Support connection pool for postgresql service

[Zheaoli]

by using BB8 https://github.com/djc/bb8

[Xuanwo]

Maybe we can store the prepared statement in the managed connection instead?

[Zheaoli]

Maybe we can store the prepared statement in the managed connection instead?

I think it would be better if we choose to use the connection pool.

In most circumstances, the connection pool will create a new connection if the old connection is invalid and this would be cause some issue because the prepared statement is binding with connection.

[Xuanwo]

the connection pool will create a new connection if the old connection is invalid and this would be cause some issue because the prepared statement is binding with connection.

We can prepare again with the newly created connection.

[Zheaoli]

the connection pool will create a new connection if the old connection is invalid and this would be cause some issue because the prepared statement is binding with connection.

We can prepare again with the newly created connection.

Not a good idea, I think. Here's the detail

let connection = pool.get().await.unwrap();
connection.query_one(&cachedStatement)

We can't guarantee the connection that is got from the pool is the same connection that is binding with the cached statement. If we choose to cache the connection at the same time, I think the connection pool would be useless

[Xuanwo]

We can't guarantee the connection that is got from the pool is the same connection that is binding with the cached statement. If we choose to cache the connection at the same time, I think the connection pool would be useless

We could implement in this way:

struct ManagedConnection {
   conn: Connection,
   client: Client,

   put_statement: OnceCell<X>, 
}

let mgd_conn = pool.get().await.unwrap();
let x = mgd_connconn.query_one(&mgd_conn.put_statement);

The statement is always alive with the connection.

[Zheaoli]

You mean we storage the connection and the statement in a same struct?