Hive currently uses PCSA (Probabilistic Counting and Stochastic Averaging) algorithm to determine distinct cardinality. The NDV value determined from the UDF is stored in the metastore instead of the actual bit vectors. This makes it impossible to estimate the overall NDV across all the partitions (or selected partitions). We should ideally store the bitvectors in the metastore and do server side merging of the bitvectors. Also we could replace the current PCSA algorithm in favour of HyperLogLog if space is a constraint.
Also Hive has a UDF for computing histogram. We can persist the histogram in the metastore so that hive optimizer can make better decisions. Also having histograms in metastore can help with order by, skew join and count distinct + group by optimizations.