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    Page Query Results

    • Paging:
      Paging is the process of fetching a subset (a page) of results from a dataset, rather than retrieving the entire results at once. This method enables processing query results one page at a time.

    • Default page size:
      If the client's query definition does Not explicitly specify the page size, the server will default to 2,147,483,647 (equivalent to int.MaxValue in C#).
      In such case, all results will be returned in a single server call.

    • Performance:
      Using paging is beneficial when handling large result datasets, contributing to improved performance.
      See paging and performance here below.

    • Paging policy:
      To prevent executing queries that do not specify a page size, you can set the throwIfQueryPageSizeIsNotSet convention, which can be useful during development or testing phases.

    • In this page:

    No paging example 

    // A simple query without paging:
// ==============================

const allResults = await session
    .query({ indexName: "Products/ByUnitsInStock" })
    .whereGreaterThan("UnitsInStock", 10)
    .all();

// Executing the query on the Northwind sample data
// will result in all 47 Product documents that match the query predicate.
    class Products_ByUnitsInStock extends AbstractJavaScriptIndexCreationTask  {
    constructor() {
        super();

        this.map("Products", p => ({
            UnitsInStock: p.UnitsInStock
        }));
    }
}
    from index "Products/ByUnitsInStock"
where UnitsInStock > 10

    Paging examples

    Retrieve a specific page:

    // Retrieve only the 3'rd page - when page size is 10:
// ===================================================

// Define an output param for getting the query stats
let stats;

const thirdPageResults = await session
    .query({ indexName: "Products/ByUnitsInStock" })
     // Get the query stats if you wish to know the TOTAL number of results
    .statistics(s => stats = s)
     // Apply some filtering condition as needed
    .whereGreaterThan("UnitsInStock", 10)
     // Call 'Skip', pass the number of items to skip from the beginning of the result set
     // Skip the first 20 resulting documents
    .skip(20)
     // Call 'Take' to define the number of documents to return
     // Take up to 10 products => so 10 is the "Page Size"
    .take(10)
    .all();

// When executing this query on the Northwind sample data,
// results will include only 10 Product documents ("products/45-A" to "products/54-A")

const totalResults = stats.totalResults;

// While the query returns only 10 results,
// `totalResults` will hold the total number of matching documents (47).
    class Products_ByUnitsInStock extends AbstractJavaScriptIndexCreationTask  {
    constructor() {
        super();

        this.map("Products", p => ({
            UnitsInStock: p.UnitsInStock
        }));
    }
}
    from index "Products/ByUnitsInStock"
where UnitsInStock > 10
limit 20, 10 // skip 20, take 10

    Page through all results:

    // Query for all results - page by page:
// =====================================

const PAGE_SIZE = 10;
let pageNumber = 0;
let pagedResults;

do {
    pagedResults = await session
        .query({ indexName: "Products/ByUnitsInStock" })
         // Apply some filtering condition as needed
        .whereGreaterThan("UnitsInStock", 10)
         // Skip the number of results that were already fetched
        .skip(pageNumber * PAGE_SIZE)
         // Request to get 'pageSize' results
        .take(PAGE_SIZE)
        .all();

    pageNumber++;

    // Make any processing needed with the current paged results here
    // ... 
}
while (pagedResults.length > 0); // Fetch next results
    class Products_ByUnitsInStock extends AbstractJavaScriptIndexCreationTask  {
    constructor() {
        super();

        this.map("Products", p => ({
            UnitsInStock: p.UnitsInStock
        }));
    }
}
    from index "Products/ByUnitsInStock"
where UnitsInStock > 10
limit 0, 10 // First loop will skip 0, take 10

// The next loops in the code will each generate the above RQL with an increased 'skip' value:
// limit 10, 10
// limit 20, 10
// limit 30, 10
// ...

    Paging and performance

    Better performance:

    It is recommended to explicitly set a page size when making a query that is expected to generate a significant number of results. This practice has several benefits:

    • Optimizes bandwidth usage by reducing data transfer between the server and client.
    • Prevents delays in response times caused by sending too much data over the network.
    • Avoids high memory consumption when dealing with numerous documents.
    • Ensures a more manageable user experience by not overwhelming users with massive datasets at once.

    Performance hints:

    • By default, if the number of returned results exceeds 2048, the server will issue a "Page size too big" notification (visible in the Studio) with information about the query.

