Add the ancestor_slice dynamic table.
This table given a slice ID finds all parents by following the chain of
|parent_id| values.
This is useful to allow queries to quickly find the parent of the given
slice (imagine joining ancestor_slice(id) ON depth = 0).
Bug: 160148605
Change-Id: I4a60b03a2f3fb8d6d2741fcc5b4c2217d70eb3fe
diff --git a/docs/analysis/trace-processor.md b/docs/analysis/trace-processor.md
index 998e678..7fc102b 100644
--- a/docs/analysis/trace-processor.md
+++ b/docs/analysis/trace-processor.md
@@ -294,6 +294,31 @@
reasons, span join does attempt to dectect and error out in this situation;
instead, incorrect rows will silently be produced.
+### Ancestor slice
+ancestor_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 direct parents above that id (i.e. given a
+slice id it will return as rows all slices that can be found by following the
+parent_id column to the top slice (depth = 0)).
+
+The returned format is the same as the
+[slice table](/docs/analysis/sql-tables#slice)
+
+For example, the following finds the top level slice given a bunch of slices of
+interest.
+
+```sql
+CREATE VIEW interesting_slices AS
+SELECT id, ts, dur, track_id
+FROM slice WHERE name LIKE "%interesting slice name%";
+
+SELECT
+ *
+FROM
+ interesting_slices LEFT JOIN
+ ancestor_slice(interesting_slices.id) AS ancestor ON ancestor.depth = 0
+```
+
## Metrics
TIP: To see how to add to add a new metric to trace processor, see the checklist
