I just noticed something that could make for a huge performance improvement in faceting.

The default query used by Datasette when faceting looks like this:

select
  country_long,
  count(*)
from (
  select * from [global-power-plants] order by rowid
)
where
  country_long is not null
group by
  country_long
order by
  count(*) desc

Here it takes 53ms: https://global-power-plants.datasettes.com/global-power-plants?sql=select%0D%0A++country_long%2C%0D%0A++count%28*%29%0D%0Afrom+%28%0D%0A++select+*+from+%5Bglobal-power-plants%5D+order+by+rowid%0D%0A%29%0D%0Awhere%0D%0A++country_long+is+not+null%0D%0Agroup+by%0D%0A++country_long%0D%0Aorder+by%0D%0A++count%28*%29+desc

Note that there's a order by rowid in there which isn't necessary - the order on that inner query doesn't matter since we're grouping and counting.

I had assumed SQLite would optimize this away - but it turns out it doesn't! Consider this version of the query, with that pointless order by removed:

select
  country_long,
  count(*)
from (
  select * from [global-power-plants]
)
where
  country_long is not null
group by
  country_long
order by
  count(*) desc

https://global-power-plants.datasettes.com/global-power-plants?sql=select%0D%0A++country_long%2C%0D%0A++count%28*%29%0D%0Afrom+%28%0D%0A++select+*+from+%5Bglobal-power-plants%5D%0D%0A%29%0D%0Awhere%0D%0A++country_long+is+not+null%0D%0Agroup+by%0D%0A++country_long%0D%0Aorder+by%0D%0A++count%28*%29+desc runs in 7.2ms!

I tried this optimization on a table with 2.5m rows in it - without the optimization it took 5 seconds, with the optimization it took 450ms. So this is a very significant improvement!