tables-java.rst

Tables

:doc:`Python <tables>` Java

Goal

Create a table data structure suitable for sparse data.

Challenge

Support efficient random access to individual cells in a table, as well as retrieval of all cells in a particular row or all cells in a particular column.

Explanation

Tables give us a representation for two-dimensional data with labeled rows and columns. (Column labels are common in data sets. For rows, a primary key, such as an entity ID, can be used.) Each cell in the table will be modeled using two key-value pairs, one in row-dominant order and one in column-dominant order.

Ordering

By storing the table in both row order and column order, we can support efficient retrieval of entire rows or columns with a single range read.

Pattern

We construct a key from a tuple containing the row and column identifiers. Unassigned cells in the tables will consume no storage, so sparse tables are stored very efficiently. As a result, a table can safely have a very large number of columns.

Using the lexicographic order of tuples, we can store the data in a row-oriented or column-oriented manner by placing either the row or column first in the tuple, respectively. Placing the row first makes it efficient to read all the cells in a particular row with a single range read; placing the column first makes reading a column efficient. We can support both access patterns by storing cells in both row-oriented and column-oriented layouts, allowing efficient retrieval of either an entire row or an entire column.

We can create a subspace for the table and nested subspaces for the row and column indexes. Setting a cell would then look like:

tr.set(rowIndex.subspace(Tuple.from(row, column)).getKey(), pack(value));
tr.set(colIndex.subspace(Tuple.from(column, row)).getKey(), pack(value));

Extensions

Higher dimensions

This approach can be straightforwardly extended to N dimensions for N > 2. Unless N is small and your data is very sparse, you probably won't want to store all N! index orders, as that could consume a prohibitive amount of space. Instead, you'll want to select the most common access patterns for direct storage.

Code

Here’s a simple implementation of the basic table pattern:

import java.util.Map;
import java.util.TreeMap;
public class MicroTable {
    private static final FDB fdb;
    private static final Database db;
    private static final Subspace table;
    private static final Subspace rowIndex;
    private static final Subspace colIndex;

    static {
        fdb = FDB.selectAPIVersion(600);
        db = fdb.open();
        table = new Subspace(Tuple.from("T"));
        rowIndex = table.subspace(Tuple.from("R"));
        colIndex = table.subspace(Tuple.from("C"));
    }

    // Packing and unpacking helper functions.
    private static byte[] pack(Object value){
        return Tuple.from(value).pack();
    }

    private static Object unpack(byte[] value){
        return Tuple.fromBytes(value).get(0);
    }

    public static void setCell(TransactionContext tcx, final String row,
                                final String column, final Object value){
        tcx.run(tr -> {
            tr.set(rowIndex.subspace(Tuple.from(row, column)).getKey(),
                    pack(value));
            tr.set(colIndex.subspace(Tuple.from(column,row)).getKey(),
                    pack(value));

            return null;
        });
    }

    public static Object getCell(TransactionContext tcx, final String row,
                                final String column){
        return tcx.run(tr -> {
            return unpack(tr.get(rowIndex.subspace(
                    Tuple.from(row,column)).getKey()).get());
        });
    }

    public static void setRow(TransactionContext tcx, final String row,
                                final Map<String,Object> cols){
        tcx.run(tr -> {
            tr.clear(rowIndex.subspace(Tuple.from(row)).range());

            for(Map.Entry<String,Object> cv : cols.entrySet()){
                setCell(tr, row, cv.getKey(), cv.getValue());
            }
            return null;
        });
    }

    public static void setColumn(TransactionContext tcx, final String column,
                                    final Map<String,Object> rows){
        tcx.run(tr -> {
            tr.clear(colIndex.subspace(Tuple.from(column)).range());
            for(Map.Entry<String,Object> rv : rows.entrySet()){
                setCell(tr, rv.getKey(), column, rv.getValue());
            }
            return null;
        });
    }

    public static TreeMap<String,Object> getRow(TransactionContext tcx,
                                                final String row){
        return tcx.run(tr -> {
            TreeMap<String,Object> cols = new TreeMap<String,Object>();

            for(KeyValue kv : tr.getRange(
                    rowIndex.subspace(Tuple.from(row)).range())){
                cols.put(rowIndex.unpack(kv.getKey()).getString(1),
                        unpack(kv.getValue()));
            }

            return cols;
        });
    }


    public static TreeMap<String,Object> getColumn(TransactionContext tcx,
                                                final String column){
        return tcx.run(tr -> {
            TreeMap<String,Object> rows = new TreeMap<String,Object>();

            for(KeyValue kv : tr.getRange(
                    colIndex.subspace(Tuple.from(column)).range())){
                rows.put(colIndex.unpack(kv.getKey()).getString(1),
                        unpack(kv.getValue()));
            }

            return rows;
        });
    }
}

That’s about all you need to store and retrieve data from simple tables.