[GitHub] [iceberg] yyanyy commented on a change in pull request #2055: Spec: add sort order to spec

[GitBox <gi...@apache.org>]

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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