For insert-heavy checkpoints this gives a much higher chance of using
the balance-quick subalgorithm instead of the more complex and slower
balance-nonroot.
Table ID is an opaque identifier that is only meaningful to the MV store.
Each checkpointed MVCC table corresponds to a single B-tree on the pager,
which naturally has a root page.
We cannot use root page as the MVCC table ID directly because:
- We assign table IDs during MVCC commit, but
- we commit pages to the pager only during checkpoint
which means the root page is not easily knowable ahead of time.
Hence, we:
- store the mapping between table id and btree rootpage
- sqlite_schema rows will have a negative rootpage column if the
table has not been checkpointed yet.
MVCC is like the annoying younger cousin (I know because I was him) that
needs to be treated differently. MVCC requires us to use root_pages that
might not be allocated yet, and the plan is to use negative root_pages
for that case. Therefore, we need i64 in order to fit this change.
performs a blocking, truncating checkpoint:
- when started, blocks any other transactions from starting
- writes all row versions to pager that havent already been
checkpointed
- flushes them to wal
- checkpoints wal to db
in the case of added/deleted rows in table id 1 (sqlite schema),
also creates/destroys btrees accordingly
