Merge remote-tracking branch 'openstreetmap/master' into mac-os-x-install

diff --cc INSTALL.md
index ceb4eccb0acc27802699f6699d69dc2168a218be,4c2c7c6c80a66137e46716b5ab9f8a4fb927204a..e414a12e425520dfc1d3e312aa83bc65a62ee78d
--- 1/INSTALL.md
--- 2/INSTALL.md
+++ b/INSTALL.md
@@@ -258,26 -224,43 +248,67 @@@ Note that the OSM map tiles you see are
  
  After installing this software, you may need to carry out some [configuration steps](CONFIGURE.md), depending on your tasks.
  
- # Ruby development install and versions<a name="rbenv"></a>
+ # Installing compiled shared library database functions (optional)
+ 
+ There are special database functions required by a (little-used) API call, the migrations and diff replication. The former two are provided as *either* pure SQL functions or a compiled shared library. The SQL versions are installed as part of the recommended install procedure above and the shared library versions are recommended only if you are running a production server making a lot of `/changes` API calls or need the diff replication functionality.
+ 
+ If you aren't sure which you need, stick with the SQL versions.
+ 
+ Before installing the functions, it's necessary to install the PostgreSQL server development packages. On Ubuntu this means:
+ 
+ ```
+ sudo apt-get install postgresql-server-dev-all
+ ```
+ 
+ On Fedora:
+ 
+ ```
+ sudo dnf install postgresql-devel
+ ```
+ 
+ The library then needs compiling.
+ 
+ ```
+ cd db/functions
+ make libpgosm.so
+ cd ../..
+ ```
+ 
+ If you previously installed the SQL versions of these functions, we'll need to delete those before adding the new ones:
+ 
+ ```
+ psql -d openstreetmap -c "DROP FUNCTION IF EXISTS maptile_for_point"
+ psql -d openstreetmap -c "DROP FUNCTION IF EXISTS tile_for_point"
+ ```
+ 
+ Then we create the functions within each database. We're using `pwd` to substitute in the current working directory, since PostgreSQL needs the full path.
+ 
+ ```
+ psql -d openstreetmap -c "CREATE FUNCTION maptile_for_point(int8, int8, int4) RETURNS int4 AS '`pwd`/db/functions/libpgosm', 'maptile_for_point' LANGUAGE C STRICT"
+ psql -d openstreetmap -c "CREATE FUNCTION tile_for_point(int4, int4) RETURNS int8 AS '`pwd`/db/functions/libpgosm', 'tile_for_point' LANGUAGE C STRICT"
+ psql -d openstreetmap -c "CREATE FUNCTION xid_to_int4(xid) RETURNS int4 AS '`pwd`/db/functions/libpgosm', 'xid_to_int4' LANGUAGE C STRICT"
+ ```
++
++# Ruby development install and versions<a name="rbenv"></a> (optional)
 +
 +For simplicity, this document explains how to install all the website dependencies as "system" dependencies. While this is simpler, and usually faster, you might want more control over the process or the ability to install multiple different versions of software alongside eachother. For many developers, [`rbenv`](https://github.com/rbenv/rbenv) is the easiest way to manage multiple different Ruby versions on the same computer - with the added advantage that the installs are all in your home directory, so you don't need administrator permissions.
 +
 +If you choose to install Ruby and Bundler via `rbenv`, then you do not need to install the system libraries for Ruby:
 +
 +* For Ubuntu, you do not need to install the following packages: `ruby2.5 libruby2.5 ruby2.5-dev bundler`,
 +* For Fedora, you do not need to install the following packages: `ruby ruby-devel rubygem-rdoc rubygem-bundler rubygems`
 +* For MacOSX, you do not need to `brew install ruby` - but make sure you've installed a version of Ruby using `rbenv` before running `gem install bundler`!
 +
 +After installing a version of Ruby with `rbenv` (the latest stable version is a good place to start), you will need to make that the default. From inside the `openstreetmap-website` directory, run:
 +
 +```
 +rbenv local $VERSION
 +```
 +
 +Where `$VERSION` is the version you installed. Then install bundler:
 +
 +```
 +gem install bundler
 +```
 +
 +You should now be able to proceed with the rest of the installation. If you're on MacOSX, make sure you set up the [config override for the libxml2 location])(#macosx-bundle-config) _after_ installing bundler.