feat: core filter selectivity (#81)

"Core" means I am handling all cases where we don't fall back to
hardcoded defaults (e.g. missing statistics, "col = col", "col =
subquery", etc.).

Note the difference between this PR and [hello world
selectivity](https://github.com/cmu-db/optd/pull/70). In this PR, the
only file that changed is `base_cost.rs`, because all the
"infrastructure" was already set up in "hello world selectivity"

I have written 20 unit tests to test this core logic in a variety of
scenarios (nulls vs no nulls, value in MCVs vs not in MCVs, reversing
the order of children, etc.) I chose to write code-based unit tests
instead of SQLPlanner-like tests because it allows fine-grained control
of the expression tree down to the order of children (and because it
takes a lot more work to modify SQLPlanner to output cardinality). I
made an effort to make the unit tests less brittle by defining helper
functions such as `bin_op()` or `const_i32()` for constructing
expression trees. If our expression tree representation changes in the
future, it is likely that only these helper functions need to be changed
to make the unit tests work again.

Some TPC-H queries require "non-core" logic, so this code doesn't
currently run with all of TPC-H. To avoid crashing, I simply return
INVALID_SELECTIVITY (0.001) for any branches that aren't a part of the
"core". I will handle "non-core" logic in a future PR.

Another case not being handled is comparisons (</<=/>=/>) between
`Value` objects. Handling this requires further discussion because not
all Values have a meaningful "order". In the meantime, I hardcoded
converting all `Value` objects to `i32` to perform comparisons.

Author: wangpatrick57

Cargo.lock

@@ -2,8 +2,12 @@
 # runs the stuff in CI.yaml locally
 # unfortunately this needs to be updated manually. just update it if you get annoyed by GHAs failing
 
-set -ex
+set -e
 
 cargo fmt --all -- --check
 cargo clippy --workspace --all-targets --all-features --locked -- -D warnings
-cargo test --no-fail-fast --workspace --all-features --locked
+cargo test --no-fail-fast --workspace --all-features --locked
+
+# %s is a workaround because printing --- doesn"t work in some shells
+# this just makes it more obvious when the CI has passed
+printf "%s\n| \033[32m\033[1mCI PASSED\033[0m |\n%s\n" "-------------" "-------------"

ci.sh

@@ -20,3 +20,4 @@ camelpaste = "0.1"
 datafusion-expr = "32.0.0"
 async-trait = "0.1"
 datafusion = "32.0.0"
+assert_approx_eq = "1.1.0"