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https://issues.apache.org/jira/browse/IMPALA-10350?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17239367#comment-17239367
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Zoltán Borók-Nagy commented on IMPALA-10350:
--------------------------------------------
Thanks, Tim. I took a quick glance at the algorithms. Lemire's algorithm
doesn't work in all cases so it falls back to string parsing (strtod_l) when it
fails.
google/wuffs algorithm depends on having the digits at hand so we'd need to
generate a string representation in this case as well.
I think a reasonable thing to do is to use Lemire's compute_float_64() because
it seems to work happily with our representation. And when it fails, I think we
should just fall back to string parsing, i.e.
std::strtod(dec_value.ToString()).
> Impala loses double precision because of DECIMAL->DOUBLE cast
> -------------------------------------------------------------
>
> 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, ramp-up
> 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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