What is the purpose of the $Group$ keyword in NCache SQL queries?

The $Group$ keyword is a special NCache SQL identifier used to specify that a query condition is targeting data categorized under a specific logical group in the cache.

How does ExecuteReader improve performance for group searches?

ExecuteReader processes queries on the server-side and transmits results to the client in manageable chunks, reducing network overhead and memory consumption on the client-side.

What happens if a searched group does not exist in the cache?

If the specified group is not found within the cache cluster, the query operation typically returns a Null value or an empty result set.

Search Group Data in Cache with SQL Query [Deprecated]

NCache provides object queries through which you use criteria given in a query to search result sets. To retrieve the data according to your specified criteria, NCache provides you with an extension of SQL. A special keyword $Group$ is used to specify that the condition under consideration uses group. A query with the searching criteria is executed using ExecuteReader. The ExecuteReader processes the query on the server-side and then sends the result in chunks (as a dictionary containing keys and values) to the client in the tabular form to the ICacheReader type of instance.

Prerequisites

To learn about the standard prerequisites required to work with all NCache client-side features, please refer to the given page on Client-Side API Prerequisites.
Custom classes/searchable attributes must be indexed as explained in Configuring Query Indexes.
For API details, refer to: ICache, ICacheReader, QueryCommand, ExecuteReader, SearchService.

Warning

If the searched group is not in the cache, the query will be returned as a Null value.

Search for Data in Groups using SQL Query

You can also search for the cache items present in groups through SQL. The following program searches for Customer IDs present in a group through SQL query.

// Precondition: Cache is already connected

// A user wants to see all the West Coast Customers using the SQL query.
// Items with groups exist in the cache
// Custom class is query indexed through the NCache Management Center or config.ncconf
// Create an SQL Query with the specified criteria
// Make sure to use the Fully Qualified Name (FQN)
string query = "SELECT CustomerID, ContactName FROM FQN.Customer WHERE $Group$ = ?";

// Use QueryCommand for query execution
var queryCommand = new QueryCommand(query);

// Providing parameters for query
queryCommand.Parameters.Add("$Group$", "West Coast Customers");

// Executing the Query
ICacheReader reader = cache.SearchService.ExecuteReader(queryCommand);

// Read results if result set is not empty
if (reader.FieldCount > 0)
{
    while (reader.Read())
    {
        // Get the value of the result set
        string customerID = reader.GetValue<string>("CustomerID");
        string customerName = reader.GetValue<string>("ContactName");

        Console.WriteLine($"Customer '{customerName}' having ID '{customerID}' belongs to West Coast.");
    }
}
else
{
    Console.WriteLine($"No customers from West Coast found");
}

// Precondition: Cache is already connected

// A user wants to see all the West Coast Customers using the SQL query.
// Items with groups exist in the cache
// Custom class is query indexed through the NCache Management Center or config.ncconf
// Create an SQL Query with the specified criteria
// Make sure to use the Fully Qualified Name (FQN)
String query = "SELECT customerID, contactName FROM FQN.Customer WHERE $Group$ = ?";

// Use QueryCommand for query execution
QueryCommand queryCommand = new QueryCommand(query);

// Providing parameters for query
queryCommand.getParameters().put("$Group$", "West Coast Customers");

// Executing the Query
CacheReader reader = cache.getSearchService().executeReader(queryCommand);

// Read results if result set is not empty
if (reader.getFieldCount() > 0) 
{
    while (reader.read()) 
    {
        // Get the value of the result set
        String customerID = reader.getValue("customerID", String.class);
        String customerName = reader.getValue("contactName", String.class);

        System.out.println("Customer '" + customerName + "' having ID '" + customerID + "' belongs to West Coast.");
    }
}
else 
{
    System.out.println("No customers from West Coast found");
}

# Precondition: Cache is already connected

# A user wants to see all the West Coast Customers using the SQL query.
# Items with groups exist in the cache
# Custom class is query indexed through the NCache Management Center or config.ncconf
# Create an SQL Query with the specified criteria
# Make sure to use the Fully Qualified Name (FQN)
query = "SELECT CustomerID, ContactName FROM FQN.Customer WHERE $Group$ = ?"

# Use QueryCommand for query execution
query_command = QueryCommand(query)

# Providing parameters for query
query_command.set_parameters({"$Group$": group_name})

# Executing the Query
search_service = cache.get_search_service()
cache_reader = search_service.execute_reader(query_command)

# Read results if result set is not empty
if cache_reader.get_field_count() > 0:

    while cache_reader.read():

        # Get the value of the result set
        customer_id = cache_reader.get_value("customer_id")
        customer_name = cache_reader.get_value("name")

        print(f"Customer '{customer_name}' having ID '{customer_id}' belongs to West Coast.")
else:
        print(f"No customers from West Coast found")

// Precondition: Cache is already connected
// This is an async method
// A user wants to see all the West Coast Customers using the SQL query.
// Items with groups exist in the cache
// Custom class is query indexed through the NCache Management Center or config.ncconf
// Provide the Fully Qualified Name (FQN) of your custom class

const query = "SELECT CustomerID, ContactName FFROM FQN.Customer WHERE $Group$ = ?";

// Use QueryCommand for query execution
const queryCommand = new QueryCommand(query);

// Providing parameters for query
const parameters = new Map();
parameters.set("$Group$", "West Coast Customers");
queryCommand.setParameters(parameters);

// Executing the Query
const searchService = await cache.getSearchService();
const cacheReader = await searchService.executeReader(queryCommand);

// Read results if result set is not empty
if (cacheReader.getFieldCount() > 0) 
{
    while (await cacheReader.read()) 
    {
        // Get the value of the result set
        const customerID = cacheReader.getValue("CustomerID");
        const customerName = cacheReader.getValue("ContactName");

        console.log(`Customer '${customerName}' having ID '${customerID}' belongs to West Coast.`
        );
    }
} 
else 
{
    console.log("No customers from West Coast found");
}

Note

To ensure the operation is fail-safe, it is recommended to handle any potential exceptions within your application, as explained in Handling Failures.

Additional Resources

NCache provides sample application for Groups on GitHub.