Use executemany from backends

[aminalaee]

Closes #284

Some of the backends support executemany and instead of running multiple single queries we can use executemany.
asyncpg, aiosqlite and aiomysql support executemany but aiopg doesn't support it.

[PrettyWood]

I would have expected a change in
https://github.com/encode/databases/blob/master/databases/core.py#L281-L286
instead of creating multiple queries to take again the first one afterwards
Do I miss something? 🤔

[aminalaee]

@PrettyWood Yeah I actually thought that would be more efficient too. If I understand your comment correctly, you want to avoid building all the queries in core and you suggest we build only the first query and pass the values with query to the backends?

I tried that before and there's a limitation.
In each database backend we're doing a _compile which builds the query and parameters from a ClauseElement instance.
https://github.com/encode/databases/blob/master/databases/backends/sqlite.py#L157

If we build only the first query in core.py, and pass the values around to the database backends, we will be calling executemany method with a compiled query string which is correct but the paramters are not coming from a compiled query meaning they're not coming from the output of _compile method.

Even if we could make this work with SQLAlchemy orm I think it won't work well with raw queries. For building raw queries we're using bindparams https://docs.sqlalchemy.org/en/14/core/sqlelement.html#sqlalchemy.sql.expression.TextClause.bindparams which doesn't seem to work with list of parameters, compared to SQLAlchemy orm .values() method that can accept list of dicts.

tldr; I think if we want to avoid building all queries and compiling queries and params, we would have to build query params ourself and not use SQLAlchemy for that. SQLAlchemy seems to favor it's own executemany style: https://docs.sqlalchemy.org/en/14/core/tutorial.html#execute-multiple

I'm really interested to improve this and get it through. So feel free to dig deeper and let me know if we can improve it.

[aminalaee]

Closing it as stale.