This patch adds the ability to compute and drop column and table statistics at partition granularity. The following commands are added. Detail about the implementation follows. COMPUTE INCREMENTAL STATS <tbl_name> [PARTITION <partition_spec>] This variant of COMPUTE STATS will, ultimately, do the same thing as the traditional COMPUTE STATS statement, but does so by caching the intermediate state of the computation for each partition in the Hive MetaStore. If the PARTITION clause is added, the computation is performed for only that partition. If the PARTITION clause is omitted, incremental stats are updated only for those partitions with missing incremental stats (e.g. one column does not have stats, or incremental stats was never computed for this partition). In this patch, incremental stats are only invalidated when a DROP STATS variant is executed. Future patches can automatically invalidate the statistics after REFRESH or INSERT queries, etc. DROP INCREMENTAL STATS <tbl_name> PARTITION <part_spec> This variant of DROP stats removes the incremental statistics for the given table. It does *not* recalculate the statistics for the whole table, so this should be used only to invalidate the intermediate state for a partition which will shortly be subject to COMPUTE INCREMENTAL STATS. The point of this variant is to allow users to notify Impala when they believe a partition has changed significantly enough to warrant recomputation of its statistics. It is not necessary for new partitions; Impala will detect that they do not have any valid statistics. -------- This is achieved by adapting the existing HLL UDA via swapping its finalize method for a new one which returns the intermediate HLL buckets, rather than aggregating and then disposing of them. This intermediate state is then returned to Impala's catalog-op-executor.cc, which then passes the intermediate state back to the frontend to be ultimately stored in the HMS. This intermediate state is computed on a per-partition basis by grouping the input to the UDA by partition. Thus, the incremental computation produces one row for each partition selected (the set of which might be quite small, if there are few partitions without valid incremental stats: this is the point of the new commands). At the same time, the query coordinator aggregates the output of the UDA to produce table-level statistics. This computation incorporates any existing (and not re-computed) intermediate partition state which is passed to the coordinator by the frontend. The resulting statistics are saved to the table as normal. Intermediate statistics are serialised to the HMS by writing a Thrift structure's serialised form to the partition's 'parameters' map. There is a schema-imposed limit of 4000 characters to the serialised string, which is exacerbated by the fact that the Thrift representation must first be base-64 encoded to avoid type errors in the HMS. The current patch breaks the encoded structure into 4k chunks, and then recombines them on read. The alltypes table (11 columns) takes about three of these chunks. This may mean that incremental stats are not suitable for particularly wide tables: these structures could be zipped before encoding for some space savings. In the meantime, the NDV estimates are run-length encoded (since they are generally sparse); this can result in substantial space savings. Change-Id: If82cf4753d19eb532265acb556f798b95fbb0f34 Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4475 Tested-by: jenkins Reviewed-by: Henry Robinson <firstname.lastname@example.org>
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Sep 16, 2014
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This folder is a sample for how to develop UDFs and UDAs for Impala. All of the files here are part of the UDF developer kit.