Mrs. Brisby wrote:
The linked-list structure of overflow storage is part of the problem.
But the fact that SQLite uses synchronous I/O is also a factor. In
order to make BLOBs fast in SQLite, I would have to change to a different
indexing technique for overflow storage *and* come up with some kind
of cross-platform, asynchronous disk read mechanism.
D.R.Morrison (1968)'s PATRICIA would certainly be faster for indexing
large objects.
A key feature of SQLite 3.0 (needed to support internationalization)
is the ability of users to specify their own comparison functions then
have SQLite use that comparison function to order indices. PATRICIA
does not support user-defined comparison functions. Keys in PATRICIA
must occur in memcmp() order, as far as I am aware.
Asynchronous read isn't necessary, but vectored reads are. Consider
readv() POSIX 1003.1-2001 -- in fact, you could probably make
result-fields return a struct iovec * that would "point" to the value
within the database.
readv() doesn't help, actually. BLOBs are stored in 1k blocks scattered
all over the file. readv() reads a continguous range of bytes - it
puts those bytes into scattered buffers but the bytes must originate
from a contiguous region of the file. I'd still have to do 1024
sequential readv()s in order to extract a 1MB blob.
--
D. Richard Hipp -- [EMAIL PROTECTED] -- 704.948.4565
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