Metallicity distribution of RRLs (light blue bins) in the known sub-structures of the MW halo. The dashed red line indicates the mean metallicity of RRLs in each sub-structure. Name of the sub-structure, number of RRLs with accurate metallicities, and mean metallicities are indicated in the legend. The uncertainties are calculated as the weighted standard deviation of the mean. For sub-structures containing only one star, the uncertainty corresponds to the photometric metallicity uncertainty of that individual star.