Add prepared statement cache to Sqlitex.Server.

[scouten]

Also adds a new function call prepare which prepares the SQL statement, adds it to the cache, and returns information about the query to the calling process. To protect encapsulation across processes, it does not return any direct reference to the prepared statement itself.

[mmmries]

Thanks for the PR! Can you provide some context on the use-case for this cache? I would hazard a guess that it's performance related, but I haven't used this library in any high-volume applications so I don't have any context for why this helps with performance.

[obmarg]

Should we document how to use a statement that has been prepared by Sqlitex.Server.prepare?

It looks from the code that I would need to call prepare with the SQL string and then any future calls to query with that same SQL string will use the pre-prepared statement. Seems like a reasonable way to do it, but it's not the way I'd have initially expected things to work. Generally would have expected that calling prepare returns something that I later use to call that prepared statement.

Maybe a docstring on the prepare function could help make things a bit more obvious?

[scouten]

Yes, that makes sense. I'll add such documentation later today or tomorrow. (I'm at work today and this project lives on my personal laptop, which is at home.)

[scouten]

@mmmries This is here to support the sqlite_ecto2 effort. Ecto really really wants to have the summary information about the prepared statement and also the signal about whether the statement would compile at all when it calls the DbConnection prepare callback.

I tried using the Sqlitex module directly in my DbConnection implementation, but that led to a different set of issues (crashes in the esqlite NIF functions when my process was killed). I believe using the Server module instead will help if not completely resolve that issue.

[scouten]

@obmarg Agree that the "typical" definition of prepare would be to return some sort of reference to the prepared statement (as, for instance, the non-Server module in Sqlitex does). I wasn't able to come up with an approach to passing back reference that didn't carry with it a risk of leaving dangling references around. Caching a pre-determined number of queries seemed like a lesser evil.

[scouten]

@obmarg Added docstring on start_link/2 and prepare/3 so hopefully future readers won't be confused. :-)

[mmmries]

Thanks @scouten for working on the ecto side of things. Hopefully the usage of Server resolves your concurrency issues in ecto

[mmmries]

released as 1.2.0 🎉 💙 💚 ❤️ 💛