Currently indexes are the bulk of the problem with `UPDATE` and
`DELETE`, while we work on fixing those it makes sense to disable
indexing since they are not stable. We want to try to make everything
else stable before we continue with indexing.
In left joins, even if the join condition is not matched, the system
must emit a row for every row of the outer table:
-- this must return t1.count() rows, with NULLs for all columns of t2
SELECT * FROM t1 LEFT JOIN t2 ON FALSE;
Our logic for clearing the null flag was to do it in Next/Prev. However,
this is problematic for a few reasons:
- If the inner table of the left join is using SeekRowid, then Next/Prev
is never called on its cursor, so the null flag doesn't get cleared.
- If the inner table of the left join is using a non-covering index seek,
i.e. it iterates its rows using an index, but seeks to the main table
to fetch data, then Next/Prev is never called on the main table, and the
main table's null flag doesn't get cleared.
What this results in is NULL values incorrectly being emitted for the
inner table after the first correct NULL row, since the null flag is
correctly set to true, but never cleared.
This PR fixes the issue by clearing the null flag whenever seek() is
invoked on the cursor. Hence, the null flag is now cleared on:
- next()
- prev()
- seek()
