How to use CouchDB Mango query (/db/_find) with an index to select multiple _id keys - couchdb

Question

I am using CouchDB 3.1.1 to perform Mango queries against a database containing a large number of documents. A very common requirement in my application is to perform queries on a very specific and dynamic set of documents. From what I understand at this moment, these are the only choices I have on how to confront my problem:
Make multiple requests to /db/_find each with a distinct "_id"
Make a single call to /db/_find
Of the ways I can accomplish the second choice:
Use an "$or" array on all the "_id": value pairs
Use an "$or" array on all the values of the "_id" key
The second choice is what I would prefer to use since making multiple POST requests would incur overhead. Unfortunately using "$or" seems to get in the way of the query engine making use of the "_id" index.
Thus, choice #1 returns with a speedy 2 ms per transaction but the results are not sorted (requiring my application to do the sorting). Choice #2, given an array of 2 _ids, regardless of the $or syntax, takes over 3 seconds to render.
What is the most efficient way to use a CouchDB Mango query index against a specific set of documents?
Fast Example: Results using a single _id
{
"selector": {
"_id": "184094"
},
"fields": [
"_id"
]
}
documents examined: 26,312
results returned: 1
execution time: 2 ms
Slow Example: Results using $or of key / value pairs
{
"selector": {
"$or": [
{
"_id": "184094"
},
{
"_id": "157533"
}
]
},
"fields": [
"_id"
]
}
documents examined: 26,312
results returned: 2
execution time: 2,454 ms
Slow Example: Results using $or array of values
{
"selector": {
"_id": {
"$or": [
"184094",
"157533"
]
}
},
"fields": [
"_id"
]
}
documents examined: 26,312
results returned: 2
execution time: 2,522 ms
Slow Example: Results using $in (which is illegal but still returns results)
{
"selector": {
"_id": {
"$in": [
"184094",
"157533"
]
}
},
"fields": [
"_id"
]
}
documents examined: 26,312
results returned: 2
execution time: 2,618 ms
Index: The registered index for _id
{
"_id": "_design/508b5b51e6085c2f96444b82aced1e5dfec986b2",
"_rev": "1-f951eb482f9a521752adfdb6718a6a59",
"language": "query",
"views": {
"foo-index": {
"map": {
"fields": {
"_id": "asc"
},
"partial_filter_selector": {}
},
"reduce": "_count",
"options": {
"def": {
"fields": [
"_id"
]
}}}}}
Explain: An 'explain' summary done to one of the slow queries. Note that the registered index was used.
{
"dbname": "dnp_person_comment",
"index": {
"ddoc": "_design/508b5b51e6085c2f96444b82aced1e5dfec986b2",
"name": "foo-index",
"type": "json",
"partitioned": false,
"def": {
"fields": [
{
"_id": "asc"
}
]
}
},
"partitioned": false,
"selector": {
"$or": [
{
"_id": {
"$eq": "184094"
}
},
{
"_id": {
"$eq": "157533"
}
}
]
},
"opts": {
"use_index": [],
"bookmark": "nil",
"limit": 25,
"skip": 0,
"sort": {},
"fields": [
"_id"
],
"partition": "",
"r": [
49
],
"conflicts": false,
"stale": false,
"update": true,
"stable": false,
"execution_stats": false
},
"limit": 25,
"skip": 0,
"fields": [
"_id"
],
"mrargs": {
"include_docs": true,
"view_type": "map",
"reduce": false,
"partition": null,
"start_key": [],
"end_key": [
"<MAX>"
],
"direction": "fwd",
"stable": false,
"update": true,
"conflicts": "undefined"
}
}