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BH Column Accessors
(Submitted as a PR: /vector/src/main/java/org/apache/drill/exec/vector/accessor/Accessors.md)
The column accessors implement a (relatively) simple API to work with a collection of vectors. They form the foundation of both the row set reader/writer and the result set loader.
The accessors started with the design of the existing "complex writers", then evolved in a distinct direction as we describe here. Like the complex writers, the column accessors somewhat follow the JSON model. While Drill provides only a complex "writer" (but no reader), the column accessors provide both readers and writers. As much as possible, the API is similar between the two.
This section describes the common structure; later pages dive into the specifics of the writers and readers.
Accessors are based on a structure, expressed here as pseudo-BNF:
Tuple : Object*
Object : Scalar | Array | Tuple | Variant
Scalar : Nullable Scalar | Non-Nullable Scalar
Array : Scalar Array | Map Array | List | Repeated List
Variant : Object*
Repeated List : Array
Scalar Array : Scalar
Map Array : Tuple
The tuple accessor represents both a row and a map and provides both name and index access to a set of columns. In the writer, the tuple is the unit of schema evolution, providing methods to add new columns on the fly.
The object accessor represents a column in Drill. That is, it represents a vector. The object accessor wraps the actual accessor, whether it be a scalar, array, map or whatever. The purpose of the object is to allow generic operations on accessors: all members of a tuple are objects, one then queries the object for its type and treats the column accordingly. In general, however, convenience methods on the accessors hide the object concept when not needed.
The object accessor for a map wraps a tuple accessor, consistent with the idea that, conceptually, maps and rows are both tuples.
The array is a generic concept and consists of two parts: the array and the "entry". The array handles indexing over the array elements. The entry handles access to each entry. The entry is just one of the other accessors. (For a repeated list, it is just another array for example.) Thus, code that works with values need not care if the value is a top-level column, a column within a map, or an entry in an array.
The variant accessor represents both a union and a (non-repeated) list. (The list is a very odd duck.) The term "variant" is borrowed from Microsoft (which borrowed it from earlier systems) and simply means a union with a type field. The Drill term "union" was avoided for two reasons. 1) Constant confusion with the UNION operator in SQL, and 2) The union implementation is quick and dirty and seems like a good candidate for rethinking.
A variant is like a map, but it is a map keyed by type, not by name. When used in a (non-repeated) list, the union can be empty, a single item (which, in the list, is not a union at all) or a true union of two or more types. (Yes, indeed the list vector is odd.)
Every accessor provides an ObjectType value that describes the above accessor types. Thus, interpreted code can get a column, check the object type, and decide how to proceed. The object type is akin to the JSON object type, but with specific types tailored to Drill.
Drill provides 30+ data types (not all of which are supported or even fully implemented.) The "classic" complex writers provide a distinct writer class for each and every type: INT, SMALLINT, nullable INT, repeated INT, and so on. The column accessors take a more restrained approach. All scalar accessors provide the same interface. The implementations differ, but client code need not care. Each implementation provides implementations for one or more of the get or set methods, throwing exceptions for the others. Further, the get and set method revolve around Java types, not Drill types. So, for example, TINYINT, SMALLINT, INT, UINT1 and UINT2 all implement the same setInt() and getInt() methods.
To allow interpreted access, each scalar reader provides a value type expressed as an enum:
ValueType valueType();
The value type identifies which get or set method is implemented. (Accessors are free to implement other methods in addition to the main one indicated by the value type.)
One open issue is how best to deal with INTERVAL and DECIMAL types. At present, these are passed as Joda objects (INTERVAL, DATE, TIME) and BigDecimal (DECIMAL types.) Production code needs a different representation that avoids object allocations for every value. This is a straightforward addition best done once we figure out the mechanism that will best work for generated code. (The current technique in generated code, a separate get() or set() call for each field, is inefficient.)
All the accessors defined above derive from a common base class, either ColumnReader or ColumnWriter. The base class provides generic services:
- Access to metadata for the column
- The object type (described above)
- Generic
Object get()andset(Object value)methods
Readers and writes implement the concept of an "index": a pointer to the current read or write position. Writers increment the index, readers allow random access.
Consider a set of three vectors: a, b, and c. In most Drill code, the application keeps track of the index. Then, the application passes its index, (rowIndex, say) to each vector for each access. For example: a.get(rowIndex) or b.set(rowIndex, value).
This code is tedious, but not hard. However, the complexity vastly increases when we consider arrays, especially map arrays. In this case, the application must keep track not only of a rowIndex, but also a repeatedMap1Index, a repeatedIntIndex and so on. Keeping all this straight is fraught with opportunities for bugs, especially since there is only exactly one right way to do the work.
The accessors take over this issue. In arrays, the accessors introduce new "local" indexes for the array. Child accessors use the "inner" index for array items. If we have multiple levels of repeated map, then we have multiple levels of index. The application does nothing other than tell the reader or writer to advance.
Readers must deal with another level of complexity: selection vectors. The reader index encapsulates this indirection: the application asks for row 7, the reader figures out that this is actually row 1234 (for an SV2), or row 4567 of batch 17 (for an SV4.)
In short, the index removes all the index complexity from operators and readers and places it into the accessors where it can be implemented and tested once and for all.
The RowSetTest class shows examples of working with readers and writers directly. The RowSetBuilder shows an example of generic (interpreted) use of the writer. RowSetPrinter and RowSetComparison show interpreted use of the reader.