    • This threshold can be customized by modifying the value of the PerformanceHints.MaxNumberOfResults configuration key.

    • As suggested by the hint, you may consider using Streaming query results instead of paging.

    Figure 1. Performance Hint

    Performance Hint

    Paging through tampered results

    • The QueryStatistics object contains the totalResults property,
      which represents the total number of matching documents found in the query results.

    • The QueryStatistics object also contains the skippedResults property.
      Whenever this property is greater than 0, that implies the server has skipped that number of results from the index.

    • The server will skip duplicate results internally in the following two scenarios:

      1. When making a Projection query with distinct.

      2. When querying a Fanout index.

    • In those cases:

      • The skippedResults property from the stats object will hold the count of skipped (duplicate) results.

      • The totalResults property will be invalidated -
        it will Not deduct the number of skipped results from the total number of results.

    • In order to do proper paging in those scenarios:
      include the skippedResults value when specifying the number of documents to skip for each page using:
      (currentPage * pageSize) + skippedResults.

    • See the following examples:

    A projection query with Distinct:

    let pagedResults;
let stats;

let totalResults = 0;
let totalUniqueResults = 0;
let skippedResults = 0;

let pageNumber = 0;
const PAGE_SIZE = 10;

do {
    pagedResults = await session
        .query({ indexName: "Products/ByUnitsInStock" })
        .statistics(s => stats = s)
        .whereGreaterThan("UnitsInStock", 10)
         // Define a projection
        .selectFields(["Category", "Supplier"])
         // Call Distinct to remove duplicate projected results
        .distinct()
         // Add the number of skipped results to the "start location"  
        .skip((pageNumber * PAGE_SIZE) + skippedResults)
         // Define how many items to return
        .take(PAGE_SIZE)
        .all();

    totalResults = stats.totalResults;         // Number of total matching documents (includes duplicates)
    skippedResults += stats.skippedResults;    // Number of duplicate results that were skipped
    totalUniqueResults += pagedResults.length; // Number of unique results returned in this server call 

    pageNumber++;
}
while (pagedResults.length > 0); // Fetch next results

// When executing the query on the Northwind sample data:
// ======================================================

// The total matching results reported in the stats is 47 (totalResults),
// but the total unique objects returned while paging the results is only 29 (totalUniqueResults)
// due to the 'distinct' usage which removes duplicates.

// This is solved by adding the skipped results count to skip().
    class Products_ByUnitsInStock extends AbstractJavaScriptIndexCreationTask  {
    constructor() {
        super();

        this.map("Products", p => ({
            UnitsInStock: p.UnitsInStock
        }));
    }
}
    from index "Products/ByUnitsInStock"
where UnitsInStock > 10
select distinct Category, Supplier
limit 0, 10  // First loop will skip 0, take 10, etc.

    Querying a Fanout index:

    let pagedResults;
let stats;

let totalResults = 0;
let totalUniqueResults = 0;
let skippedResults = 0;

let pageNumber = 0;
const PAGE_SIZE = 50;

do {
    pagedResults = await session
        .query({ indexName: "Orders/ByProductName" })
        .statistics(s => stats = s)
         // Add the number of skipped results to the "start location"  
        .skip((pageNumber * PAGE_SIZE) + skippedResults)
        .take(PAGE_SIZE)
        .all();

    totalResults = stats.totalResults;
    skippedResults += stats.skippedResults;
    totalUniqueResults += pagedResults.length;

    pageNumber++;
}
while (pagedResults.length > 0); // Fetch next results

// When executing the query on the Northwind sample data:
// ======================================================

// The total results reported in the stats is 2155 (totalResults),
// which represent the multiple index-entries generated as defined by the fanout index.

// By adding the skipped results count to the Skip() method,
// we get the correct total unique results which is 830 Order documents.
    // A fanout index - creating MULTIPLE index-entries per document:
// ==============================================================

class Orders_ByProductName extends AbstractJavaScriptIndexCreationTask {
    constructor() {
        super();

        this.map("Orders", order => {
            return order.Lines.map(line => {
                return {
                    ProductName: line.ProductName
                };
            });
        });
    }
}
    from index "Orders/ByProductName"
limit 0, 50  // First loop will skip 0, take 50, etc.

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