issue_comments: 1321460293
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| 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 |