Re: lmdb: is read operation permitted inside the write transaction?

[Timur Kristóf <timur.kristof@gmail.com>]

On Tue, 2015-05-19 at 17:05 +0100, Howard Chu wrote:
> Timur Kristóf wrote:
> > On Mon, 2015-05-18 at 23:03 +0200, Hallvard Breien Furuseth wrote:
> > > On 12/05/15 19:36, Howard Chu wrote:
> > > > Dmytro Milinevskyy wrote:
> > > > > Basically I need to have an opportunity to call mdb_get while 
> > > > > in
> > > > > write
> > > > > transaction.
> > > > > The sequence is smth like:
> > > > >    - txn = mdb_txn_begin(flags=0)
> > > > >    for i in 0..x:
> > > > >      - v = mdb_get(txn, i)
> > > > >      - mdb_put(txn, x+i, v)
> > > > >      - mdb_put(txn, i, v+1)
> > > > >    - mdb_txn_commit(txn)
> > > > > 
> > > > > Will it be always valid data?
> > > > 
> > > > Yes.
> > > 
> > > ...No.  I expect he means v = the MDB_val returned by mdb_get().
> > > mdb_put() can modify the data it points at.
> 
> Since he's using "v+1" in his example I assumed he's making a local 
> copy of 
> the returned values. Anyway, the plain answer to the question in the 
> Subject 
> is Yes.
> 
> > > See the MDB_val documentation in ldmb.h:
> > >    * Values returned from the database are valid only until a
> > > subsequent
> > >    * update operation, or the end of the transaction. Do not 
> > > modify or
> > >    * free them, they commonly point into the database itself.
> 
> > What exactly does a subsequent update operation mean? Overwriting 
> > the
> > value of the same key,
> 
> If we meant "same key" we would have said so.

Okay, thanks for the clarification!

> 
> > or any mdb_put or mdb_cursor_put operation at
> > all, regardless of which key they touch?
>