fix: sqlite pragma order for auto_vacuum

[jasonish]

Setting the auto_vacuum pragma must come before setting the journal mode otherwise it won't apply.

Simple test app:

use sqlx::sqlite::SqliteAutoVacuum;
use sqlx::sqlite::SqliteJournalMode;
use sqlx::sqlite::SqliteSynchronous;
use sqlx::Connection;
use sqlx::SqliteConnection;

#[tokio::main]
async fn main() {
    let options = sqlx::sqlite::SqliteConnectOptions::new()
        .create_if_missing(true)
        .filename("./test.sqlite")
        .synchronous(SqliteSynchronous::Off)
        .auto_vacuum(SqliteAutoVacuum::Full)
        .journal_mode(SqliteJournalMode::Wal);
    let mut conn = SqliteConnection::connect_with(&options).await.unwrap();
    let auto_vacuum: u8 = sqlx::query_scalar("PRAGMA auto_vacuum")
        .fetch_one(&mut conn)
        .await
        .unwrap();
    println!("auto_vacuum = {auto_vacuum}");
}

In current git main, if journal_mode option is set, auto_vacuum will not take affect. Work-around is to execute a PRAGMA auto_vacuum = 1; VACUUM on the connection, but that shouldn't be needed for a newly created database.

[abonander]

The SQLite docs don't say anything about the relative ordering with journal_mode, but they do say the following: https://www.sqlite.org/pragma.html#pragma_auto_vacuum

The database connection can be changed between full and incremental autovacuum mode at any time. However, changing from "none" to "full" or "incremental" can only occur when the database is new (no tables have yet been created) or by running the VACUUM command. To change auto-vacuum modes, first use the auto_vacuum pragma to set the new desired mode, then invoke the VACUUM command to reorganize the entire database file. To change from "full" or "incremental" back to "none" always requires running VACUUM even on an empty database.

[jasonish]

Yeah, I went looking for the docs on this but it's not mentioned. But this issue is visible with the SQLite command line tool as well.

For example:

$ rm -f ./new.db && sqlite3 ./new.db
SQLite version 3.42.0 2023-05-16 12:36:15
Enter ".help" for usage hints.
sqlite> pragma auto_vacuum = 1;
sqlite> pragma auto_vacuum;
1

Now enable wal mode first:

$ rm -f ./new.db && sqlite3 ./new.db
SQLite version 3.42.0 2023-05-16 12:36:15
Enter ".help" for usage hints.
sqlite> pragma journal_mode = 'wal';
wal
sqlite> pragma auto_vacuum = 1;
sqlite> pragma auto_vacuum;
0

So by choosing the ordering that SQLx is choosing, auto_vacuum may be enabled, but not always. Which is probably not expected behavior. If I'm opening a new database and select the options for auto_vacuum of FULL, and journal_mode of wal, I wouldn't expect to also have to go run a vacuum immediately - which is what I'm doing as a workaround, but only if the database file doesn't exist already on startup.

[abonander]

I can reproduce that, but going back to what the SQLite docs say, if I then run a vacuum it actually updates the auto_vacuum pragma:

$ sqlite3 ./test.db
SQLite version 3.31.1 2020-01-27 19:55:54
Enter ".help" for usage hints.
sqlite> pragma auto_vacuum;
0
sqlite> pragma journal_mode = 'wal';
wal
sqlite> pragma auto_vacuum = 1;
sqlite> pragma auto_vacuum;
0
sqlite> vacuum;
sqlite> pragma auto_vacuum;
1

I think this is SQLite behaving as documented: changing the journal mode marks the database as "dirty" and so it needs an explicit vacuum for the change to the auto_vacuum pragma to apply.

[abonander]

The exact same behavior occurs when doing anything else in the database that marks it as dirty (using a fresh database file here):

$ sqlite3 ./test.db
SQLite version 3.31.1 2020-01-27 19:55:54
Enter ".help" for usage hints.
sqlite> create table foo(bar integer, baz text);
sqlite> pragma auto_vacuum;
0
sqlite> pragma auto_vacuum = 1;
sqlite> pragma auto_vacuum;
0
sqlite> vacuum;
sqlite> pragma auto_vacuum;
1

[jasonish]

I think there is an expecation that if I'm only setting options, and creating a new database, then I don't need to follow the note about existing databases. I'd expect to if I had created tables.

Perhaps a documentation update on auto_vacuum:

For existing databases, or new databases combined with options such as setting a journal mode, a change to this value does not take effect unless a VACUUM command is executed.

[abonander]

Yeah, I suppose there's no reason not to send the auto_vacuum pragma first either way.