Currently in the main translation logic after planning and optimization,
we don't _really_ need to pass a &mut Vec<WhereTerm> around anymore, except
for the fact that virtual table constraint resolution is done ad-hoc in
`init_loop()`. Even there, the only thing we mutate is `WhereTerm::consumed`
which is a boolean indicating that the term has been "used up" by the optimizer
and shouldn't be evaluated as a normal where clause condition anymore.
In the upcoming branch for WHERE clause subqueries, I want to store immutable
references to WHERE clause expressions in `Resolver`, but this is unfortunately
not possible if we still use the aforementioned mutable references.
Hence, we can temporarily make `WhereTerm::consumed` a `Cell<bool>` which allows
us to pass an immutable reference to `init_loop()`, and the `Cell` can be removed
once the virtual table constraint resolution is moved to an earlier part of the
query processing pipeline.
Currently our "table id"/"table no"/"table idx" references always
use the direct index of the `TableReference` in the plan, e.g. in
`SelectPlan::table_references`. For example:
```rust
Expr::Column { table: 0, column: 3, .. }
```
refers to the 0'th table in the `table_references` list.
This is a fragile approach because it assumes the table_references
list is stable for the lifetime of the query processing. This has so
far been the case, but there exist certain query transformations,
e.g. subquery unnesting, that may fold new table references from
a subquery (which has its own table ref list) into the table reference
list of the parent.
If such a transformation is made, then potentially all of the Expr::Column
references to tables will become invalid. Consider this example:
```sql
-- Assume tables: users(id, age), orders(user_id, amount)
-- Get total amount spent per user on orders over $100
SELECT u.id, sub.total
FROM users u JOIN
(SELECT user_id, SUM(amount) as total
FROM orders o
WHERE o.amount > 100
GROUP BY o.user_id) sub
WHERE u.id = sub.user_id
-- Before subquery unnesting:
-- Main query table_references: [users, sub]
-- u.id refers to table 0, column 0
-- sub.total refers to table 1, column 1
--
-- Subquery table_references: [orders]
-- o.user_id refers to table 0, column 0
-- o.amount refers to table 0, column 1
--
-- After unnesting and folding subquery tables into main query,
-- the query might look like this:
SELECT u.id, SUM(o.amount) as total
FROM users u JOIN orders o ON u.id = o.user_id
WHERE o.amount > 100
GROUP BY u.id;
-- Main query table_references: [users, orders]
-- u.id refers to table index 0 (correct)
-- o.amount refers to table index 0 (incorrect, should be 1)
-- o.user_id refers to table index 0 (incorrect, should be 1)
```
We could ofc traverse every expression in the subquery and rewrite
the table indexes to be correct, but if we instead use stable identifiers
for each table reference, then all the column references will continue
to be correct.
Hence, this PR introduces a `TableInternalId` used in `TableReference`
as well as `Expr::Column` and `Expr::Rowid` so that this kind of query
transformations can happen with less pain.
```sql
-- This PR does effectively this transformation:
select
sum(l_extendedprice* (1 - l_discount)) as revenue
from
lineitem,
part
where
(
p_partkey = l_partkey
and p_brand = 'Brand#22'
and p_container in ('SM CASE', 'SM BOX', 'SM PACK', 'SM PKG')
and l_quantity >= 8 and l_quantity <= 8 + 10
and p_size between 1 and 5
and l_shipmode in ('AIR', 'AIR REG')
and l_shipinstruct = 'DELIVER IN PERSON'
)
or
(
p_partkey = l_partkey
and p_brand = 'Brand#23'
and p_container in ('MED BAG', 'MED BOX', 'MED PKG', 'MED PACK')
and l_quantity >= 10 and l_quantity <= 10 + 10
and p_size between 1 and 10
and l_shipmode in ('AIR', 'AIR REG')
and l_shipinstruct = 'DELIVER IN PERSON'
)
or
(
p_partkey = l_partkey
and p_brand = 'Brand#12'
and p_container in ('LG CASE', 'LG BOX', 'LG PACK', 'LG PKG')
and l_quantity >= 24 and l_quantity <= 24 + 10
and p_size between 1 and 15
and l_shipmode in ('AIR', 'AIR REG')
and l_shipinstruct = 'DELIVER IN PERSON'
);
-- Same query with common conjuncts (ANDs) extracted:
select
sum(l_extendedprice* (1 - l_discount)) as revenue
from
lineitem,
part
where
p_partkey = l_partkey
and l_shipmode in ('AIR', 'AIR REG')
and l_shipinstruct = 'DELIVER IN PERSON'
and (
(
p_brand = 'Brand#22'
and p_container in ('SM CASE', 'SM BOX', 'SM PACK', 'SM PKG')
and l_quantity >= 8 and l_quantity <= 8 + 10
and p_size between 1 and 5
)
or
(
p_brand = 'Brand#23'
and p_container in ('MED BAG', 'MED BOX', 'MED PKG', 'MED PACK')
and l_quantity >= 10 and l_quantity <= 10 + 10
and p_size between 1 and 10
)
or
(
p_brand = 'Brand#12'
and p_container in ('LG CASE', 'LG BOX', 'LG PACK', 'LG PKG')
and l_quantity >= 24 and l_quantity <= 24 + 10
and p_size between 1 and 15
)
);
```
