big-r81 commented on issue #3998:
URL: https://github.com/apache/couchdb/issues/3998#issuecomment-1102812121
I think the answer is much simpler:
There is a special defualt index only for `_id` which is for me:
```json
{
"total_rows": 2,
"indexes": [
{
"ddoc": null,
"name": "_all_docs",
"type": "special",
"def": { "fields": [{ "_id": "asc" }] }
},
{
"ddoc": "_design/c3f8e5d6df2ef807ee1016949098ac4e6287d31c",
"name": "tags-json-index",
"type": "json",
"partitioned": false,
"def": { "fields": [{ "_id": "asc" }, { "shri": "asc" }] }
}
]
}
```
If I only sort for `_id` the "special" index is used. If I use
```json
"sort" : ["_id", "shri"]
```
in the query, with the following ddoc
```json
{
"_id": "_design/c3f8e5d6df2ef807ee1016949098ac4e6287d31c",
"_rev": "3-bbb25abe2218156af2ad8a5d6648ce1d",
"language": "query",
"views": {
"tags-json-index": {
"map": {
"fields": {
"_id": "asc",
"shri": "asc"
},
"partial_filter_selector": {}
},
"reduce": "_count",
"options": {
"def": {
"fields": [
"_id",
"shri"
]
}
}
}
}
}
```
then the the mango query uses this index...
```json
{
"dbname": "test",
"index": {
"ddoc": "_design/c3f8e5d6df2ef807ee1016949098ac4e6287d31c",
"name": "tags-json-index",
"type": "json",
"partitioned": false,
"def": {
"fields": [
{
"_id": "asc"
},
{
"shri": "asc"
}
]
}
},
"partitioned": false,
"selector": {
"$and": [
{
"shri.tags": {
"$elemMatch": {
"": {
"$eq": "java"
}
}
}
},
{
"_id": {
"$gte": "0"
}
}
]
},
"opts": {
"use_index": [],
"bookmark": "nil",
"limit": 501,
"skip": 0,
"sort": {
"_id": "asc",
"shri": "asc"
},
"fields": [
"_id"
],
"partition": "",
"r": [
49
],
"conflicts": false,
"stale": false,
"update": true,
"stable": false,
"execution_stats": false
},
"limit": 501,
"skip": 0,
"fields": [
"_id"
],
"mrargs": {
"include_docs": true,
"view_type": "map",
"reduce": false,
"partition": null,
"start_key": [
"0"
],
"end_key": [
"<MAX>"
],
"direction": "fwd",
"stable": false,
"update": true,
"conflicts": "undefined"
}
}
```
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