Norbert Nemec created COUCHDB-3280:
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Summary: 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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