Postgresql – How to use FOR UPDATE with a view that is built using LEFT JOIN

join;postgrespostgresqlpostgresql-10updateview

Assuming a simple table, e.g.

\d+ task

      Column       |           Type           |                     Modifiers                     | Storage  |
-------------------+--------------------------+---------------------------------------------------+----------+
 id                | integer                  | not null default nextval('task_id_seq'::regclass) | plain    |
 parent_task_id    | integer                  |                                                   | plain    |
 cinema_id         | integer                  | not null                                          | plain    |
 venue_id          | integer                  |                                                   | plain    |
 movie_id          | integer                  |                                                   | plain    |
 event_id          | integer                  |                                                   | plain    |
 result            | json                     |                                                   | extended |
 context           | json                     |                                                   | extended |
 guide             | json                     |                                                   | extended |
 started_at        | timestamp with time zone |                                                   | plain    |
 ended_at          | timestamp with time zone |                                                   | plain    |
 created_at        | timestamp with time zone | not null default now()                            | plain    |
 updated_at        | timestamp with time zone | not null default now()                            | plain    |
Indexes:
    "task_pkey" PRIMARY KEY, btree (id)
    "public_task_cinema_id1_idx" btree (cinema_id)
    "public_task_event_id4_idx" btree (event_id)
    "public_task_movie_id3_idx" btree (movie_id)
    "public_task_parent_task_id0_idx" btree (parent_task_id)
    "public_task_started_at6_idx" btree (started_at)
    "public_task_venue_id2_idx" btree (venue_id)
Foreign-key constraints:
    "task_parent_task_id_fkey" FOREIGN KEY (parent_task_id) REFERENCES task(id) ON DELETE CASCADE
    "task_cinema_id_fkey" FOREIGN KEY (cinema_id) REFERENCES cinema(id) ON DELETE CASCADE
    "task_event_id_fkey" FOREIGN KEY (event_id) REFERENCES event(id) ON DELETE CASCADE
    "task_movie_id_fkey" FOREIGN KEY (movie_id) REFERENCES movie(id) ON DELETE CASCADE
    "task_venue_id_fkey" FOREIGN KEY (venue_id) REFERENCES venue(id) ON DELETE CASCADE
Referenced by:
    TABLE "task" CONSTRAINT "task_parent_task_id_fkey" FOREIGN KEY (parent_task_id) REFERENCES task(id) ON DELETE CASCADE
Triggers:
    update_task_updated_at BEFORE UPDATE ON task FOR EACH ROW EXECUTE PROCEDURE update_updated_at_column()

Assuming a simple view that selects tasks that either do not have parent task or the parent task is complete:

CREATE VIEW outstanding_task AS
  SELECT
    t1.*
  FROM task t1
  LEFT JOIN task t2 ON t2.id = t1.parent_task_id
  WHERE
    t2.id IS NULL OR
    t2.result IS NOT NULL;

I am attempting to use this view with FOR UPDATE:

SELECT *
FROM outstanding_task
LIMIT 1
FOR UPDATE SKIP LOCKED;

However, in postgres v10.4 this produces an error:

ERROR: FOR UPDATE cannot be applied to the nullable side of an outer join

How to use FOR UPDATE with a view that is built using LEFT JOIN?

Note 1: The question is specific to views. Without a view I could use FOR UPDATE OF t1.

Best Answer

Untested, as I don't currently have postgres working, but the concept should be clear:

Try using the view in another inner join against the base table, then update the base table.

select *
from task as t
inner join outstanding_tasks as o on t.id=o.id
for update of task

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Postgresql – Postgres multiple joins slow query, how to store default child record

You write:

Each customer can have multiple sites, but only one should be displayed in this list.

Yet, your query retrieves all rows. That would be a point to optimize. But you also do not define which site is to be picked.

Either way, it does not matter much here. Your EXPLAIN shows only 5026 rows for the site scan (5018 for the customer scan). So hardly any customer actually has more than one site. Did you ANALYZE your tables before running EXPLAIN?

From the numbers I see in your EXPLAIN, indexes will give you nothing for this query. Sequential table scans will be the fastest possible way. Half a second is rather slow for 5000 rows, though. Maybe your database needs some general performance tuning?

Maybe the query itself is faster, but "half a second" includes network transfer? EXPLAIN ANALYZE would tell us more.

If this query is your bottleneck, I would suggest you implement a materialized view.

After you provided more information I find that my diagnosis pretty much holds.

The query itself needs 27 ms. Not much of a problem there. "Half a second" was the kind of misunderstanding I had suspected. The slow part is the network transfer (plus ssh encoding / decoding, possibly rendering). You should only retrieve 100 rows, that would solve most of it, even if it means to execute the whole query every time.

If you go the route with a materialized view like I proposed you could add a serial number without gaps to the table plus index on it - by adding a column row_number() OVER (<your sort citeria here>) AS mv_id.

Then you can query:

SELECT *
FROM   materialized_view
WHERE  mv_id >= 2700
AND    mv_id <  2800;

This will perform very fast. LIMIT / OFFSET cannot compete, that needs to compute the whole table before it can sort and pick 100 rows.

pgAdmin timing

When you execute a query from the query tool, the message pane shows something like:

Total query runtime: 62 ms.

And the status line shows the same time. I quote pgAdmin help about that:

The status line will show how long the last query took to complete. If a dataset was returned, not only the elapsed time for server execution is displayed, but also the time to retrieve the data from the server to the Data Output page.

If you want to see the time on the server you need to use SQL EXPLAIN ANALYZE or the built in Shift + F7keyboard shortcut or Query -> Explain analyze. Then, at the bottom of the explain output you get something like this:

Total runtime: 0.269 ms

Postgresql – Improve performance on concurrent UPDATEs for a timestamp column in Postgres

Would a different kind of column be faster? For example an integer

No. timestamp and timestamptz are just unsigned 64-bit integers internally anyway.

Is there some way to not lock the column?

It doesn't lock the column. It takes weak table lock that doesn't really block anything except DDL, and takes a row level lock on the row you're updating.

There is no way to prevent the row level lock. It exists because without it behaviour and ordering concurrent updates would be undefined. We don't like undefined behaviour in RDBMSs.

It only blocks concurrent updates of the same row anyway.

Any other tips to improve this?

Not with the detail provided. There's likely a better way to do what you're trying to do, but it'll probably involve taking a few steps back and looking for a different strategy for solving the underlying problem.

In the specific case of cache invalidation I think you might want to look into LISTEN and NOTIFY. Again though, there just isn't enough info here to go on.

Best Answer

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pgAdmin timing

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