Re: [HACKERS] Pluggable storage - Mailing list pgsql-hackers

On Thu, Jun 22, 2017 at 8:00 PM, Alexander Korotkov
<a.korotkov@postgrespro.ru> wrote:
> On Wed, Jun 21, 2017 at 10:47 PM, Robert Haas <robertmhaas@gmail.com> wrote:
>>
>> On Mon, Jun 12, 2017 at 9:50 PM, Haribabu Kommi
>> <kommi.haribabu@gmail.com> wrote:
>> > Open Items:
>> >
>> > 1. The BitmapHeapScan and TableSampleScan are tightly coupled with
>> > HeapTuple and HeapScanDesc, So these scans are directly operating
>> > on those structures and providing the result.
>> >
>> > These scan types may not be applicable to different storage formats.
>> > So how to handle them?
>>
>> I think that BitmapHeapScan, at least, is applicable to any table AM
>> that has TIDs.   It seems to me that in general we can imagine three
>> kinds of table AMs:
>>
>> 1. Table AMs where a tuple can be efficiently located by a real TID.
>> By a real TID, I mean that the block number part is really a block
>> number and the item ID is really a location within the block.  These
>> are necessarily quite similar to our current heap, but they can change
>> the tuple format and page format to some degree, and it seems like in
>> many cases it should be possible to plug them into our existing index
>> AMs without too much heartache.  Both index scans and bitmap index
>> scans ought to work.
>
>
> If #1 is only about changing tuple and page formats, then could be much
> simpler than the patch upthread?  We can implement "page format access
> methods" with routines for insertion, update, pruning and deletion of tuples
> *in particular page*.  There is no need to redefine high-level logic for
> scanning heap, doing updates and so on...

If you are changing tuple format then you do need to worry about
places wherever we are using HeapTuple like TupleTableSlots,
Visibility routines, etc. (just think if somebody changes tuple
header, then all such places are susceptible to change).

Similarly,
if the page format is changed you need to consider all page scan API's
like heapgettup_pagemode/heapgetpage.