multi.md

Multi Backend

The multi backend is designed for custom table structures as an alternative to the standard pgsql backend tables. It is intended to allow the configuration of a custom set of tables with hopefully fewer rows and fewer columns. This can be beneficial to queries in which some context (eg. zoom level) could limit the number of tables that need to be queried. Addtionaly it would allow more tables to be queried in parallel.

Database Layout

It connects to a PostgreSQL database and stores the data in one or more tables. Each table is configured in a way similar to that of the pgsql backend. That is essentially why it was named multi because it's basically multiple pgsql backends each with its own set of options and only a single table.

Table Configuration

As sample configuration may resemble the following:

[
  {
    "name": "building",
    "type": "polygon",
    "tagtransform": "building.lua",
    "tagtransform-node-function": "nodes_proc",
    "tagtransform-way-function": "ways_proc",
    "tagtransform-relation-function": "rels_proc",
    "tagtransform-relation-member-function": "rel_members_proc",
    "tags": [
      {"name": "building", "type": "text"},
      {"name": "shop", "type": "text"},
      {"name": "amenity", "type": "text"}
    ]
  },
  ...
]

Note that each table has a name and can target a single type of geometry by setting the type to one of point, line or polygon. tagtransform is used to set the name of the lua script to be used for custom tag processing. Within the lua script you may define several methods that will be called when processing various tags, these can be named via tagtransform-node-function, tagtransform-way-function, tagtransform-relation-function, and tagtransform-relation-member-function. As with the normal top level options within osm2pgsql you can specify any of the following: tablespace-index, tablespace-data, enable-hstore, enable-hstore-index, enable-multi, hstore-match-only. Hstore column names may be specified via an array of strings named hstores. Finally standard columns may be specified via an array of objects named tags with each object containing a name and a postgres type. Note you may also set flags on each tag as with the standard osm2pgsql style file. flags is formated exactly as in the style file as a string of flag names separated by commas.

Polygons

Area handling differs slightly from the traditional osm2pgsql C and Lua transforms where if a way is added to the polygon or line table depends on if it is closed, not just its tagging. All tables with the multi-backend are independent.

All ways in line tables are turned into linestrings and what the tag transform sets the polygon flag to does not matter. In polygon tables ways that can be formed into polygons are, and if the polygon flag is not set by the tag transform then other ways become linestrings. If the polygon flag is set, then linestrings will not be added to the table.

Example

An example based on the above is in multi.lua and multi.style.json. It creates three tables, one for bus stops, one for buildings, and one for highways. Some Lua processing is done to unify tagging values.

Importing

See: Importing.