Building heights from PostGIS

You don't say how you are loading the data into a PostGIS database; there are several ways (osm2pgsql, osmosis, imposm, ogr2ogr in descending order of popularity) and each will result in different data schemas. Assuming that you have used osm2pgsql, your buildings will be in planet_osm_polygons, and the building heights will only be available, in the tags column, if you have imported with --hstore.

To ease further processing, I would then create a table with all building centroids of buildings that have a height, like this

SELECT ST_CENTROID(way) geom, tags->'height' as height
INTO mybuildings
FROM planet_osm_polygon
WHERE building is not null and tags?'height';

Ideally you would also want to convert the height value to a floating point value during this step, however simply casting it will fail as soon as someone has added a non-numeric value to the field. People typically use a regular expression match for that:

SELECT ST_CENTROID(way) geom, (tags->'height')::numeric as height
INTO mybuildings
FROM planet_osm_polygon
WHERE building is not null and tags->'height' ~ '^\s*[0-9]+(\.[0-9]+)?\s*$';

Next, you want a spatial index on your new table

CREATE INDEX foo ON mybuildings USING gist(geom);

Now, you'll proceed almost as planned, but be sure to use ST_DWITHIN since it can benefit from the index whereas the ST_CONTAINS/ST_BUFFER construct cannot:

SELECT     points_table.id,
           AVG(mybuildings.height)
FROM       points_table
LEFT JOIN  mybuildings
ON         ST_DWITHIN(st_makeline(point_a, pointb), geom, 100)
GROUP BY  points_table.id;