html_url,issue_url,id,node_id,user,created_at,updated_at,author_association,body,reactions,issue,performed_via_github_app
https://github.com/simonw/datasette/issues/1884#issuecomment-1321460293,https://api.github.com/repos/simonw/datasette/issues/1884,1321460293,IC_kwDOBm6k_c5Ow-JF,15178711,2022-11-21T04:40:55Z,2022-11-21T04:40:55Z,CONTRIBUTOR,"Counting any virtual tables can be pretty tricky. On one hand, counting a [CSV virtual table](https://www.sqlite.org/csv.html) would return the number of rows in the CSV, which is helpful (but can be I/O intensive). Counting a [FTS5 virtual table](https://www.sqlite.org/fts5.html) would return the number of entries in the FTS index, which is kindof helpful, but can be misleading in some cases.

On the other hand, arbitrarily running `COUNT(*)` on some virtual tables can be incredibly expensive. SQLite offers new shortcuts/pushdowns on `COUNT(*)` queries for virtual tables, and instead calls the underlying vtab implementation and iterates through all rows in the table without discretion. For example, a virtual table that's backed by a Postgres table would call `select * from pg_table`, which would use up a lot of network and CPU calls. Or a virtual table backed by a [google sheet](https://github.com/0x6b/libgsqlite) would make network/API requests to get all the rows from the sheet just to make a count.

The [`pragma_table_list`](https://www.sqlite.org/pragma.html#pragma_table_list) pragma tells you when a table is a regular table or virtual (in the `type` column), but was only added in version 3.37.0 (2021-11-27). 


Personally, I wouldnt try to `COUNT(*)` virtual tables - it depends on how the virtual table is implemented, it requires that the connection has the proper extensions loaded, and it may accientally cause perf issues for new-age extensions. A few extensions that I'm writing have virtual tables that wouldn't benefit much from `COUNT(*)`, and the fact that SQLite iterates through all rows in a table to count just makes things worse. ","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",1439009231,