Add “Timeseries and Aggregations” section to “Creating a subgraph” page (#663)

Co-authored-by: marcusrein <[email protected]>

Author: benface

website/pages/en/developing/creating-a-subgraph.mdx

@@ -1298,3 +1298,65 @@ Handlers for File Data Sources cannot be in files which import `eth_call` contra
 #### References
 
 [GIP File Data Sources](https://forum.thegraph.com/t/gip-file-data-sources/2721)
+
+## Timeseries and Aggregations
+
+### Overview
+
+Timeseries and aggregations enable your subgraph to track statistics like daily average price, hourly total transfers, etc.
+
+This feature introduces two new types of subgraph entity. Timeseries entities record data points with timestamps. Aggregation entities perform pre-declared calculations on the Timeseries data points on an hourly or daily basis, then store the results for easy access via GraphQL.
+
+#### Example Schema
+
+```graphql
+type Data @entity(timeseries: true) {
+  id: Int8!
+  timestamp: Timestamp!
+  price: BigDecimal!
+}
+
+type Stats @aggregation(intervals: ["hour", "day"], source: "Data") {
+  id: Int8!
+  timestamp: Timestamp!
+  sum: BigDecimal! @aggregate(fn: "sum", arg: "price")
+}
+```
+
+### Defining Timeseries and Aggregations
+
+Timeseries entities are defined with `@entity(timeseries: true)` in schema.graphql. Every timeseries entity must have a unique ID of the int8 type, a timestamp of the Timestamp type, and include data that will be used for calculation by aggregation entities. These Timeseries entities can be saved in regular trigger handlers, and act as the “raw data” for the Aggregation entities.
+
+Aggregation entities are defined with `@aggregation` in schema.graphql. Every aggregation entity defines the source from which it will gather data (which must be a Timeseries entity), sets the intervals (e.g., hour, day), and specifies the aggregation function it will use (e.g., sum, count, min, max, first, last). Aggregation entities are automatically calculated on the basis of the specified source at the end of the required interval.
+
+#### Available Aggregation Intervals
+
+- `hour`: sets the timeseries period every hour, on the hour.
+- `day`: sets the timeseries period every day, starting and ending at 00:00.
+
+#### Available Aggregation Functions
+
+- `sum`: Total of all values.
+- `count`: Number of values.
+- `min`: Minimum value.
+- `max`: Maximum value.
+- `first`: First value in the period.
+- `last`: Last value in the period.
+
+#### Example Aggregations Query
+
+```graphql
+{
+  stats(interval: "hour", where: { timestamp_gt: 1704085200 }) {
+    id
+    timestamp
+    sum
+  }
+}
+```
+
+Note:
+
+To use Timeseries and Aggregations, a subgraph must have a spec version ≥1.1.0. Note that this feature might undergo significant changes that could affect backward compatibility.
+
+[Read more](https://github.com/graphprotocol/graph-node/blob/master/docs/aggregations.md) about Timeseries and Aggregations.