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https://issues.apache.org/jira/browse/IMPALA-10350?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17237560#comment-17237560
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Tim Armstrong commented on IMPALA-10350:
----------------------------------------
I think the problem is this function which is used in the DECIMAL -> DOUBLE
cast:
https://github.com/apache/impala/blob/45c105d/be/src/runtime/decimal-value.inline.h#L725
Here's a small test program indicating that it seems to be where the truncation
happens.
[^test.c]
{noformat}
$ gcc test.c -o test -lm && ./test
-0.43149576573887316 -0.43149576573900000
-0.43149576573887310
{noformat}
> Impala loses double precision on the write side
> -----------------------------------------------
>
> Key: IMPALA-10350
> URL: https://issues.apache.org/jira/browse/IMPALA-10350
> Project: IMPALA
> Issue Type: Bug
> Components: Backend
> Reporter: Zoltán Borók-Nagy
> Priority: Major
> Labels: correctness
> Attachments: test.c
>
>
> Impala might loses presision of double values. Reproduction:
> {noformat}
> create table double_tbl (d double) stored as textfile;
> insert into double_tbl values (-0.43149576573887316);
> {noformat}
> Then inspect the data file:
> {noformat}
> $ hdfs dfs -cat
> /test-warehouse/double_tbl/424097c644088674-c55b910100000000_175064830_data.0.txt
> -0.4314957657388731{noformat}
> The same happens if we store our data in Parquet.
> Hive writes don't lose precision. If the data was written by Hive then Impala
> can read the values correctly:
> {noformat}
> $ bin/run-jdbc-client.sh -t NOSASL -q "select * from double_tbl;"
> Using JDBC Driver Name: org.apache.hive.jdbc.HiveDriver
> Connecting to: jdbc:hive2://localhost:21050/;auth=noSasl
> Executing: select * from double_tbl
> ----[START]----
> -0.43149576573887316
> ----[END]----{noformat}
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