Github user cloud-fan commented on a diff in the pull request:
https://github.com/apache/spark/pull/13680#discussion_r73814104
--- Diff:
sql/catalyst/src/main/java/org/apache/spark/sql/catalyst/expressions/UnsafeArrayData.java
---
@@ -25,55 +25,57 @@
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 array 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.
+ * The `numElements` is 8 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. It can only be calculated by
knowing the total bytes of
--- End diff --
@davies, are you saying that we should make each variable-length element in
the `variable length portion` word-aligned? Then we need to follow `UnsafeRow`
and store both offset and length in `offset region`
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