id,node_id,number,title,user,state,locked,assignee,milestone,comments,created_at,updated_at,closed_at,author_association,pull_request,body,repo,type,active_lock_reason,performed_via_github_app,reactions,draft,state_reason
751195017,MDU6SXNzdWU3NTExOTUwMTc=,1111,Accessing a database's `.json` is slow for very large SQLite files,15178711,open,0,,,3,2020-11-26T00:27:27Z,2021-01-04T19:57:53Z,,CONTRIBUTOR,,"I have a SQLite DB that's pretty large, 23GB and something like 300 million rows. I expect that most queries I run on it will be slow, which is fine, but there are some things that Datasette does that makes working with the DB very slow. Specifically, when I access the `.json` metadata for a table (which I believe it comes from `datasette/views/database.py`, it takes 43 seconds for the request to come in:

```bash
$ time curl localhost:9999/out.json
{""database"": ""out"", ""size"": 24291454976, ""tables"": [{""name"": ""PageviewsHour"", ""columns"": [""file"", ""code"", ""page"", ""pageviews""], ""primary_keys"": [], ""count"": null, ""hidden"": false, ""fts_table"": null, ""foreign_keys"": {""incoming"": [], ""outgoing"": [{""other_table"": ""PageviewsHourFiles"", ""column"": ""file"", ""other_column"": ""file_id""}]}, ""private"": false}, {""name"": ""PageviewsHourFiles"", ""columns"": [""file_id"", ""filename"", ""sha256"", ""size"", ""day"", ""hour""], ""primary_keys"": [""file_id""], ""count"": null, ""hidden"": false, ""fts_table"": null, ""foreign_keys"": {""incoming"": [{""other_table"": ""PageviewsHour"", ""column"": ""file_id"", ""other_column"": ""file""}], ""outgoing"": []}, ""private"": false}, {""name"": ""sqlite_sequence"", ""columns"": [""name"", ""seq""], ""primary_keys"": [], ""count"": 1, ""hidden"": false, ""fts_table"": null, ""foreign_keys"": {""incoming"": [], ""outgoing"": []}, ""private"": false}], ""hidden_count"": 0, ""views"": [], ""queries"": [], ""private"": false, ""allow_execute_sql"": true, ""query_ms"": 43340.23213386536}
real	0m43.417s
user	0m0.006s
sys	0m0.016s
```

I suspect this is because a `COUNT(*)` is happening under the hood, which, when I run it through sqlite directly, does take around the same time:

```bash
$ time sqlite3 out.db < <(echo ""select count(*) from PageviewsHour;"")
362794272

real	0m44.523s
user	0m2.497s
sys	0m6.703s
```

I'm using the `.json` request in the [Observable Datasette Client](https://observablehq.com/@asg017/datasette-client) to 1) verify that a link passed in is a reachable Datasette instance, and 2) a quick way to look at metadata for a db. A few different solutions I can think of:

1. Have some other endpoint, like `/-/datasette.json` that the Observable Datasette client can fetch from to verify that the passed in URL is a valid Datasette (doesnt solve the slow problem, feel free to split this issue into 2)
2. Have a way to turn off table counts when accessing a database's `.json` view, like `?no_count=1` or something
3. Maybe have a timeout on the `table_counts()` function if it takes too long. which is odd, because it seems like it already does that (I think?), I can debug a little more if that's the case

More than happy to debug further, or send a PR if you like one of the proposals above!",107914493,issue,,,"{""url"": ""https://api.github.com/repos/simonw/datasette/issues/1111/reactions"", ""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,