Re: unlogged tables vs. GIST

Hi Robert / Tom

I think to support GiST with unlogged table we need to do three things:

1. Teach the buffer manager that the LSN of a page not marked
BM_PERMANENT can be ignored

2. Teach GetXLogRecPtrForTemp() to allocate fake LSNs for GiST buffers
using a 64-bit, counter that is persisted across clean shutdowns

3. Remove the error checks that prevent creating an unlogged GiST
index and update the documentation accordingly

PFA, patch which try to do above things.

For (2), I have added a new function called, GetXLogRecPtrForUnloogedRel()
which returns a fake LSN for GiST indexes. However, I have removed
GetXLogRecPtrForTemp() function and added its functionality inside this new
function itself to avoid complexity.

With this patch I am able to create GiST for unlogged tables.
It works well with my examples.
Also make check has no issues.

Please have a look and let me know your views.

Thanks

On Sat, Dec 18, 2010 at 7:20 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:

> On Fri, Dec 17, 2010 at 4:17 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> >> Since these bits will only be set/cleared when the buffer mapping is
> >> changed, can we examine this bit without taking the spinlock?
> >
> > Only if you're willing for the result to be unreliable.
>
> If we read the bits while someone else is writing them, we'll get
> either the old or the new value, but the BM_FLUSH_XLOG bit will be the
> same either way. When FlushBuffer() is called, a shared content log
> and a pin are held, so the buffer can't be renamed under us. If
> that's really unacceptable, we can do something like the attached, but
> I think this is unnecessarily gross. We already assume in other
> places that the read or write of an integer is atomic. In this case
> we don't even need it to be fully atomic - we just need the particular
> byte that contains this bit not to go through some intermediate state
> where the bit is unset. Is there really a memory architecture out
> there that's crazy enough to let such a thing happen? If so, I'd
> expect things to be breaking right and left on that machine anyway.
>
> --
> Robert Haas
> EnterpriseDB: http://www.enterprisedb.com
> The Enterprise PostgreSQL Company
>
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