Zoltan Ivanfi created PARQUET-1222:
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Summary: Definition of float and double sort order is ambigious
Key: PARQUET-1222
URL: https://issues.apache.org/jira/browse/PARQUET-1222
Project: Parquet
Issue Type: Bug
Components: parquet-format
Reporter: Zoltan Ivanfi
Currently parquet-format specifies the sort order for floating point numbers as
follows:
{code}
* FLOAT - signed comparison of the represented value
* DOUBLE - signed comparison of the represented value
{code}
The problem is that the comparison of floating point numbers is only a partial
ordering with strange behaviour in specific corner cases. For example,
according to IEEE 754, -0 is neither less nor more than \+0 and comparing NaN
to anything always returns false. This ordering is not suitable for statistics.
Additionally, the Java implementation already uses a different (total) ordering
that handles these cases correctly but differently than the C\+\+
implementations, which leads to interoperability problems.
We should explicitly require implementations to follow a specific comparison
logic for these types. The candidates are:
* The [Java
implementation|http://hg.openjdk.java.net/jdk8/jdk8/jdk/file/687fd7c7986d/src/share/classes/java/lang/Double.java#l999]
which looks easy and efficient to implement in any language.
* The [IEEE 754 totalOrder
predicate|https://github.com/rust-lang/rust/issues/5585] which is rather
complicated to the extent that it is hard to tell whether the Java
implementation adheres to it.
* The [IEEE 754-2008 min and max
operations|https://en.wikipedia.org/wiki/IEEE_754_revision#min_and_max] which
may be hard to use for comparison, so components could not use sorting to
achieve the smallest possible min/max ranges.
An additional problem is dealing with existing data. One possibility is to
specify legacy rules, like "if the stats contain NaN and the file was written
by Impala, it should be ignored", but that would complicate the specs and be a
burden on implementors. In fact, `min_value` and `max_value` were introduced
because we did not want to define similar legacy rules for `min` and `max`.
Another alternative is to deprecate `min_value` and `max_value` as well and
introduce `yet_another_min` and `yet_another_max` fields instead (with nicer
names, naturally).
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