Github user davies commented on a diff in the pull request:
https://github.com/apache/spark/pull/13680#discussion_r73569132
--- 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 --
If the total length of the array is aligned to 8-byte, we should have N+1
offset in order to know the length of last element.
Another trick could be we store the end of each element instead of offset,
because we could know the offset of the first element (elementOffset + 4 * n),
it's a little bit confusing.
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