No need to find statement in cached statement map?

[kingluo]

To execute each query, sqlx needs to find the cached statement even for statement object already provided.

For example:

Statement provided:

let stmt = conn.prepare_with("select 1", &[]);
let stmt = stmt.await.unwrap();
let row: (i32,) = stmt.query_as().fetch_one(&mut conn).await.unwrap();

Statement not provided:

let row: (i32,) = sqlx::query_as("SELECT 1")
    .fetch_one(&mut conn)
    .await
    .unwrap();

And see get_or_prepare:
https://github.com/launchbadge/sqlx/blob/master/sqlx-core/src/postgres/connection/executor.rs#L173

If the map is big, then the find would be an unnecessary overhead.

[abonander]

Unless you're actually running into a situation where the hashmap lookup for the prepared statement is a significant source of overhead, I don't think it's worth the effort. Being a hashmap, lookups are going to be quite fast (relatively speaking) up to probably several hundred thousand elements.

Where a significant overhead is likely to occur on maps of that size is on insert, when the map needs to resize itself and thus rehash all its elements.

However, you'd need to execute that many unique queries on a single connection for this to become an issue, which is only likely to come up if you're dynamically generating queries. In that case I think we just need a way to say "prepare this statement, but don't cache it" which I already have in mind for implementing #875 and we can expose that for users to set as well.