mirror of
https://github.com/sqlite/sqlite.git
synced 2024-11-25 08:59:33 +01:00
77 lines
3.4 KiB
Plaintext
77 lines
3.4 KiB
Plaintext
*** Throughout this document, a page is deemed to have been synced
|
|
automatically as soon as it is written when PRAGMA synchronous=OFF.
|
|
Otherwise, the page is not synced until the xSync method of the VFS
|
|
is called successfully on the file containing the page.
|
|
|
|
*** Definition: A page of the database file is said to be "overwriteable" if
|
|
one or more of the following are true about the page:
|
|
|
|
(a) The original content of the page as it was at the beginning of
|
|
the transaction has been written into the rollback journal and
|
|
synced.
|
|
|
|
(b) The page was a freelist leaf page at the start of the transaction.
|
|
|
|
(c) The page number is greater than the largest page that existed in
|
|
the database file at the start of the transaction.
|
|
|
|
(1) A page of the database file is never overwritten unless one of the
|
|
following are true:
|
|
|
|
(a) The page and all other pages on the same sector are overwriteable.
|
|
|
|
(b) The atomic page write optimization is enabled, and the entire
|
|
transaction other than the update of the transaction sequence
|
|
number consists of a single page change.
|
|
|
|
(2) The content of a page written into the rollback journal exactly matches
|
|
both the content in the database when the rollback journal was written
|
|
and the content in the database at the beginning of the current
|
|
transaction.
|
|
|
|
(3) Writes to the database file are an integer multiple of the page size
|
|
in length and are aligned to a page boundary.
|
|
|
|
(4) Reads from the database file are either aligned on a page boundary and
|
|
an integer multiple of the page size in length or are taken from the
|
|
first 100 bytes of the database file.
|
|
|
|
(5) All writes to the database file are synced prior to the rollback journal
|
|
being deleted, truncated, or zeroed.
|
|
|
|
(6) If a master journal file is used, then all writes to the database file
|
|
are synced prior to the master journal being deleted.
|
|
|
|
*** Definition: Two databases (or the same database at two points it time)
|
|
are said to be "logically equivalent" if they give the same answer to
|
|
all queries. Note in particular the content of freelist leaf
|
|
pages can be changed arbitarily without effecting the logical equivalence
|
|
of the database.
|
|
|
|
(7) At any time, if any subset, including the empty set and the total set,
|
|
of the unsynced changes to a rollback journal are removed and the
|
|
journal is rolled back, the resulting database file will be logical
|
|
equivalent to the database file at the beginning of the transaction.
|
|
|
|
(8) When a transaction is rolled back, the xTruncate method of the VFS
|
|
is called to restore the database file to the same size it was at
|
|
the beginning of the transaction. (In some VFSes, the xTruncate
|
|
method is a no-op, but that does not change the fact the SQLite will
|
|
invoke it.)
|
|
|
|
(9) Whenever the database file is modified, at least one bit in the range
|
|
of bytes from 24 through 39 inclusive will be changed prior to releasing
|
|
the EXCLUSIVE lock.
|
|
|
|
(10) The pattern of bits in bytes 24 through 39 shall not repeat in less
|
|
than one billion transactions.
|
|
|
|
(11) A database file is well-formed at the beginning and at the conclusion
|
|
of every transaction.
|
|
|
|
(12) An EXCLUSIVE lock must be held on the database file before making
|
|
any changes to the database file.
|
|
|
|
(13) A SHARED lock must be held on the database file before reading any
|
|
content out of the database file.
|