cosmosdb为什么汇总计数功能中未使用索引？

Question

以下查询可以直接从索引中回答，因为“ partition”是分区键（并且在默认索引策略中是范围索引）。

"SELECT value count(1) FROM c WHERE c.partition = 'xxx'"

但是，查询指标显示未使用索引。相反，它会产生大量的RUs费用...

任何想法可能是什么问题？

var client = DatabaseClient.Client;
        IDocumentQuery<dynamic> query = client.CreateDocumentQuery(
            UriFactory.CreateDocumentCollectionUri(DatabaseClient.DatabaseName, DatabaseClient.CollectionName),
            "SELECT count(1) FROM c WHERE c.partition = 'xxx',
            new FeedOptions
            {
                PartitionKey = new PartitionKey("xxx"),
                PopulateQueryMetrics = true,
                MaxItemCount = -1,
                MaxDegreeOfParallelism = -1,
                EnableCrossPartitionQuery = false
            }).AsDocumentQuery();
        FeedResponse<dynamic> result = await query.ExecuteNextAsync();

        // Returns metrics by partition key range Id 
        IReadOnlyDictionary<string, QueryMetrics> metrics = result.QueryMetrics;

指标显示索引使用率为0。