issues: 944870799
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| 944870799 | MDU6SXNzdWU5NDQ4NzA3OTk= | 1394 | Big performance boost on faceting: skip the inner order by | 9599 | closed | 0 | 4 | 2021-07-14T23:32:29Z | 2021-07-16T02:23:32Z | 2021-07-15T00:05:50Z | OWNER | 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: ```sql 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! | 107914493 | issue | {"url": "https://api.github.com/repos/simonw/datasette/issues/1394/reactions", "total_count": 2, "+1": 1, "-1": 0, "laugh": 0, "hooray": 1, "confused": 0, "heart": 0, "rocket": 0, "eyes": 0} | completed |