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https://issues.apache.org/jira/browse/IMPALA-6527?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Lars Volker updated IMPALA-6527:
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Summary: NaN values lead to incorrect statistics filtering under certain
circumstances (was: NaN values lead to incorrect filtering under certain
circumstances)
> NaN values lead to incorrect statistics filtering under certain circumstances
> -----------------------------------------------------------------------------
>
> Key: IMPALA-6527
> URL: https://issues.apache.org/jira/browse/IMPALA-6527
> Project: IMPALA
> Issue Type: Bug
> Components: Backend
> Affects Versions: Impala 2.11.0
> Reporter: Zoltan Ivanfi
> Assignee: Zoltán Borók-Nagy
> Priority: Blocker
> Labels: correctness, parquet
> Fix For: Impala 3.0, Impala 2.12.0
>
>
> h1. Summary
> If the first number in a row group written by Impala is NaN, then Impala
> writes incorrect statistics in the metadata. This will result in incorrect
> results when filtering the data.
> h1. Reproduction
> First, create a Parquet table with a double column:
> {noformat}
> create table test_nan(val double) stored as parquet;
> {noformat}
> Insert two values in a single statement, the first of which is a NaN:
> {noformat}
> insert into test_nan values (cast('NaN' as double)), (42);
> {noformat}
> Check that both values are actually present in the table:
> {noformat}
> select * from test_nan;
> +-----+
> | val |
> +-----+
> | NaN |
> | 42 |
> +-----+
> Fetched 2 row(s) in 0.13s
> {noformat}
> Filter using a condition that should match the regular number:
> {noformat}
> select * from test_nan where val > 0;
> Fetched 0 row(s) in 0.13s
> {noformat}
> *Expectation*: The row with the regular number should be returned.
> *Actual result*: No rows are returned.
> h1. Explanation
> Parquet files contain statistics metadata including the fields {{min}} and
> {{max}} or {{min_value}} and {{max_value}} (depending on the Impala version).
> If the first number is a NaN, the minimum and maximum values that Impala
> writes in the metadata are NaN. Based on this metadata, the row group can not
> contain any value that matches the condition, thereby Impala discards its
> contents without checking the individual entries. The problem is that the
> statistics were incorrectly written in the first place. (This can be and has
> been checked by using {{parquet-tools meta}} on the Parquet file.)
> What follows are just my assumptions without checking the actual code: While
> writing data, Impala keeps track of the smallest and largest value
> encountered so far. Let's call them min_so_far and max_so_far, respectively.
> Initially, the first (non_NULL) value is set as both the min_so_far and
> max_so_far. Then each new value is compared against min_so_far and
> max_so_far, updating each one if necessary. In pseudo_code:
> {code:java}
> if (new_value < min_so_far) {
> min_so_far = new_value;
> }
> {code}
> The problem is that any comparison involving NaN returns false, thereby if
> NaN is already in min_so_far, then no value can ever replace it and NaN will
> be stuck there.
> On the positive side, min_so_far can only become NaN if the first value in
> the row group is NaN. If the first value is not NaN, then NaN can never
> replace min_so_far, since the comparison will always return false when it
> involves a NaN.
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