In PouchDB, MapReduce functions are used to perform queries and data aggregation on the documents stored in a database. MapReduce is a powerful paradigm for processing and analyzing large datasets efficiently. Here's an overview of how MapReduce functions work in PouchDB:
Map Function :
* The Map function is responsible for processing individual documents in the database and emitting key-value pairs based on specific criteria.
* The Map function is invoked once for each document in the database, allowing you to extract and transform data as needed.
* Inside the Map function, you can access the document properties and emit key-value pairs using the emit() function.
function mapFunction(doc) {
if (doc.type === 'user') {
emit(doc.name, 1);
}
}?
Reduce Function :
* The Reduce function is optional and is used to aggregate and summarize the results produced by the Map function.
* The Reduce function takes a set of key-value pairs emitted by the Map function and combines them to produce a single result.
* The Reduce function is typically used for operations such as counting, summing, averaging, and grouping data.
function reduceFunction(keys, values, rereduce) {
return sum(values);
}?
Querying with MapReduce :
* Once you have defined the Map and optionally the Reduce functions, you can use them to query the database and retrieve results.
* PouchDB provides the query() method to execute MapReduce queries on the database. You can specify the Map and Reduce functions, as well as additional options such as key ranges and sorting criteria.
db.query({
map: mapFunction,
reduce: reduceFunction,
group: true
}).then(function(result) {
console.log('Query result:', result);
}).catch(function(err) {
console.error('Query error:', err);
});?
Grouping and Sorting :
* MapReduce queries in PouchDB support grouping and sorting of results based on keys emitted by the Map function.
* You can use the group option to group results by key and the descending option to sort results in descending order.
db.query({
map: mapFunction,
reduce: reduceFunction,
group: true,
descending: true
}).then(function(result) {
console.log('Query result:', result);
}).catch(function(err) {
console.error('Query error:', err);
});?
