Re: [ANNOUNCE] IMCS: In Memory Columnar Store for PostgreSQL - Mailing list pgsql-hackers

On Thu, Jan 9, 2014 at 12:21 AM, Robert Haas <robertmhaas@gmail.com> wrote:
> On Tue, Jan 7, 2014 at 10:20 PM, Amit Kapila <amit.kapila16@gmail.com> wrote:
>> On Tue, Jan 7, 2014 at 2:46 AM, Robert Haas <robertmhaas@gmail.com> wrote:
>>>
>>> Well, right now we just reopen the same object from all of the
>>> processes, which seems to work fine and doesn't require any of this
>>> complexity.  The only problem I don't know how to solve is how to make
>>> a segment stick around for the whole postmaster lifetime.  If
>>> duplicating the handle into the postmaster without its knowledge gets
>>> us there, it may be worth considering, but that doesn't seem like a
>>> good reason to rework the rest of the existing mechanism.
>>
>> I think one has to try this to see if it works as per the need. If it's not
>> urgent, I can try this early next week?
>
> Anything we want to get into 9.4 has to be submitted by next Tuesday,
> but I don't know that we're going to get this into 9.4.

Using DuplicateHandle(), we can make segment stick for Postmaster
lifetime. I have used below test (used dsm_demo module) to verify:
Session - 1
select dsm_demo_create('this message is from session-1');dsm_demo_create
-----------------      827121111

Session - 2
-----------------
select dsm_demo_read(827121111);      dsm_demo_read
----------------------------this message is from session-1
(1 row)

Session-1
\q

-- till here it will work without DuplicateHandle as well

Session -2
select dsm_demo_read(827121111);      dsm_demo_read
----------------------------this message is from session-1
(1 row)

Session -2
\q

Session -3
select dsm_demo_read(827121111);      dsm_demo_read
----------------------------this message is from session-1
(1 row)

-- above shows that handle stays around.

Note -
Currently I have to bypass below code in dam_attach(), as it assumes
segment will not stay if it's removed from control file.

/*
* If we didn't find the handle we're looking for in the control
* segment, it probably means that everyone else who had it mapped,
* including the original creator, died before we got to this point.
* It's up to the caller to decide what to do about that.
*/
if (seg->control_slot == INVALID_CONTROL_SLOT)
{
dsm_detach(seg);
return NULL;
}


Could you let me know what exactly you are expecting in patch,
just a call to DuplicateHandle() after CreateFileMapping() or something
else as well?

With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com