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https://issues.apache.org/jira/browse/COUCHDB-3280?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15863139#comment-15863139
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Adam Kocoloski commented on COUCHDB-3280:
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Hi Tony, that's largely how I'd interpreted this idea. The key is that Mango is
not trying to inspect the custom user code, but simply allowing for queries
against the index created by that custom code to be specified using the Mango
syntax. As I mentioned on dev@ I think this is a very good idea.
> Computed Indices in Mango Query Server
> --------------------------------------
>
> Key: COUCHDB-3280
> URL: https://issues.apache.org/jira/browse/COUCHDB-3280
> Project: CouchDB
> Issue Type: Wish
> Components: Mango
> Reporter: Norbert Nemec
>
> The Mango Query Server aims at offering a simpler alternative to MapReduce
> queries. As it stands now, it is also fundamentally limited in terms of
> expressiveness an performance. Indices can be defined only over a plain set
> of fields with none of the possibilities that a map function offers.
> Selectors allow powerful combinations, but require to perform much of the
> computational effort at query time. The $regex operator, for example even
> contains a warning about this in the documentation.
> My proposal would be to add an alternative way to define indices by
> explicitly providing a design document map function. The 'fields' argument of
> such a "computed index" would not have to exist as an actual field in the
> database, but would be made available as a "computed field" for regular use
> in subsequent find requests.
> Ultimately, this approach would bridge the gap between the simple-but-limited
> Mango queries and the powerful-but-unwieldy MapReduce queries. Rather than
> having to decide between both approaches, developers could start with the
> simple Mango approach and then just learn one more concept if they need the
> full power.
> (It is my understanding that the currently recommended approach is to add
> computed fields to the documents directly at creation time. Though this is a
> workaround for the limitations of selectors, it does not offer any guarantees
> for internal consistency of a database and mixes the concerns of content
> creation with those of retrieval.)
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