This PR implements two things:
* Getting the value of a conjunction of elements separated by `AND` using `expr::evaluate`
* Preparing conjunctions using `prepare_expression`
---
`NULL` is treated as an "unkown value" - maybe `true` maybe `false`.
`TRUE AND NULL` evaluates to `NULL` because it might be `true` but also might be `false`.
`FALSE AND NULL` evaluates to `FALSE` because no matter what value `NULL` acts as, the result will still be `FALSE`.
Unset and empty values are not allowed.
Usually in CQL the rule is that when `NULL` occurs in an operation the whole expression becomes `NULL`, but here we decided to deviate from this behavior.
Treating `NULL` as an "unkown value" is the standard SQL way of handing `NULLs` in conjunctions.
It works this way in MySQL and Postgres so we do it this way as well.
The evaluation short-circuits. Once `FALSE` is encountered the function returns `FALSE` immediately without evaluating any further elements.
It works this way in Postgres as well, for example:
`SELECT true AND NULL AND 1/0 = 0` will throw a division by zero error,
but `SELECT false AND 1/0 = 0` will successfully evaluate to `FALSE`.
Closes#12300
* github.com:scylladb/scylladb:
expr_test: add unit tests for prepare_expression(conjunction)
cql3: expr: make it possible to prepare conjunctions
expr_test: add tests for evaluate(conjunction)
cql3: expr: make it possible to evaluate conjunctions
3fa230fee4