My current query is:
SELECT (ST_Dump( ST_Union( way ) )).geom AS way, 10000 AS way_pixels FROM ( SELECT way, way_pixels, FROM polygons ) AS test
it works fine to obtain and dissolve and split into indivisual polygons. But is there a reasonable way to compute sum of way_pixels attribute over merged polygons?
For example polygons A, B, C were merged. A has way_pixels value of 20, B of 120 and C of 19. So I want merged polygon to have way_pixels set to 159, not 10000 as it is done currently.
Computing sum has a known problem. With overlapping rather than joining polygons it is going to produce misleading values. But it is preferable to assuming that size is always 10000.
ST_Area is not solution as way_pixels values are not real area but projected area (in Web Mercator).