[Tile-serving] [openstreetmap/osm2pgsql] Fix multistage processing (#1208)

[Jochen Topf]

@joto commented on this pull request.



> @@ -247,6 +246,19 @@ it is not, use stage 1 to store information about OSM objects you will need
 in stage 2 in some global variable. In stage 2 you can read this information
 again and use it to decide where and how to store the data in the database.
 
+Processing in two stages can add quite a bit of overhead. Because this feature
+is new, there isn't much operational experience with it. So be a bit careful
+when you are experimenting and watch memory and disk space consumption and
+any extra time you are using. Keep in mind that:
+
+* All data stored in stage 1 for use in stage 2 in your Lua script will use
+  main memory.
+* Keeping track of way ids marked in stage 1 needs some memory.
+* To do the extra processing in stage 2, time is needed to get objects out
+  of the object store and re-process them.
+* Osm2pgsql will create an id index on all way tables to look up ways that are
+  re-processed.

Fixed in next version of PR.

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