mergeEntry = function(entry, entryTables) {
    if (entry.isPrefixApplied_) {
      var payload = entry.row.payload();
      for (var prefix in payload) {
        result[prefix] = payload[prefix];
      }
    } else {
      goog.asserts.assert(
          !result.hasOwnProperty(entryTables[0]),
          'Attempted to join table with itself, without using table alias, ' +
          'or same alias ' + entryTables[0] + 'is reused for multiple tables.');

      // Since the entry is not prefixed, all attributes come from a single
      // table.
      result[entryTables[0]] = entry.row.payload();
    }
  }
Combines two entries into a single entry.
@param {!lf.proc.RelationEntry} leftEntry
@param {!Array<string>} leftEntryTables The names of all source tables for
    the attributes in leftEntry.
@param {!lf.proc.RelationEntry} rightEntry
@param {!Array<string>} rightEntryTables The names of all source tables for
    the attributes in rightEntry.
@return {!lf.proc.RelationEntry} The combined entry.