CosmosDB Distinct query - slow performance - performance

Question

We are using CosmosDB and I'm running a Distinct query as follows
Select Distinct c.SomeType, c.SomeName
From c
Where c.ptkey = 'WHATEVERPTKEY'
And c.SomeCategory = 'WhateverCategory'
...where ptkey is the field that holds the partition key. The above works but takes around 1-1.5 minutes to complete (I'm assuming because some/many of the documents are very large) - I've tried filtering on the partition's unique key (id), using a "Group By", and played with "Order By" (restrictions apply when you combine the two and/or only one field is allowed in the Order By unless you have a composite key), but not much changes.
The one thing that does make a big difference is creating an Indexing Policy as follows
"compositeIndexes": [
[
{
"path": "/ptkey"
},
{
"path": "/SomeCategory"
},
{
"path": "/SomeType"
},
{
"path": "/SomeName"
}
]
]
...however, my question is how do I limit this composite key definition to only apply to the specific partition key the above query is for ('WHATEVERPTKEY' - as we have around a dozen partition keys within our database/collection) and secondly is there any alternative/better option (other than re-modelling our data)
Note my Query Stats when running the query in Azure CosmosDB Data Explorer without an index are as follows
Query Statistics
746.96 RUs
1 - 12
Retrieved document count 0
Retrieved document size 0 bytes
Output document count 0
Output document size 1789 bytes
Index hit document count 0
Index lookup time 1.1900000000000002 ms
Document load time 366.48990000000003 ms
Query engine execution time 29.0101 ms
System function execution time 0.76 ms
User defined function execution time 0 ms
Document write time 0.02 ms
1
UPDATE
full indexing policy of the collection is below
{
"indexingMode": "consistent",
"automatic": true,
"includedPaths": [
{
"path": "/*"
}
],
"excludedPaths": [
{
"path": "/\"_etag\"/?"
}
],
"compositeIndexes": [
[
{
"path": "/ptkey",
"order": "ascending"
},
{
"path": "/SomeCategory",
"order": "ascending"
},
{
"path": "/SomeType",
"order": "ascending"
},
{
"path": "/SomeName",
"order": "ascending"
}
]
]
}

Answer 1

So the query metrics in the portal can sometimes not show accurate data when there are LOTS of multiple pages of results but it does often work.
With DISTINCT the cost can depend on how many results you're dealing with. If you're expecting just a few results the impact on cost is low. If you're into thousands, it can get very expensive. Work is happening to make that less expensive. It's a ways out before it will be released.
Can you try again with just this as your index policy?
{
"indexingMode": "consistent",
"automatic": true,
"includedPaths": [
{
"path": "/*"
}
],
"excludedPaths": [
{
"path": "/\"_etag\"/?"
}
],
"compositeIndexes": [
[
{
"path": "/ptkey",
"order": "ascending"
},
{
"path": "/SomeCategory",
"order": "ascending"
}
]
]
}