Re: Parallel Seq Scan

On Sat, Feb 7, 2015 at 10:36 PM, Amit Kapila <amit.kapila16@gmail.com> wrote:
>> Well, I agree with you, but I'm not really sure what that has to do
>> with the issue at hand.  I mean, if we were to apply Amit's patch,
>> we'd been in a situation where, for a non-parallel heap scan, heapam.c
>> decides the order in which blocks get scanned, but for a parallel heap
>> scan, nodeParallelSeqscan.c makes that decision.
>
> I think other places also decides about the order/way heapam.c has
> to scan, example the order in which rows/pages has to traversed is
> decided at portal/executor layer and the same is passed till heap and
> in case of index, the scanlimits (heap_setscanlimits()) are decided
> outside heapam.c and something similar is done for parallel seq scan.
> In general, the scan is driven by Scandescriptor which is constructed
> at upper level and there are some API's exposed to derive the scan.
> If you are not happy with the current way nodeParallelSeqscan has
> set the scan limits, we can have some form of callback which do the
> required work and this callback can be called from heapam.c.

I thought about a callback, but what's the benefit of doing that vs.
hard-coding it in heapam.c?  If the upper-layer wants to impose a TID
qual or similar then heap_setscanlimits() makes sense, but that's
effectively a filter condition, not a policy decision about the access
pattern.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company