MongoDB中group() mapReduce() aggregate()之比較

對於SQL而言，如果從users表裏查詢每個team所有成員的number，查詢語句如下：

SELECT team, no FROM users GROUP BY team                             (1)

但是對於Mongodb而言，實現這樣的功能，則比較複雜。

從mongodb2.2之後，有了三個function可以實現這個功能，他們按照產生的順序，分別是group(), mapReduce()和aggregate().

他們之間的區別有哪些呢？參照stack overflow上討論http://stackoverflow.com/questions/12337319/mongodb-aggregation-comparison-group-group-and-mapreduce整理如下：

1.     db.collection.group().

定義爲:

Db.collection.group(
			key,
			reduce,
			initial,
			keyf,
			cond,
			finalize).

特徵爲：

• Simple syntax and functionality for grouping .. analogous to GROUP BY in SQL.
• Returns result set inline (as an array of grouped items).
• Implemented using the JavaScript engine; custom reduce() functions can be written in JavaScript.
• Current Limitations
  • Will not group into a result set with more than 10,000 keys.
  • Results must fit within the limitations of a BSON document (currently 16Mb).
  • Takes a read lock and does not allow any other threads to execute JavaScript while it is running.
  • Does not work with sharded collections.

Ex: 如果需要實現語句1的功能，實現如下：

db.users.group({key: {team: 1}, initial: {members: []}, reduce: function(cur, result){result.members.push(cur.no);}});

2.     db.collection.mapReduce().

據說增加mapreduce是爲了迎合mapreduce的流行。

db.collection.mapReduce(
                         <mapfunction>,
                        <reducefunction>,
                         {
                           out: <collection>,
                           query: <document>,
                           sort: <document>,
                           limit: <number>,
                           finalize: <function>,
                           scope: <document>,
                           jsMode: <boolean>,
                           verbose: <boolean>
                         }
                       )

特徵爲：

• Implements the MapReduce model for processing large data sets.
• Can choose from one of several output options (inline, new collection, merge, replace, reduce)
• MapReduce functions are written in JavaScript.
• Supports non-sharded and sharded input collections.
• Can be used for incremental aggregation over large collections.
• MongoDB 2.2 implements much better support for sharded map reduce output.
• Current Limitations
  • There is a JavaScript lock so a mongod server can only execute one JavaScript function at a point in time .. however, most steps of the MapReduce are very short so locks can be yielded frequently.
  • MapReduce functions can be difficult to debug. You can use print() and printjson() to include diagnostic output in the mongod log.
  • MapReduce is generally not intuitive for programmers trying to translate relational query aggregation experience.

由於需要用到js engine，所以速度是比較慢的，具體的可以參照http://technicaldebt.com/?p=1157

 

Ex: 如果需要實現語句1的功能，實現如下：

var map = function(){ emit(this.team, this.no); }; 
var reduce = function(key, value){ return {team: key, members: value}; };
db.users.mapReduce(map, reduce, {out: "team_member"});

3.     db.collection.aggregate().

For simplertasks, mapReduce is big hammer. And avoid overhead of JavaScript engine, alsoselect matching subdocuments and arrays. Aggregate framework is implementedwithpipelinein C++.

Pipeline 定義的操作有:

$match – query predicate as a filter.

$project – use a sample document todetermine the shape of the result.

$unwind – hands out array elements oneat a time.

$group – aggregates items into bucketsdefined by a key.

$sort – sort document.

$limit – allow the specified number ofdocuments to pass

$skip – skip over the specified numberof documents.

特徵如下：

• New feature in the MongoDB 2.2.0 production release (August, 2012).
• Designed with specific goals of improving performance and usability.
• Returns result set inline.
• Supports non-sharded and sharded input collections.
• Uses a "pipeline" approach where objects are transformed as they pass through a series of pipeline operators such as matching, projecting, sorting, and grouping.
• Pipeline operators need not produce one output document for every input document: operators may also generate new documents or filter out documents.
• Using projections you can add computed fields, create new virtual sub-objects, and extract sub-fields into the top-level of results.
• Pipeline operators can be repeated as needed (for example, multiple $project or $groupsteps.
• Current Limitations
  • Results are returned inline, so are limited to the maximum document size supported by the server (16Mb)
  • Doesn't support as many output options as MapReduce
  • Limited to operators and expressions supported by the Aggregation Framework (i.e. can't write custom functions)
  • Newest server feature for aggregation, so has more room to mature in terms of documentation, feature set, and usage.

Ex: 如果需要實現語句1的功能，實現如下：

db.users.aggregate({$project: {team: 1, no: 1}}, {$group: { _id: "$team", memebers: {$addToSet: "$no"}}});

Refs:

http://docs.mongodb.org/manual/aggregation/#Aggregation-Examples

http://docs.mongodb.org/manual/reference/method/db.collection.group/

http://technicaldebt.com/?p=1157

http://stackoverflow.com/questions/12337319/mongodb-aggregation-comparison-group-group-and-mapreduce