When using Oracle 12c with the GeoRaster data type and the Image Mosaic JDBC plugin to visualize a 32bit raster image, the background of the result seems to contain values around 1.0 instead of a transparent color.
Unfortunately with this bug I can't use GeoServer as a map server with a relational database as data store, because heat maps or height layers typically contain 32 bit floats, which are not rendered correctly and can't be used in combination with other layers because the background is not transparent.
Here are the steps to reproduce the problem with an Oracle 12c database and GeoServer 2.8.0, but I assume that it's reproducible with other databases too. I used the sfdem.tif geoTiff, which comes with GeoServer in this example.
1.) Import sfdem.tif into database
2.) Create config file for Image Mosaic JDBC plugin
3.) Create new data store and layer with default settings in GeoServer
4.) Add new SLD style with color maps and assign it to the layer
5.) View the result of the WMS call, background settings doesn't affect the result
The background (area outside of bbox) is red, because the 1.0 value is mapped to a red color in the sld style. The expected behaviour would be that the background is transparent, which is the case when the layer is created from the file directly. I couldn't find any acceptable workaround to overcome this issue.
Tested with Oracle 12c GeoRaster and GeoServer 2.8.0