When applying the limit and order by, the where clause also needs to be
applied to yield the correct results. This can be seen with an example.
Assuming a data set where there are 100 sessions, with 10 sessions
belonging to a user with id `u_user1` and 90 sessions belonging to
`u_user2`. If the constructed query is like:
select *
from
(select public_id from session limit 10) s,
session_list ss
where
s.public_id = ss.public_id
and s.user_id = 'u_user1'
order by create_time asc;
The initial sub-query with the limit could return sessions only
belonging to `u_user2`, then it would apply the where clause and exclude
all of the rows.
Instead if the query is:
with
session_ids as (
select public_id
from sessions as s
where
s.user_id = 'u_user1'
order by create_time asc
limit 10
)
select *
from session_list
where
session_list.public_id in (select * from session_ids)
order by create_time asc;
The CTE applies the where clause ensuring that only sessions for
`u_user1` are used for the final select from the view.
Note that the CTE is use primarily for readability.
This also adds an index on `session (create_time)` to aide the query
planner in performing the order by.
Timothy Messier
f00af362dc