We need a read only phase and a commit phase. Otherwise we will never
be able to rollback changes properly. We currently do that, but we
do that in the view. Before we move to circuits, this needs to be
internalized by the operator.
I am 100% sure they are total bullshit by now, since we don't implement
the join operator yet. The code evolved a lot, and in every turn there
are issues with aggregators, projectors, filters... some subtle, some
not so subtle.
We keep having to patch join slightly as we make changes to the API, but
we don't truly exercise whether or not they keep working because there
is no support for them in the views. Therefore: let's remove it. We'll
bring it back later.
min/max require O(N) storage because of deletions. It is easy to see
why: if you *add* a new row, you can quickly and incrementally check
if it is smaller / larger than the previous accumulator.
But when you *delete* a row you can't do that and have to check the
previous values.
Feldera uses something called "traces" which to me look a lot like
indexes. When we implement materialization, this is easy to do. But to
avoid having something broken, we'll just disable min / max until then.
The operator itself should handle deletions and updates that change
the rowid by consolidating its state.
Our current materialized views track state themselves, so we don't
see this problem now. But it becomes apparent once we switch the
views to use circuits.
My goal with this patch is to be able to implement the ProjectOperator
for DBSP circuits using VDBE for expression evaluation.
*not* doing so is dangerous for the following reason: we will end up
with different, subtle, and incompatible behavior between SQLite
expressions if they are used in views versus outside of views.
In fact, even in our prototype had them: our projection tests, which
used to pass, were actually wrong =) (sqlite would return something
different if those functions were executed outside the view context)
For optimization reasons, we single out trivial expressions: they don't
have go through VDBE. Trivial expressions are expressions that only
involve Columns, Literals, and simple operators on elements of the same
type. Even type coercion takes this out of the realm of trivial.
Everything that is not trivial, is then translated with translate_expr -
in the same way SQLite will, and then compiled with VDBE.
We can, over time, make this process much better. There are essentially
infinite opportunities for optimization here. But for now, the main
warts are:
* VDBE execution needs a connection
* There is no good way in VDBE to pass parameters to a program.
* It is almost trivial to pollute the original connection. For example,
we need to issue HALT for the program to stop, but seeing that halt
will usually cause the program to try and halt the original program.
Subprograms, like the ones we use in triggers are a possible solution,
but they are much more expensive to execute, especially given that our
execution would essentially have to have a program with no other role
than to wrap the subprogram.
Therefore, what I am doing is:
* There is an in-memory database inside the projection operator (an
obvious optimization is to share it with *all* projection operators).
* We obtain a connection to that database when the operator is created
* We use that connection to execute our VDBE, which offers a clean, safe
and isolated way to execute the expression.
* We feed the values to the program manually by editing the registers
directly.
