A LevelUP plugin to merge multiple ranges into a single sorted stream using a subkey for ordering. Useful when doing range queries against multiple key-based secondary indexes. Supports standard LevelUP stream options and naïve pagination via a skip option.

npm install level-merged-stream

Given a LevelUP instance db:

var mergedStream = require('level-merged-stream');

db = mergedStream(db);

• db.createMergedReadStream()
• db.createMergedKeyStream()
• db.createMergedValueStream()

As per db.createReadStream() but with the following additional options:

• 'ranges' (array): The start/end ranges as you'd provide to createReadStream. Each range is streamed and merged according to the value privided by subkey. Each range must be ordered by subkey, i.e. any range prefix must be constant for the entire range for overall ordering to be consistent.

• 'subkey' (function, default: identity): Selects the subkey from the key that is used for ordering the results.

• 'comparator' (function, default: primitive comparison): A subkey comparator function if primitive comparison is insufficient, .e.g. for Buffers or custom keys.

• 'dedupe' (boolean/function, default: false): If true, when two or more subkeys are considered equal, only the first result will be streamed. The key and/or value returned could come from any underlying range stream. If a function, it should return true if two keys passed are considered equal for deduplication purposes.

• 'skip' (number, default: 0): The number of results to skip in the merged stream. Useful for pagination when also using limit.

As per db.createKeyStream() this streams only the keys.

As per db.createValueStream() this streams only the values.

examples/readme.js:

var path = require('path');
var level = require('level');
var mergedStream = require('level-merged-stream');

var location = path.join(__dirname, '/.db');
var db = level(location);
db = mergedStream(db);

db.batch()
  .put('a1', '1')
  .put('b2', '2')
  .put('c3', '3')
  .put('d4', '4')
  .put('a5', '5')
  .put('b6', '6')
  .put('c7', '7')
  .put('d8', '8')
  .write(function () {
    db.mergedReadStream({
      // Only stream the 'a's and 'c's
      ranges: [
        { start: 'a', end: 'b' },
        { start: 'c', end: 'd' }
      ],
      // Ignore the first character for sorting
      subkey: function (key) {
        return key.slice(1);
      },
      skip: 1,
      limit: 2
    })
      .on('data', console.log)
      .on('end', function () {
        db.close(function () {
          level.destroy(location);
        });
      });
  });

Output:

{ key: 'c3', value: '3' }
{ key: 'a5', value: '5' }

MIT