issue_comments

Counting any virtual tables can be pretty tricky. On one hand, counting a CSV virtual table would return the number of rows in the CSV, which is helpful (but can be I/O intensive). Counting a FTS5 virtual table 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 would make network/API requests to get all the rows from the sheet just to make a count.

The 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.

I'm realizing I don't know if a virtual table will ever return a count. Maybe it depends on the implementation. For these three, just checking now, it'll always return zero.

That said, I'm not sure there's any downside to having them return zero and caching that. (They're hidden, too.)

It does work, though I think it's probably still worth excluding virtual tables that will always be zero. Here's the same inspection as before, now with --load-extension spatialite:

json { "alltheplaces": { "hash": "0843cfe414439ab903c22d1121b7ddbc643418c35c7f0edbcec82ef1452411df", "size": 963375104, "file": "alltheplaces.db", "tables": { "spatial_ref_sys": { "count": 6215 }, "spatialite_history": { "count": 18 }, "sqlite_sequence": { "count": 2 }, "geometry_columns": { "count": 3 }, "spatial_ref_sys_aux": { "count": 6164 }, "views_geometry_columns": { "count": 0 }, "virts_geometry_columns": { "count": 0 }, "geometry_columns_statistics": { "count": 3 }, "views_geometry_columns_statistics": { "count": 0 }, "virts_geometry_columns_statistics": { "count": 0 }, "geometry_columns_field_infos": { "count": 0 }, "views_geometry_columns_field_infos": { "count": 0 }, "virts_geometry_columns_field_infos": { "count": 0 }, "geometry_columns_time": { "count": 3 }, "geometry_columns_auth": { "count": 3 }, "views_geometry_columns_auth": { "count": 0 }, "virts_geometry_columns_auth": { "count": 0 }, "data_licenses": { "count": 10 }, "sql_statements_log": { "count": 0 }, "states": { "count": 56 }, "counties": { "count": 3234 }, "idx_states_geometry_rowid": { "count": 56 }, "idx_states_geometry_node": { "count": 3 }, "idx_states_geometry_parent": { "count": 2 }, "idx_counties_geometry_rowid": { "count": 3234 }, "idx_counties_geometry_node": { "count": 98 }, "idx_counties_geometry_parent": { "count": 97 }, "idx_places_geometry_rowid": { "count": 1236796 }, "idx_places_geometry_node": { "count": 38163 }, "idx_places_geometry_parent": { "count": 38162 }, "places": { "count": 1332609 }, "SpatialIndex": { "count": 0 }, "ElementaryGeometries": { "count": 0 }, "KNN": { "count": 0 }, "idx_states_geometry": { "count": 56 }, "idx_counties_geometry": { "count": 3234 }, "idx_places_geometry": { "count": 1236796 } } } }

Here's how to get a list of virtual tables: https://stackoverflow.com/questions/46617118/how-to-fetch-names-of-virtual-tables