feat(docs): describe rate with window functions

schema/patterns.md → docs/patterns.md

@@ -42,3 +42,33 @@ FROM (
 GROUP BY metric, time
 ORDER BY 1, 2
 ```
+
+## Window Functions
+
+Window functions allow aggregates to view more than one row at a time, often using the previous and/or next row
+to calculate change over time. Larger windows may calculate ordered aggregates like percentiles, but also need to
+look at a larger set of rows and may not scale well.
+
+Some utility functions are provided in [the `rate_` family](../schema/utils/rate.sql) to calculate change between
+samples, handle counter resets, and adjust for time.
+
+Time buckets should be run before the window function and may be done in a sub-select to help deduplicate:
+
+```sql
+SELECT
+  metric,
+  bucket AS time,
+  rate_time(value, lag(value) OVER w, '$__interval') AS value
+FROM (
+  SELECT
+    CONCAT(labels->>'instance', labels->>'job') AS metric,
+    $__timeGroup("time", $__interval) AS bucket,
+    MAX(value) AS value
+  FROM metrics
+  WHERE
+    $__timeFilter("time") AND
+    name = 'some_metric'
+) AS m
+WINDOW w AS (PARTITION BY metric ORDER BY time)
+ORDER BY metric, time;
+```