Cloiudant using $nin There is no index available for this selector - cloudant

Question

I created a JSON index in cloudant on _id like so:
{
"index": {
"fields": [ "_id"]
},
"ddoc": "mydesigndoc",
"type": "json",
"name": "myindex"
}
First off, unless I specified the index name, somehow cloudant could not differentiate between the index I created and the default text based index for _id (if that is truly the case, then this is a bug I believe)
I ran the following query against the _find endpoint of my db:
{
"selector": {
"_id": {
"$nin":["v1","v2"]
}
},
"fields":["_id", "field1", "field2"],
"use_index": "mydesigndoc/myindex"
}
The result was this error:
{"error":"no_usable_index","reason":"There is no index available for this selector."}
if I change "$nin":["v1","v2"] to "$eq":"v1" then it works fine, but that is not the query I am after.
So in order to get what I want, I had to this to my selector "_id": {"$gt":null}, which now looks like:
{
"selector": {
"_id": {
"$nin":["v1","v2"],
"$gt":null
}
},
"fields":["_id", "field1", "field2"],
"use_index": "mydesigndoc/myindex"
}
Why is this behavior? This seems to be only happening if I use the _id field in the selector.
What are the ramifications of adding "_id": {"$gt":null} to my selector? Is this going to scan the entire table rather than use the index?
I would appreciate any help, thank you

Answer 1

Cloudant Query can use Cloudant's pre-existing primary index for selection and range querying without you having to create your own index in the _id field.
Unfortunately, the index doesn't really help when using the $nin operator - Cloudant would have to scan the entire database to check for documents which are not in your list - the index doesn't really get it any further forward.
By changing the operator to $eq you are playing to the strengths of the index which can be used to locate the record you need quickly and efficiently.
In short, the query you are attempting is inefficient. If your query was more complex e.g. the equivalent of WHERE colour='red' AND _id NOT IN ['a','b'] then a Cloudant index on colour could be used to reduce the data set to a reasonable level before doing the $nin operation on the remaining data.