Gabor Kaszab created IMPALA-9942:
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Summary: DataSketches HLL shouldn't take empty strings as distinct
values
Key: IMPALA-9942
URL: https://issues.apache.org/jira/browse/IMPALA-9942
Project: IMPALA
Issue Type: Improvement
Components: Backend
Affects Versions: Impala 4.0
Reporter: Gabor Kaszab
Assignee: Gabor Kaszab
Let's consider a table that has string, char and varchar columns and some of
the values in these columns are empty strings.
{code:java}
select * from strings;
+-----+------------+-----+
| s | c | v |
+-----+------------+-----+
| | | |
| abc | abc | abc |
| | | |
+-----+------------+-----+
{code}
If I query the # of distinct values by DataSketches HLL then the empty string
add +1 to the end result.
{code:java}
select ds_hll_estimate(ds_hll_sketch(s)), ds_hll_estimate(ds_hll_sketch(c)),
ds_hll_estimate(ds_hll_sketch(v)) from strings;
+------------+----------+-------------+
| hll_string | hll_char | hll_varchar |
+------------+----------+-------------+
| 2 | 2 | 2 |
+------------+----------+-------------+
{code}
However, Hive's implementation omits empty strings so for this particular
example above Hive would return 1 for each column.
I assume omits empty strings because of this line:
https://github.com/apache/incubator-datasketches-java/blob/master/src/main/java/org/apache/datasketches/hll/BaseHllSketch.java#L351
First step of this task would be to decide which approach is the correct one,
and as a second step do the adjustment in Impala if we decide that way.
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