Merge pull request #49385 from SnehaGunda/issues

Updating query reference docs

Author: itechedit

articles/cosmos-db/indexing-policies.md

@@ -33,6 +33,22 @@ After reading this article, you'll be able to answer the following questions:
 * How do I make changes to a collection’s indexing policy?
 * How do I compare storage and performance of different indexing policies?
 
+## <a id="Indexing"></a> Cosmos DB indexing
+
+The purpose of database indexes is to serve queries in their various forms and shapes with minimum resource consumption (like CPU and input/output) while providing good throughput and low latency. Often, the choice of the right index for querying a database requires much planning and experimentation. This approach poses a challenge for schema-less databases where the data doesn’t conform to a strict schema and evolves rapidly. 
+
+Therefore, when we designed the Cosmos DB indexing subsystem, we set the following goals:
+
+* Index documents without requiring schema: The indexing subsystem does not require any schema information or make any assumptions about schema of the documents.  
+
+* Support for efficient, rich hierarchical, and relational queries: The index supports the Cosmos DB query language efficiently, including support for hierarchical and relational projections.  
+
+* Support for consistent queries in face of a sustained volume of writes: For high write throughput workloads with consistent queries, the index is updated incrementally, efficiently, and online in the face of a sustained volume of writes. The consistent index update is crucial to serve the queries at the consistency level in which the user configured the document service.  
+
+* Support for multi-tenancy: Given the reservation-based model for resource governance across tenants, index updates are performed within the budget of system resources (CPU, memory, and input/output operations per second) allocated per replica.  
+
+* Storage efficiency: For cost effectiveness, the on-disk storage overhead of the index is bounded and predictable. This is crucial because Cosmos DB allows the developer to make cost-based tradeoffs between index overhead in relation to the query performance.  
+
 ## Customize the indexing policy of a collection <a id="CustomizingIndexingPolicy"></a>  
 You can customize the trade-offs between storage, write and query performance, and query consistency by overriding the default indexing policy on an Azure Cosmos DB collection. You can configure the following aspects: