On Thu, Apr 21, 2016 at 8:20 PM, Dan Kennedy <danielk1977 at gmail.com> wrote:
> Generally speaking, no matter how they are created, a given in-memory >> database has one and only one connection. You cannot, for example, use a >> URI ?filename? with mode=memory to open the same in-memory database more >> than once (I assume that?s what you mean by ?by name??).[...] >> > Yes you can Jay, see Dan's answer. And https://www.sqlite.org/sharedcache.html section 6.0: Enabling shared-cache for an in-memory database allows two or more database connections in the same process to have access to the same in-memory database. An in-memory database in shared cache is automatically deleted and memory is reclaimed when the last connection to that database closes. >> You can see that even though I?ve opened the same >> ?file:data.db?mode=memory? database more than once, it is actually three >> distinct databases. I?m pretty sure that when mode=memory, the >> path/filename are ignored. > > > If you open the databases using "file:data.db?mode=memory&cache=shared" > then db1, db2 and db3 will all refer to the same in-memory database. Which > will fail, as you cannot attach the same db to a single handle more than > once. > But, if your process has two separate database handles and they both > attach "file:data.db?mode=memory&cache=shared" then both handles will be > connected to the same in-memory database. This is exactly what we are doing. Now I keep on reading in this list cache=shared is for constrained devices and one shouldn't use it ever. But how else to use an in-memory database from multiple threads and leverage table-level locking as opposed to DB-level locking? We are heavy users of vtables and in-memory DBs, plural, within the same process. And would like to have some //ism on tables. I'd even wish for WAL-type MVCC for in-memory databases to avoid locks even more. Is WAL mode supported for in-memory DBs? What is "wrong" with shared cache that people keep saying don't use it? Dan, Richard, is shared cache broken somehow? Thanks, --DD
