With this change, the following two queries are considered equivalent:
```sql
SELECT value FROM generate_series(5, 50);
SELECT value FROM generate_series WHERE start = 5 AND stop = 50;
```
Arguments passed in parentheses to the virtual table name are now
matched to hidden columns.
Column references are still not supported as table-valued function
arguments. The only difference is that previously, a query like:
```sql
SELECT one.value, series.value
FROM (SELECT 1 AS value) one, generate_series(one.value, 3) series;
```
would cause a panic. Now, it returns a proper error message instead.
Adding support for column references is more nuanced for two main
reasons:
- We need to ensure that in joins where a TVF depends on other tables,
those other tables are processed first. For example, in:
```sql
SELECT one.value, series.value
FROM generate_series(one.value, 3) series, (SELECT 1 AS value) one;
```
the one table must be processed by the top-level loop, and series must
be nested.
- For outer joins involving TVFs, the arguments must be treated as ON
predicates, not WHERE predicates.
In SQLite, the field equivalent to `constraint_usage` (`aConstraintUsage`
from `sqlite3_index_info`) is used to request arguments that are later
passed to the `xFilter` method. In Limbo, this behavior applies to
virtual tables, but not to table-valued functions. Currently, TVFs have
dedicated handling that passes all function arguments to the filter
method and doesn't use information provided in the `constraint_usage`
field.
This commit is a step toward unifying the handling of virtual tables and
TVFs.
