Couchdb super slow view, 100% cpu usage - couchdb

Question

There is one account doc. This doc has ~1k seats. For each seat, we emit a doc. Naturally, you'd expect this to be slow. The map function runs like this:
function(doc) {
if (doc.type == 'account') {
doc.seats.map(function(seat) {
emit(seat.userID, doc))
}
}
}
However deleting doc.seats, then emitting the much smaller doc didn't seem to help.
function(doc) {
if (doc.type == 'account') {
doc.seats.map(function(seat) {
delete doc.seats
emit(seat.userID, doc))
}
}
}
Does anyone understand why deleting the seats doesn't speed this up? The only way we could speed it up was by not emitting the doc object, and just emitting an id.
function(doc) {
if (doc.type == 'account') {
doc.seats.map(function(seat) {
emit(seat.userID, doc.id))
}
}
}
Is this a problem with looping over a doc's array in a couch view map?

Answer 1

tldr;
Use a permanent view if you care about performance
doc is immutable from a view. You can't even add to it without making a copy.
It's almost always better to emit the _id and use include_docs than it is to emit an entire doc as your value.
explanation
Here are a couple of points to your question, using your example document which contains an array called seats with 1K entries.
Emitting the entire doc here is a bad idea. If this is a permanent view (which you should always use if performance is at all an issue), you've taken one copy of doc, and then made 1000 copies and indexed them by the seat.userID. This isn't efficient. It's worse as a temporary view because then it's generated on the fly, in memory, each time the view is called.
AFAIK the doc is totally immutable as accessed via view, so the way you're attempting to delete the seats field doesn't work. Therefore the deleting doc.seats shouldn't provide any performance gain since you're still going to complete the loop and create 1000 copies of the original doc. You can however make a deep copy of doc that doesn't have seats included and pass that through emit.
For example:
function(doc) {
var doc_without_seats = JSON.parse(JSON.stringify(doc))
doc_without_seats['seats'] = null;
doc.seats.map( function (seat){
emit(seat.userID, doc_without_seats);
});
}
You're certainly on the right track with emitting the doc._id instead of the doc. The index you're building in this case is at largest, 1/ 1000th the size. If you still need to access the entire document, you can pass the option include_docs=true to the view when you query it. This keeps the whole doc from being copied in the index.
Another potential optimization would be to just emit the things you'll want to reference when looking something up by seat.userID. If that's still large and unwieldy, use the include_docs method.