fix: use i64 for COUNT(*) result on MySQL and SQLite

[devgony]

Summary

• Use i64 consistently for COUNT(*) results across all database backends (MySQL, PostgreSQL, SQLite)
• Remove unnecessary db_backend variable that was only used for type selection

Problem

MySQL's COUNT(*) returns BIGINT (64-bit), but SeaORM was reading it as i32, which could cause overflow for datasets with more than ~2 billion rows.

Historical Context

The original code (before commit 9c865cb) used i32 for all databases:

let num_items = result.try_get::<i32>("", "num_items")? as usize;

In August 2021, commit 9c865cbb ("Try to fix count()") added PostgreSQL-specific handling because PostgreSQL's COUNT(*) returns BIGINT which cannot be decoded as i32:

let num_items = match self.db {
    #[cfg(feature = "sqlx-postgres")]
    DatabaseConnection::SqlxPostgresPoolConnection(_) => 
        result.try_get::<i64>("", "num_items")? as usize,
    _ => result.try_get::<i32>("", "num_items")? as usize
};

However, MySQL also returns BIGINT for COUNT(*). The fix only addressed PostgreSQL because:

1. PostgreSQL's sqlx driver strictly enforces type matching (i64 required)
2. MySQL's sqlx driver was more lenient and could decode small values as i32
3. Test datasets were small enough that overflow never occurred

Solution

Use i64 for all backends since:

• PostgreSQL COUNT(*) → BIGINT (i64)
• MySQL COUNT(*) → BIGINT (i64)
• SQLite handles dynamic integer sizing and works with i64

let num_items = result.try_get::<i64>("", "num_items")? as u64;

Changes

• src/executor/paginator.rs (async version)
• sea-orm-sync/src/executor/paginator.rs (sync version)

[tyt2y3]

hi, thank you for your contribution. I realize we don't actually have test coverage for it in our test suite. can you add some test cases in a new file paginator_tests, with content like https://github.com/SeaQL/sea-orm/blob/master/tests/cursor_tests.rs ? this would give us more confidence, thanks!

[tyt2y3]

thank you! looks good, can merge if all tests passed

[github-actions]

🎉 Released In 2.0.0-rc.32 🎉

Huge thanks for the contribution!
This feature has now been released, so it's a great time to upgrade.
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