CODEWITHSUNDEEP
couchdb
cllpse
cllpse

Reputation: 21727

CouchDB - Parameter and views - What goes on behind the scenes, and is it fast/faster than temporary views?

Considering these three documents...

[
    {
        _id: "...",
        _rev: "...",
        title: "Foo",
        body: "..."
    },
    {
        _id: "...",
        _rev: "...",
        title: "Bar",
        body: "..."
    },
    {
        _id: "...",
        _rev: "...",
        title: "Hello World!",
        body: "..."
    },
]

And this view...

byTitle: {
    map: function (document)
    {
        emit(document.title, document);
    }
}

What goes on behind the scenes, when I query the view?...

GET /database/_design/posts/_view/byTitle?key="Foo"


I've asked a few questions on views lately... questions about what I phrased as "dynamic parameters"... Essentially I wanted to know how to do the equivalent of SELECT ... WHERE field = parameter

All answers steered me towards using temporary views, which are really slow, and should not be used in production. So my second question is... is the above method for querying by title, fit for use in production? Or am I forcing CouchDB to do unspeakable horrors, performance-wise?... am I essentially doing the same as using a temporary view?

Upvotes: 2

Views: 665

Answers (2)

Marcello Nuccio
Marcello Nuccio

Reputation: 3901

I think you have misinterpreted some answer. You can use a temporary view to test various map/reduce functions. When you are satisfied with the code you should put it into a design document and use it for querying.

Temporary views are slow because the index is built and deleted for every query. Putting it into a design document, tells CouchDB to not delete the index and to keep it updated (this is done on query time).

So

GET /database/_design/posts/_view/byTitle?key="Foo"

is the fastest way to query by title because it is indexed.

As a side note: you can use

byTitle: {
    map: function (document)
    {
        emit(document.title, null);
    }
}

and query with include_docs=true to save some disk space.

Upvotes: 6

chris polzer
chris polzer

Reputation: 3367

For answering your question, a few things have to be cleared out (and I hope I get it all right):

Permanent vs. temporary views: The difference between permanent and temporary views is, that permanent views are stored permanently.

In order to understand the storing part, you need to know, that CouchDB's storage engine relies on a B+ Tree offering very powerful indexing capabilities that enable us to find data in that storage by key in a "logarithmic amortized time" (CouchDB book).

CouchDB is handling documents in an "append only" manner. That means it is not like in the most relational DBMS where single values within a table row get updated and locking occurs. If a document is updated, it simply incrementally is set a new revision (_rev) and is appended to the storage.

When you are creating a permanent view, upon querying it the first time, for each document in your database, your new view is executed, storing that data to a new B+ tree file for that view, thus providing a new index to aggregate data according to the key you defined in your view.

Upon updating documents that are handled by that view, not the whole permanent view needs to be recomputed, but only the updated documents.

Now you should be able to understand why temporary views are nice for developing or testing in Futon, but since they have to be computed new for all your documents are not recommended for anything else than development.

Anyways. Marcello is right. If you are intending to just pass back complete documents, it is are encouraged to query with "include_docs=true". Why? Because the B-tree for your permanent view will just need to store the copied data next to your indexing key.

@Marcello-Nuccio I am not sure although if it is correct to say, that dynamic views have no index? As I understood, they have an index, but it makes no sense as they are computed new upon every query? Ok, now my brbain is hurting!

Upvotes: 2

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