count（）是计算对象数量的最有效方法吗？

Question

I need to count the number of objects of a particular model in my database and display the number with a context processor for the user to view on every page.

Right now I'm simply doing Model.objects.count() . It works, but as there are now more than 400,000 objects in the db, it has noticeably slowed things down.

I'm running on my development server, so maybe once I push to dedicated servers this won't be a problem, but I'm not sure... I'm worried what will happen once we get into the millions or beyond. Any tips?

Answer 1

If the estimate you get from pg_class.reltuples is good enough, go for it. That would be the simplest solution.

If you need exact numbers, you could do that with a trigger. The following example keeps a count of the table mytab in mytab_count :

CREATE TABLE mytab_count (
   c bigint NOT NULL
);  

-- make sure there cannot be more than one row
CREATE UNIQUE INDEX mytab_count_singleton
   ON mytab_count ((1));

INSERT INTO mytab_count
   SELECT count(*) FROM mytab;

CREATE OR REPLACE FUNCTION count_trig() RETURNS trigger
   LANGUAGE plpgsql AS
$$BEGIN
   CASE TG_OP
      WHEN 'INSERT' THEN
         UPDATE mytab_count SET c = c + 1;
         RETURN NEW;
      WHEN 'DELETE' THEN
         UPDATE mytab_count SET c = c - 1;
         RETURN OLD;
   END CASE;
END;$$;

CREATE TRIGGER count_trig AFTER INSERT OR DELETE ON mytab
   FOR EACH ROW EXECUTE PROCEDURE count_trig();