Add the descendant_slice table.
This allows you to given a slice find all descendants related to it. This
can be useful for metrics to traverse the stack and gain information
about what is happening in the trace.
Bug: 160148605
Change-Id: I54869e78d02fd18c18acf7808e4d1ba385f6c6ac
diff --git a/docs/analysis/trace-processor.md b/docs/analysis/trace-processor.md
index 7fc102b..a6fa6cf 100644
--- a/docs/analysis/trace-processor.md
+++ b/docs/analysis/trace-processor.md
@@ -319,6 +319,35 @@
ancestor_slice(interesting_slices.id) AS ancestor ON ancestor.depth = 0
```
+### Descendant slice
+descendant_slice is a custom operator table that takes a
+[slice table's id column](/docs/analysis/sql-tables#slice) and computes all
+slices on the same track that are nested under that id (i.e. all slices that
+are on the same track at the same time frame with a depth greater than the given
+slice's depth.
+
+The returned format is the same as the
+[slice table](/docs/analysis/sql-tables#slice)
+
+For example, the following finds the number of slices under each slice of
+interest.
+
+```sql
+CREATE VIEW interesting_slices AS
+SELECT id, ts, dur, track_id
+FROM slice WHERE name LIKE "%interesting slice name%";
+
+SELECT
+ *
+ (
+ SELECT
+ COUNT(*) AS total_descendants
+ FROM descendant_slice(interesting_slice.id)
+ )
+FROM interesting_slices
+```
+
+
## Metrics
TIP: To see how to add to add a new metric to trace processor, see the checklist
