Question

I imported ~1000 paths from somewhere into my postgis database to a linestring field.

(EDIT) My table is like this

+---------+---------------+------------------+
| id(int) | name(varchar) | path(LINESTRING) |
+---------+---------------+------------------+
| 123     | foo           | 000002...        |
| 124     | bar           | 000002...        |

I had the problem that each of the paths were divided into chunks, and those chunks were mixed up in some cases.

Suppose a linestring that was divided at point number 50 and 70:

1. Chunk A: points 1-50
2. Chunk B: points 51-70
3. Chunk C: points 71-100

When I migrated that into my database, they got mixed, so the resulted linestring could end up like this:

1. Chunk A: points 1-50
2. Chunk C: points 71-100
3. Chunk B: points 51-70

So that produces a jump from 50 to 71 and another one from 100 to 51

(EDIT) When I imported those paths divided in chunks I supposed that they were ordered, but the fact was that some were mixed, and that made some of my linestrings to be with their points ordered like the second example.

I want to be able to reorder those chunks (of points) so I would like to construct a SQL query to detect which paths have mixed points, then I could manually (with a tool made using openlayers) rearrange them.

It would be desirable to have a SQL update query to solve this problem, but I think that detection is easier (I presume there are ~5% or less of paths with errors)

EDIT3: I think the script for detection could check if a path contains a pair of consecutive points too far away. Maybe a SQL that orders paths from the path that contains the longest segment, would be good.

How could I make a function to get the length of the max segment in a linestring?

Here I show an example:
This is how it is in the database

This is how I want it to be fixed

EDIT4: Like I planned on EDIT3, a function could be written to find the longest distance between two consecutive points in a linestring iterating through the points of a linestring using ST_NPoints() and ST_PointN(), then a query could be made to order paths with that longest distance. It’s highly probable that the linestrings that have this distance too long present the problem described. That way I would be able to detect them, and fix them manually.

The result from the detection SQL would be something like this:

                                             |ordered by this|
+---------+---------------+------------------+---------------+
| id(int) | name(varchar) | path(LINESTRING) |  msbtcp(int)  |
+---------+---------------+------------------+---------------+
| 123     | foo           | 000002...        | 1000          |
| 124     | bar           | 000002...        | 800           |

*msbtcp would be the result of the function: max_separation_between_two_consecutive_points(path)

Answer 1

This sounds a bit convoluted, however if you’re just after the max distance between two consecutive points in a linestring:

CREATE OR REPLACE FUNCTION max_distance_in_linestring(line geometry) RETURNS float as $BODY$
DECLARE
    i integer;
    n integer;
    d float;
    m float;
BEGIN
    d := 0;
    n := ST_NPoints(line);
    i := 2;
    LOOP
        EXIT WHEN i >= n;
        m := ST_Distance(ST_PointN(line,i-1),ST_PointN(line,i));
        -- use for lon,lats:
        -- m := ST_Distance(ST_PointN(line,i-1)::geography,ST_PointN(line,i)::geography);
        IF m > d THEN
            d := m;
        END IF;
        i := i + 1;
    END LOOP;
    RETURN d;
END;
$BODY$
LANGUAGE plpgsql;

SELECT max_distance_in_linestring('LINESTRING(0 0, 1 1, 2 2)'::geometry);
SELECT max_distance_in_linestring('LINESTRING(0 0, 4 3, 2 2)'::geometry);

you may want to re-cast the ST_PointN calls to ::geography for getting distance in meters.

The SQL would be like this:

SELECT
  name, path 
FROM
  paths
ORDER BY
  max_distance_in_linestring(path) DESC