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From davies <...@git.apache.org>
Subject [GitHub] spark pull request #13680: [SPARK-15962][SQL] Introduce implementation with ...
Date Thu, 04 Aug 2016 17:34:42 GMT
Github user davies commented on a diff in the pull request:

    https://github.com/apache/spark/pull/13680#discussion_r73567215
  
    --- Diff: sql/catalyst/src/main/java/org/apache/spark/sql/catalyst/expressions/UnsafeArrayData.java
---
    @@ -25,30 +25,36 @@
     import org.apache.spark.sql.types.*;
     import org.apache.spark.unsafe.Platform;
     import org.apache.spark.unsafe.array.ByteArrayMethods;
    +import org.apache.spark.unsafe.bitset.BitSetMethods;
     import org.apache.spark.unsafe.hash.Murmur3_x86_32;
     import org.apache.spark.unsafe.types.CalendarInterval;
     import org.apache.spark.unsafe.types.UTF8String;
     
     /**
      * An Unsafe implementation of Array which is backed by raw memory instead of Java objects.
      *
    - * Each tuple has three parts: [numElements] [offsets] [values]
    + * Each tuple has four parts: [numElements][null bits][values or offset][variable length
portion]
      *
      * The `numElements` is 4 bytes storing the number of elements of this array.
      *
    - * In the `offsets` region, we store 4 bytes per element, represents the relative offset
(w.r.t. the
    - * base address of the array) of this element in `values` region. We can get the length
of this
    - * element by subtracting next offset.
    - * Note that offset can by negative which means this element is null.
    + * In the `null bits` region, we store 1 bit per element, represents whether a element
has null
    + * Its total size is ceil(numElements / 8) bytes, and  it is aligned to 8-byte word boundaries.
      *
    - * In the `values` region, we store the content of elements. As we can get length info,
so elements
    - * can be variable-length.
    + * In the `values or offset` region, we store the content of elements. For fields that
hold
    + * fixed-length primitive types, such as long, double, or int, we store the value directly
    + * in the field. For fields with non-primitive or variable-length values, we store a
relative
    + * offset (w.r.t. the base address of the array) that points to the beginning of
    + * the variable-length field into int. its length can be got by subtracting 2 adjacent
offsets
    --- End diff --
    
    For the length of last var-length object, it can only be calculated by know the total
bytes of the array, could you also add a comment for this?


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