Make /admin lighter

[matschaffer]

We have railsadmin set up at /admin but it tries to count measurements when you load it (very slow, too expensive to load on dev envs).

It's possible #529 might also fix this if we do an ActiveRecord count cache and RailsAdmin uses that by default.

Either way, we can't count the measurement table for ever admin page load. Too expensive in general.

[matschaffer]

Looks like this is largely due to railsadmin attempting to count large tables.

One query I caught was SELECT COUNT(*) FROM "bgeigie_logs"; which the db says looks like this:

safecast=> explain analyze SELECT COUNT(*) FROM "bgeigie_logs";
                                                             QUERY PLAN

-------------------------------------------------------------------------------------------------------------------------
------------
 Aggregate  (cost=3223832.62..3223832.63 rows=1 width=0) (actual time=142717.206..142717.206 rows=1 loops=1)
   ->  Seq Scan on bgeigie_logs  (cost=0.00..3013346.50 rows=84194450 width=0) (actual time=0.007..123746.505 rows=841585
42 loops=1)
 Planning time: 0.589 ms
 Execution time: 142717.299 ms
(4 rows)

railsadminteam/rails_admin#2968 mentions a limited_pagination setting we may be able to place on our larger tables to avoid trying to perfectly number all pages in the display.

https://wiki.postgresql.org/wiki/Count_estimate is also another interesting technique, but I suspect it'd be hard to wedge into rails admin.

But it does seem to offer a row estimate that's < 1% off from the actual row count:

safecast=> SELECT COUNT(*) FROM "bgeigie_logs";
  count
----------
 84164952
(1 row)

safecast=> explain SELECT COUNT(*) FROM "bgeigie_logs";
                                  QUERY PLAN
-------------------------------------------------------------------------------
 Aggregate  (cost=3224082.05..3224082.06 rows=1 width=0)
   ->  Seq Scan on bgeigie_logs  (cost=0.00..3013579.64 rows=84200964 width=0)
(2 rows)

[matschaffer]

We'll need to upgrade rails admin to at least https://github.com/sferik/rails_admin/blob/master/CHANGELOG.md#130---2018-02-18 for that, but probably time for an upgrade (still on 0.8.1)

[matschaffer]

Once we're upgraded it looks like the config is probably this:

  config.model 'BgeigieLog' do
    list do
      limited_pagination
    end
  end

  config.model 'Measurement' do
    list do
      limited_pagination
    end
  end

[robouden]

This is already implemented?

[matschaffer]

Not yet, just gathering details. Looks like if we do the above we can probably get /admin working without bogging down the DB