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This function receives a set of serialized Apache DataSketches KLL
sketches produced by ds_kll_sketch() and merges them into a single
sketch.
An example usage is to create a sketch for each partition of a table,
write these sketches to a separate table and based on which partition
the user is interested of the relevant sketches can be union-ed
together to get an estimate. E.g.:
SELECT
ds_kll_quantile(ds_kll_union(sketch_col), 0.5)
FROM sketch_tbl
WHERE partition_col=1 OR partition_col=5;
Testing:
- Apart from the automated tests I added to this patch I also
tested ds_kll_union() on a bigger dataset to check that
serialization, deserialization and merging steps work well. I
took TPCH25.linelitem, created a number of sketches with grouping
by l_shipdate and called ds_kll_union() on those sketches.
Change-Id: I020aea28d36f9b6ef9fb57c08411f2170f5c24bf
Reviewed-on: http://gerrit.cloudera.org:8080/16267
Reviewed-by: Impala Public Jenkins <impala-public-jenkins@cloudera.com>
Tested-by: Impala Public Jenkins <impala-public-jenkins@cloudera.com>