Related to
https://trac.osgeo.org/postgis/ticket/2200 support multiple bands issue
If the $RASTER has bands like ARRAY['8BUI', '16BUI'], the result of
```
ST_Union(
$RASTER,
ARRAY[
ROW(1, 'SUM'),
ROW(2, 'MAX')
]::unionarg[]
)
```
will be like ARRAY['8BUI', ' 8BUI'].
ST_Union always uses the first band class to process the 2nd band and ignore the original class.