[EMAIL PROTECTED] wrote:
I'm trying to write some defensive code that is able to recover from
database corruption. The idea is that if a disk fails and a database
becomes corrupt it can be detected and synchronised from a backup copy.
To this end, I've just been trying to write a function that returns true
only when it is sure the database is ok. I use PRAGMA integrity check; and
compare the returned string with "ok". When I tried this with a few random
database changes, though, I had a hard time trying to get the corruption
to trigger. I did the following:
CREATE TABLE foo(bar);
INSERT INTO foo VALUES("bar");
I then went in with a text editor and started modifying the bar record. I
changed "bar" to "car", but the change was not detected. I started
modifying characters to the left and right of the "car" string, but still
no corruption. I was able to get corruption to be detected when I
truncated the file.
Can I take it from this behaviour that there isn't any checksum checking
going on apart from headers and the BTrees themselves? Will the
integrity_check at least guarantee me that I won't at some later stage get
an SQLITE_CORRUPT return?
PRAGMA integrity_check does a good job of testing the
integrity of the BTree layer in the file. (See
http://www.sqlite.org/fileformat.html for a definition
of the "btree layer".) The pragma also checks to make
sure that all indices are correct. But other than that,
changes to the database can easily go undetected.
Note that if you had had an index on the foo table,
your change of "bar" to "car" would have been detected
by the index checks.
--
D. Richard Hipp -- [EMAIL PROTECTED] -- 704.948.4565
---------------------------------------------------------------------
To unsubscribe, e-mail: [EMAIL PROTECTED]
For additional commands, e-mail: [EMAIL PROTECTED]
