[HIVE-9689] Store histograms and distinct value estimator's bit vectors in metastore

• Type: New Feature
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Labels: gsoc gsoc2015 hive java
• Assignee: Unassigned
• Reporter: Prasanth Jayachandran
• Created: 13/Feb/15 20:21
• Updated: 28/Feb/16 21:05

Description

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.

Issue Links

• is related to: HIVE-9905 Investigate ways to improve NDV calculations during stats aggregation [hbase-metastore branch]
• relates to: HIVE-9647 Discrepancy in cardinality estimates between partitioned and un-partitioned tables