Simple implementation for aggregation functions. The code added is
purposely so that we can add things like `CEILING(avg(n))` in the future. Aggregation function
impose a higher level of complexity that requires us to plan ahead
several things:
* simple avg: `select avg(id) from users`
* nested avg: `select ceiling(avg(id)) from users`
* avg with other columns: `select ceiling(avg(id)), * from users` (yes,
this is valid sqllite). For now I'm nullifying extra columns for
simplicity.
* avg with other agg functions: `select avg(id), sum(id) from users`.
This should be supported.
* At last -- Order By is a painful enough case to treat alone (not done
in this pr)
Signed-off-by: Pere Diaz Bou <pere-altea@hotmail.com>
SQLite special-cases `LIMIT 0` by emitting an explicit `Goto` to avoid
executing any `ResultRow` statements. We, however, hacked around this in
DecrJumpZero but also the placement of the generated instruction.
Let's follow SQLite codegen here. We still need to fix `LIMIT 0`,
though.
Introduced pragma statement parsing and update in memory of default page cache size.
There are some more "improvements" to the print insn procedure — I couldn't decide what was the preferred way in rust to do printing on different int types so I went with the stupidest I could think of at the moment.
