How to re-write this self join update using a CTE

Question

I would like to improve my performance on this update statement and would like to try re-writing using a CTE:
UPDATE "usr_sessions" "a" SET "is_ended_at_trustable" = 't'
      WHERE (
        EXISTS (
          SELECT 1
          FROM "usr_sessions" "b"
          WHERE "a"."ended_at" = "b"."started_at"
            AND "a"."usr_space_id" = "b"."usr_space_id"
            AND "a"."account_id" = "b"."account_id"
      ) ) AND "a"."is_ended_at_trustable" IS NULL
Any help is greatly appreciated!  Open to other suggestions as well if there is a better way!

Answer 1

If I understood your description correctly, here's a way to accomplish the same thing, while dodging the need for the self join.   The update itself won't be any faster, but the overall query leading to the update will likely be faster sans self-join.
  (If my interpretation wasn't exactly what you meant, tweak the "partition by" clause).
MERGE is generally considered better then UPDATE, but your particular update isn't at risk for the shortcomings of update (still, Merge is newer, cooler, and more trustworthy).
Setup_Example_Data:
Declare @Usr_Sessions table (account_id int, usr_space_id int, is_ended_at_Trustable Char(1), started_at varchar(99), ended_at varchar(99))
Insert @Usr_Sessions
Select 1, 10, 't', 'A1', 'A1'
UNION ALL Select 2, 20, 'f', 'B1', 'B2'
UNION ALL Select 3, 30, NULL, 'C1', 'C1'
UNION ALL Select 4, 40, NULL, 'D1', 'D2'
UNION ALL Select 5, 50, NULL, 'E1', 'E2'
UNION ALL Select 5, 51, NULL, 'E3', 'E3'
UNION ALL Select 6, 61, NULL, 'F1', 'F2'
UNION ALL Select 6, 62, 't', 'F3', 'F3'
UNION ALL Select 6, 62, 'f', 'F4', 'F4'
Select 'Before', * from @Usr_Sessions
OP_Query:
BEGIN TRAN
UPDATE A SET is_ended_at_trustable = 't' from @usr_Sessions A-- Select * from @Usr_Sessions "a" --
WHERE (
EXISTS (
SELECT 1
FROM @usr_sessions "b"
WHERE "a"."ended_at" = "b"."started_at"
AND "a"."usr_space_id" = "b"."usr_space_id"
AND "a"."account_id" = "b"."account_id"
) ) AND "a"."is_ended_at_trustable" IS NULL
Select 'After 1', * from @Usr_Sessions
ROLLBACK TRAN /* Just to reset test data to original form, so second query below runs against original data */
Dodge_Self_Join:
With X as
Select *
, count(case when started_at = ended_at and is_ended_at_trustable is null then 'x' else null end)
over(partition by account_id, usr_space_id) as Updatable
From @Usr_Sessions
Update X
set is_ended_at_Trustable = 'T'
where Updatable > 0 -- EDIT -- fixed error, previously said "updatable = 1"
Select 'After 2', * from @Usr_Sessions