Working through the changes for pgRouting 4.0. New version is functionally equivilent to the legacy version with improved names

Author: rustprooflabs

docs/src/routing-3.md

@@ -1,10 +1,72 @@
-# Routing with PgOSM Flex and PgRouting prior to 4.0 
-
+# Routing with PgRouting 3
 
 > If you are using a pgRouting 4.0 or later see [Routing with pgRouting 4](./routing-4.md).
 
 
-To prepare the OpenStreetMap roads data for routing, run the
+## Clean the data
+
+The following query does the initial cleanup for preparing OpenStreetMap roads
+for routing. The following code is converting multi-linestrings to standard
+linestrings for subsequent processing steps.
+
+```sql
+CREATE TABLE routing.road_line AS
+WITH a AS (
+-- Remove as many multi-linestrings as possible with ST_LineMerge() 
+SELECT osm_id, osm_type, maxspeed, oneway, layer,
+        route_foot, route_cycle, route_motor, access,
+        ST_LineMerge(geom) AS geom
+    FROM osm.road_line
+), extra_cleanup AS (
+-- Pull out those that are still multi, use ST_Dump() to pull out parts
+SELECT osm_id, osm_type, maxspeed, oneway, layer,
+        route_foot, route_cycle, route_motor, access,
+        (ST_Dump(geom)).geom AS geom
+    FROM a 
+    WHERE ST_GeometryType(geom) = 'ST_MultiLineString'
+), combined AS (
+-- Combine two sources
+SELECT osm_id, osm_type, maxspeed, oneway, layer,
+        route_foot, route_cycle, route_motor, access,
+        geom
+    FROM a
+    WHERE ST_GeometryType(geom) != 'ST_MultiLineString'
+UNION
+SELECT osm_id, osm_type, maxspeed, oneway, layer,
+        route_foot, route_cycle, route_motor, access,
+        geom
+    FROM extra_cleanup
+    -- Some data may be lost here if multi-linestring somehow
+    -- persists through the extra_cleanup query
+    WHERE ST_GeometryType(geom) != 'ST_MultiLineString'
+)
+-- Calculate a new surrogate ID for key
+SELECT ROW_NUMBER() OVER (ORDER BY geom) AS id, *
+    FROM combined
+    ORDER BY geom
+;
+```
+
+The above query creates the `routing.road_line` table.  The next step
+adds some database best practices to the table:
+
+* Explain why a surrogate ID was added
+* Primary key on the `id` column
+* Index on `osm_id`
+
+
+```sql
+COMMENT ON COLUMN routing.road_line.id IS 'Surrogate ID, cannot rely on osm_id being unique after converting multi-linestrings to linestrings.';
+ALTER TABLE routing.road_line
+    ADD CONSTRAINT pk_routing_road_line PRIMARY KEY (id)
+;
+CREATE INDEX ix_routing_road_line_osm_id
+    ON routing.road_line (osm_id)
+;
+```
+
+To prepare the OpenStreetMap roads data for routing with the older pgRouting
+installation, run the
 pgRouting functions `pgr_nodeNetwork()`, `pgr_createTopology()`,
 and `pgr_analyzeGraph()`.