We don't want something like `BEGIN IMMEDIATE` to start a subtransaction,
so instead we will open it if:
- Statement is write, AND
a) Statement has >0 table_references, or
b) The statement is an INSERT (INSERT doesn't track table_references in
the same way as other program types)
The subjournal is a temporary file where stmt subtransactions write an
'undo log' of pages before modifying them. If a stmt subtransaction
rolls back, the pages are restored from the subjournal.
1. The number of deferred FK violations when the statement started.
When a statement subtransaction rolls back, the connection's
deferred violation counter will be reset to this value.
2. The number of immediate FK violations that occurred during the
statement. In practice we just need to know whether this number
is nonzero, and if it is, the statement subtransaction will roll
back.
Statement subtransactions will be implemented in future commits.
Every transaction was reading page 1 from the WAL to check the schema cookie
in op_transaction, causing unnecessary WAL lookups.
This commit caches the schema_cookie in Pager as AtomicU64, similar to how
page_size and reserved_space are already cached. The cache is updated when the
header is read/modified and invalidated in begin_read_tx() when WAL changes
are detected from other connections.
This matches SQLite's approach of caching frequently accessed header fields to
avoid repeated page 1 reads. Improves write throughput by 5% in our
benchmarks.
Depends on #3584 to use the most up-to-date implementation of
`ThreadRng`
- Add `fill_bytes` method to `IO`
- use `thread_rng` instead of `getrandom`, as `getrandom` is much slower
and `thread_rng` offers enough security
- modify `exec_randomblob`, `exec_random` and random_rowid generation to
use methods from IO for determinism
- modified simulator IO to implement `fill_bytes`
This the PRNG for sqlite if someone is curious. It is similar to
`thread_rng`:
```c
/* Initialize the state of the random number generator once,
** the first time this routine is called.
*/
if( wsdPrng.s[0]==0 ){
sqlite3_vfs *pVfs = sqlite3_vfs_find(0);
static const u32 chacha20_init[] = {
0x61707865, 0x3320646e, 0x79622d32, 0x6b206574
};
memcpy(&wsdPrng.s[0], chacha20_init, 16);
if( NEVER(pVfs==0) ){
memset(&wsdPrng.s[4], 0, 44);
}else{
sqlite3OsRandomness(pVfs, 44, (char*)&wsdPrng.s[4]);
}
wsdPrng.s[15] = wsdPrng.s[12];
wsdPrng.s[12] = 0;
wsdPrng.n = 0;
}
assert( N>0 );
while( 1 /* exit by break */ ){
if( N<=wsdPrng.n ){
memcpy(zBuf, &wsdPrng.out[wsdPrng.n-N], N);
wsdPrng.n -= N;
break;
}
if( wsdPrng.n>0 ){
memcpy(zBuf, wsdPrng.out, wsdPrng.n);
N -= wsdPrng.n;
zBuf += wsdPrng.n;
}
wsdPrng.s[12]++;
chacha_block((u32*)wsdPrng.out, wsdPrng.s);
wsdPrng.n = 64;
}
sqlite3_mutex_leave(mutex);
```
Reviewed-by: Pere Diaz Bou <pere-altea@homail.com>
Closes#3799
We have almost 1000 lines of code related to Completions the
`io/mod.rs`. My intention here is to declutter the file so that we can
focus more on what is relevant for the `IO` and `File` trait
Closes#3800
