I have a Flask API that has 2 endpoints.

When the user clicks a button, the front-end first calls a /calculate route, which computes an expensive query and appends the results to a table called results.

This table is indexed as follows:

 create index if not exists results_tbl_idx on results
              (customer_id, result_id, team_id, team_name, territory_name, specialty, id);

When /calculate returns, it then gets a handle to the result_id, and then calls a /get_results endpoint, which runs the following query on the results table:

with distinct_users_team_tbl as (
    select team_id,
        count(distinct id) as distinct_users_team,
        count(distinct id) filter ( where targeted_users > 0 ) as targeted_distinct_users_team
    from tam_results_14
    where result_id = 201 and customer_id = '1'
    group by team_id
distinct_users_territory_tbl as (
    select team_id, territory_name,
        count(distinct id) as distinct_users_territory,
        count(distinct id) filter ( where targeted_users > 0 ) as targeted_distinct_users_territory
    from tam_results_14
    where result_id = 201 and customer_id = '1'
    group by team_id, territory_name
distinct_users_specialty_tbl as (
    select team_id, team_name, territory_name, specialty,
        sum(tam) as tam,
        sum(sam) as sam,
        count(distinct id) as  distinct_users_specialty,
        count(distinct id) filter ( where targeted_users > 0 ) as  targeted_distinct_users_specialty
    from tam_results_14
    where result_id = 201 and customer_id = '1'
    group by team_id, team_name, territory_name, specialty

select *
from distinct_users_specialty_tbl
join distinct_users_team_tbl
    on distinct_users_specialty_tbl.team_id = distinct_users_team_tbl.team_id
join distinct_users_territory_tbl
    on distinct_users_specialty_tbl.team_id = distinct_users_territory_tbl.team_id
    and distinct_users_specialty_tbl.territory_name = distinct_users_territory_tbl.territory_name

Sometimes, just after running /calculate, when fetching /get_results, the query hangs forever.

Typically, the /get_results query executes in < 4s. And even while it's hanging, I can paste the exact same query that is hanging into a console and run it in < 4s.

I have checked pg_stat_activity and can see the query running with no wait_event and state = active.

I then checked pg_locks and for that pid, I can see:


My current hunch

Is it that database hasn't finished indexing the new entries to the results table before I query it? But that it has finished adding items to the index by the time I paste the hung query into the terminal and run which can use the index?

Is there some way I can force the table to fully index before running the /get_results query? Is that even possible?

Thanks for any help!!!

  • Please edit your question and tag for the version of Postgres you are using. Mar 19 '21 at 6:47
  • 1
    Inserts and index updates won’t block your query (writers don’t block readers). Your query just won’t see those records if you start the query before the transaction has committed. I also hope you aren’t dropping and recreating the index all the time? Mar 19 '21 at 6:50
  • And if you are dropping and recreating the index and you start the query before the index creation has committed, then your query is probably doing full table scans of everything -- and maybe three times, once for each CTE depending on your Postgres version -- as the index won't exist according to your query. Mar 19 '21 at 7:22
  • @Colin'tHart I am calling CREATE INDEX IF NOT EXISTS each time /calculate is called. I never explicity drop the index. In fact, if the schema changes, I just change the results table name to a new version (results_v2) for example, and then create a new index on that if it didn't exist.
    – iamyojimbo
    Mar 19 '21 at 10:26
  • 1
    If this is just a cache table and you don't care about the contents, you can create it as UNLOGGED. If the server crashes it will be truncated, but writes will be much faster.
    – bobflux
    Mar 19 '21 at 19:09

Add an explicit

ANALYZE results;

at the end of \calculate so that the other function has accurate table statistics.

  • I ended up doing VACUUM ANALYZE results in the end. What's the effect if I skip VACUUM?
    – iamyojimbo
    Mar 19 '21 at 10:28
  • VACUUM is probably not necessary, and it uses way more resources. Mar 19 '21 at 10:52

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