m = Unix epoch time
CREATE TABLE t (id int NOT NULL, geom geometry(LineStringZM,4326), CONSTRAINT t_pkey PRIMARY KEY (id));
INSERT INTO t VALUES (1, 'SRID=4326;LINESTRING ZM(30 10 5 1620980688, 30 15 10 1618388688, 30 20 15 1615710288, 30 25 20 1620980688)'::geometry);
INSERT INTO t VALUES (2, 'SRID=4326;LINESTRING ZM(50 10 5 1620980688, 50 15 10 1618388688, 50 20 15 1615710288, 50 25 20 1620980688)'::geometry);
INSERT INTO t VALUES (3, 'SRID=4326;LINESTRING ZM(20 10 5 1620980688, 20 15 10 1618388688, 20 20 15 1615710288, 20 25 20 1620980688)'::geometry);
Use case A: Simplify the geometry based on x, y and z
This can be accomplished by e.g. ST_SimplifyVW which keep the m dimension after simplification.
Use case B: Query geometry based on the m dimension
I have a set of linestringzm which I want to query based on my time dimension (m). The result is either the full geometry if all m is between e.g.1618388000 and 1618388700 or the part of the geometry which satisfies the predicate. What is the most efficient way to query the data?

Answer 1

If you want to check every single point of your LineString you could ST_DumpPoints them and get the M dimension with ST_M. After that extract the subset as a LineString containing the overlapping M values and apply ST_MakeLine with a GROUP BY:
WITH j AS (
SELECT id,geom,(ST_DumpPoints(geom)).geom AS p
FROM t
)
SELECT id,ST_AsText(ST_MakeLine(p))
FROM j
WHERE ST_M(p) BETWEEN 1618388000 AND 1618388700
GROUP BY id;
Demo: db<>fiddle
Note: Depending on your table and LineString sizes this query may become pretty slow, as values are being parsed in query time and therefore aren't indexed. Imho a more elegant alternative would be ..
.. 1) to create a tstzrange column
ALTER TABLE t ADD COLUMN line_interval tstzrange;
.. 2) to properly index it
CREATE INDEX idx_t_line_interval ON t USING gist (line_interval);
.. and 3) to populate it with the time of geom's first and last points:
UPDATE t SET line_interval =
tstzrange(
to_timestamp(ST_M(ST_PointN(geom,1))),
to_timestamp(ST_M(ST_PointN(geom,ST_NPoints(geom)))));
After that you can speed things up by checking wether the indexed column overlaps with a given interval. This will significantly improve query time:
SELECT * FROM t
WHERE line_interval && tstzrange(
to_timestamp(1618138148),
to_timestamp(1618388700));
Demo: db<>fiddle
Further reading:
ST_M
ST_PointN
ST_NPoints
PostgreSQL Built-in Range Types