What's regular blocking arrangement ? (GDAL)

I am using GDAL tools to retrieve PostGiS raster. The code(command line) I am using is as follows :

gdal_translate -of AAIGrid "PG:host=localhost dbname='database' user='postgres' password='admin' schema='public' table='rastertable' where='rid=2'" postgis.asc

And it shows error: rastertable doesn’t seem to have regular blocking arrangement. Only tables with regular blocking arrangement can be read from now.

I never heard the blocking arrangement before and I am sure the table in my postgis is a raster table. Any ideas? Thanks.