Delete Aged PostgreSQL Rows with Tricky Condition

postgresqlpostgresql-9.3

I have two postgresql tables I need to clean up as part of maintenance. The tables, as described in my Django front-end, are:

class Group(models.Model):
    owner = models.ForeignKey(User)
    unique = models.CharField(max_length=36, unique=True)
    created_at = models.DateTimeField(auto_now_add=True)

class Reply(models.Model):
    text = models.TextField(validators=[MaxLengthValidator(500)])
    which_group = models.ForeignKey(Group)
    writer = models.ForeignKey(User)
    submitted_on = models.DateTimeField(db_index=True, auto_now_add=True)

I essentially want to delete all groups where the most recent reply was submitted more than 14 days ago. What will be the correct sql queries to accomplish that? I'm thinking of:

DELETE FROM links_group WHERE id IN (SELECT which_group_id FROM links_reply WHERE "submitted_on" < now() - interval '14 days');

But this is missing the logic of most recent reply. How do go about with this?

Relevant portions of \d+ links_group; are:

                                                          Table "public.links_group"
     Column     |           Type           |                        Modifiers                         | Storage  | Stats target | Description 
----------------+--------------------------+----------------------------------------------------------+----------+--------------+-------------
 id             | integer                  | not null default nextval('links_group_id_seq'::regclass) | plain    |              |  
 owner_id       | integer                  | not null                                                 | plain    |              |  
 unique         | character varying(36)    | not null                                                 | extended |              | 
 created_at     | timestamp with time zone | not null                                                 | plain    |              |
Indexes:
    "links_group_pkey" PRIMARY KEY, btree (id)
    "links_group_unique_key" UNIQUE CONSTRAINT, btree ("unique")
    "links_group_owner_id" btree (owner_id)
    "links_group_unique_like" btree ("unique" varchar_pattern_ops)
Foreign-key constraints:
    "links_group_owner_id_fkey" FOREIGN KEY (owner_id) REFERENCES auth_user(id) DEFERRABLE INITIALLY DEFERRED
Referenced by:
    TABLE "links_reply" CONSTRAINT "links_reply_which_group_id_fkey" FOREIGN KEY (which_group_id) REFERENCES links_group(id) DEFERRABLE INITIALLY DEFERRED
Has OIDs: no

Relevant portions of \d+ links_reply; are:

                                                          Table "public.links_reply"
     Column     |           Type           |                        Modifiers                         | Storage  | Stats target | Description 
----------------+--------------------------+----------------------------------------------------------+----------+--------------+-------------
 id             | integer                  | not null default nextval('links_reply_id_seq'::regclass) | plain    |              | 
 text           | text                     | not null                                                 | extended |              | 
 which_group_id | integer                  | not null                                                 | plain    |              | 
 writer_id      | integer                  | not null                                                 | plain    |              | 
 submitted_on   | timestamp with time zone | not null                                                 | plain    |              |
Indexes:
    "links_reply_pkey" PRIMARY KEY, btree (id)
    "links_reply_submitted_on" btree (submitted_on)
    "links_reply_which_group_id" btree (which_group_id)
    "links_reply_writer_id" btree (writer_id)
Foreign-key constraints:
    "links_reply_which_group_id_fkey" FOREIGN KEY (which_group_id) REFERENCES links_group(id) DEFERRABLE INITIALLY DEFERRED
    "links_reply_writer_id_fkey" FOREIGN KEY (writer_id) REFERENCES auth_user(id) DEFERRABLE INITIALLY DEFERRED
Has OIDs: no

Best Answer

You need something like this (the table and column names may differ, check the actual SQL CREATE TABLE statements):

DELETE FROM links_group AS g 
WHERE g.id IN 
      ( SELECT r.which_group_id
        FROM links_reply AS r
          -- WHERE r.which_group IS NOT NULL    
          -- not needed, the column is not nullable
        GROUP BY r.which_group_id
        HAVING MAX(r.submitted_on) < NOW() - INTERVAL '14 days'
      ) ;

Since there is a foreign key from links_reply that references links_group and does not have an ON DELETE CASCADE attribute set, the above will of course fail. To delete from both tables, you either need to run two statements or combine them in one with a modifying CTE:

WITH 
  groups_to_delete AS
    ( SELECT r.which_group_id
      FROM links_reply AS r
      GROUP BY r.which_group_id
      HAVING MAX(r.submitted_on) < NOW() - INTERVAL '14 days'
    ),
  delete_replies AS
    ( DELETE FROM links_reply AS r 
      WHERE r.which_group_id IN (TABLE groups_to_delete)
      RETURNING which_group_id
    )
DELETE FROM links_group AS g 
WHERE g.id IN (TABLE delete_replies) ;

Related Solutions

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

Related Solutions

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

pgAdmin timing

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

Related Question