Add documentation for TS source (elastic#134373)

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---------

Co-authored-by: Marci W <[email protected]>
Co-authored-by: Liam Thompson <[email protected]>
Co-authored-by: Felix Barnsteiner <[email protected]>

Author: 4 people

docs/reference/query-languages/esql/_snippets/commands/layout/ts.md

@@ -1,12 +1,16 @@
 ```yaml {applies_to}
-serverless: ga
-stack: ga
+serverless: preview
+stack: preview 9.2.0
 ```
 
-The `TS` command is similar to the `FROM` source command,
-but with two key differences: it targets only [time-series indices](docs-content://manage-data/data-store/data-streams/time-series-data-stream-tsds.md)
-and enables the use of time-series aggregation functions
-with the [STATS](/reference/query-languages/esql/commands/stats-by.md) command.
+**Brief description**
+
+The `TS` source command is similar to the [`FROM`](/reference/query-languages/esql/commands/from.md)
+source command, with the following key differences:
+
+ - Targets only [time series indices](docs-content://manage-data/data-store/data-streams/time-series-data-stream-tsds.md)
+ - Enables the use of [time series aggregation functions](/reference/query-languages/esql/functions-operators/time-series-aggregation-functions.md) inside the
+   [STATS](/reference/query-languages/esql/commands/stats-by.md) command
 
 **Syntax**
 
@@ -22,10 +26,92 @@ TS index_pattern [METADATA fields]
 `fields`
 :   A comma-separated list of [metadata fields](/reference/query-languages/esql/esql-metadata-fields.md) to retrieve.
 
+**Description**
+
+The `TS` source command enables time series semantics and adds support for
+[time series aggregation functions](/reference/query-languages/esql/functions-operators/time-series-aggregation-functions.md) to the `STATS` command, such as
+[`AVG_OVER_TIME()`](/reference/query-languages/esql/functions-operators/time-series-aggregation-functions.md#esql-avg_over_time),
+or [`RATE`](/reference/query-languages/esql/functions-operators/time-series-aggregation-functions.md#esql-rate).
+These functions are implicitly evaluated per time series, then aggregated by group using a secondary aggregation
+function. For example:
+
+```esql
+TS metrics
+  | WHERE @timestamp >= now() - 1 hour
+  | STATS SUM(RATE(search_requests)) BY TBUCKET(1 hour), host
+```
+
+This query calculates the total rate of search requests (tracked by the `search_requests` counter) per host and hour. The `RATE()`
+function is applied per time series in hourly buckets. These rates are summed for each
+host and hourly bucket (since each host can map to multiple time series).
+
+This paradigm—a pair of aggregation functions—is standard for time series
+querying. For supported inner (time series) functions per
+[metric type](docs-content://manage-data/data-store/data-streams/time-series-data-stream-tsds.md#time-series-metric), refer to
+[](/reference/query-languages/esql/functions-operators/time-series-aggregation-functions.md). These functions also
+apply to downsampled data, with the same semantics as for raw data.
+
+::::{note}
+If a query is missing an inner (time series) aggregation function,
+[`LAST_OVER_TIME()`](/reference/query-languages/esql/functions-operators/time-series-aggregation-functions.md#esql-last_over_time)
+is assumed and used implicitly. For instance, the following two queries are
+equivalent, returning the average of the last memory usage values per time series:
+
+```esql
+TS metrics | STATS AVG(memory_usage)
+
+TS metrics | STATS AVG(LAST_OVER_TIME(memory_usage))
+```
+
+To calculate the average memory usage across per-time-series averages, use
+the following query:
+
+```esql
+TS metrics | STATS AVG(AVG_OVER_TIME(memory_usage))
+```
+::::
+
+Use regular (non-time-series)
+[aggregation functions](/reference/query-languages/esql/functions-operators/aggregation-functions.md),
+such as `SUM()`, as outer aggregation functions. Using a time series aggregation
+in combination with an inner function causes an error. For example, the
+following query is invalid:
+
+```esql
+TS metrics | STATS AVG_OVER_TIME(RATE(memory_usage))
+```
+
+::::{note}
+A [time series](/reference/query-languages/esql/functions-operators/time-series-aggregation-functions.md)
+aggregation function must be wrapped inside a
+[regular](/reference/query-languages/esql/functions-operators/aggregation-functions.md)
+aggregation function. For instance, the following query is invalid:
+
+```esql
+TS metrics | STATS RATE(search_requests)
+```
+::::
+
+**Best practices**
+
+- Avoid aggregating multiple metrics in the same query when those metrics have different dimensional cardinalities.
+  For example, in `STATS max(rate(foo)) + rate(bar))`, if `foo` and `bar` don't share the same dimension values, the rate
+  for one metric will be null for some dimension combinations. Because the + operator returns null when either input
+  is null, the entire result becomes null for those dimensions. Additionally, queries that aggregate a single metric
+  can filter out null values more efficiently.
+- Use the `TS` command for aggregations on time series data, rather than `FROM`. The `FROM` command is still available
+  (for example, for listing document contents), but it's not optimized for procesing time series data and may produce
+  unexpected results.
+- The `TS` command can't be combined with certain operations (such as
+  [`FORK`](/reference/query-languages/esql/commands/fork.md)) before the `STATS` command is applied. Once `STATS` is
+  applied, you can process the tabular output with any applicable ES|QL operations.
+- Add a time range filter on `@timestamp` to limit the data volume scanned and improve query performance.
+
 **Examples**
 
 ```esql
 TS metrics
-| STATS sum(last_over_time(memory_usage))
+| WHERE @timestamp >= now() - 1 day
+| STATS SUM(AVG_OVER_TIME(memory_usage)) BY host, TBUCKET(1 hour)
 ```