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