yyanyy commented on a change in pull request #2055:
URL: https://github.com/apache/iceberg/pull/2055#discussion_r679522667
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File path: site/docs/spec.md
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@@ -254,6 +254,24 @@ Notes:
2. The width, `W`, used to truncate decimal values is applied using the scale
of the decimal column to avoid additional (and potentially conflicting)
parameters.
+### Sorting
+
+Users can sort their data within partitions by columns to gain performance.
The information on how the data is sorted can be declared per data or delete
file, by a **sort order**.
+
+A sort order is defined by an sort order id and a list of sort fields. The
order of the sort fields within the list defines the order in which the sort is
applied to the data. Each sort field consists of:
+
+* A **source column id** from the table's schema
+* A **transform** that is used to produce values to be sorted on from the
source column. This is the same transform as described in [partition
transforms](#partition-transforms).
+* A **sort direction**, that can only be either `asc` or `desc`
+* A **null order** that describes the order of null values when sorted. Can
only be either `nulls-first` or `nulls-last`
+
+Order id `0` is reserved for the unsorted order.
+
+Sorting floating-point numbers should produce the following behavior: `-NaN` <
`-Infinity` < `-value` < `-0` < `0` < `value` < `Infinity` < `NaN`. This aligns
with the implementation of Java floating-point types comparisons.
Review comment:
Thanks for the input! You are right that in Java `-Double.NaN` equals
`Double.NaN` so I think we should drop the mentioning of `-NaN` in spec. For
the interpretation of having the sign bit set, I did some quick reading and it
seems like NaN can be represented by a lot of ways (from
[here](https://stackoverflow.com/a/2154512) it says anything that looks like
`x111 1111 1axx xxxx xxxx xxxx xxxx xxxx` where x means anything would be NaN),
and the sign bit for NaN seems to have [no
meaning](https://en.wikipedia.org/wiki/IEEE_754). In this case I think we
probably want to avoid mentioning `-NaN` in spec completely.
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