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Couchdb Mango Performance vs Map Reduce Views

问题描述:

I've just noticed that in the release notes of Couchdb 2.0, it is mentionned that Mango queries are recommended for new applications. It is also mentionned that apparently Mango indexes are from 2x to x10 faster than javascript queries which really surprised me, as such I have a number of questions :

  • Are Map/Reduce views being phased out ? I'm expecting the answer to be no since it seems to me that Mango does not cover all the use cases of Map/Reduce (the easiest example being Reduce itself), and the flexibility of this querying style seems to be more limited too. But m prefer to ask because of the recommendation :

We recommend all new apps start using Mango as a default.

  • We know that Map/Reduce views rely on B-trees, but I can't find any insight, in the doc or the mailing list regarding the magic behind Mango. Mango essentially is white magic for me at the minute. Yet I can tell that having an in-depth knowledge of how the javascript views are indexed behind the scenes was massively helpful to avoid pitfalls, naive implementations as well as to optimize performances. Does anyone have any insight on how Mango works ? Are the indexes B-trees too ? When are the indexes updated since there is no longer design documents ? Where do the performance gains come from ? (these gains are counter-intuitive to me, since in my understanding, the performance of javascript queries came from the precomputed nature of Map functions)

What I'm essentially after is on the one hand some insight regarding Mango and on the other hand, an overview of how Mango and Map/Reduce are supposed to live together in the 2.x era.

网友答案:

I recently tried to switch my app over to using Mango queries, with the result of scrapping it completely and switching back to map/reduce. Here are a few of my reasons:

  1. Mango is buggy when dealing with queries that do not exactly specify the index to use. This one drove me batty for a while last weekend. If you don't specify the index, sometimes an alternate index will be selected and return no (or incorrect) results.
  2. Mango performance is not 'magic'. Many types of queries will end up doing in memory searches. Couch will select the best fit index then march through all those records in memory to fit the corner cases. Cloudant hand waves over some of these issues by saying to use 'text' based searches, which aren't available in Couchdb.
  3. As you pointed out, Mango searches simply cannot handle some types of query constructions well. I wouldn't consider my app to be overly complicated yet I ran into several situations where I could not construct a suitable Mango query for the task at hand. A major one here is searching arrays to find tags (for example, searching to see what users are members of a group). Mango cannot index array elements so resorts to doing full scans in memory.
    1. Views have some very powerful features for transformation of search results in the form of Lists. That doesn't exist in Mango.

Your mileage may vary, but just wanted to leave a warning that this is still quite new features.

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