Re: Pluggable storage

On Thu, Sep 14, 2017 at 8:17 AM, Haribabu Kommi <kommi(dot)haribabu(at)gmail(dot)com>
wrote:

> Instead of modifying the Bitmap Heap and Sample scan's to avoid referring
> the internal members of the HeapScanDesc, I divided the HeapScanDesc
> into two parts.
>
> 1. StorageScanDesc
> 2. HeapPageScanDesc
>
> The StorageScanDesc contains the minimal information that is required
> outside
> the Storage routine and this must be provided by all storage routines. This
> structure contains minimal information such as relation, snapshot, buffer
> and
> etc.
>
> The HeapPageScanDesc contains other extra information that is required for
> Bitmap Heap and Sample scans to work. This structure contains the
> information
> of blocks, visible offsets and etc. Currently this structure is used only
> in
> Bitmap Heap and Sample scan and it's supported contrib modules, except
> the pgstattuple module. The pgstattuple needs some additional changes.
>
> By adding additional storage API to return HeapPageScanDesc as it required
> by the Bitmap Heap and Sample scan's and this API is called only in these
> two scan's. And also these scan methods are choosen by the planner only
> when the storage routine supports to returning of HeapPageScanDesc API.
> Currently Implemented the planner support only for Bitmap, yet to do it
> for Sample scan.
>
> With the above approach, I removed all the references of HeapScanDesc
> outside the heap. The changes of this approach is available in the
> 0008-Remove-HeapScanDesc-usage-outside-heap.patch
>
> Suggestions/comments with the above approach.
>

For me, that's an interesting idea. Naturally, the way BitmapHeapScan and
SampleScan work even on very high-level is applicable only for some storage
AMs (i.e. heap-like storage AMs). For example, index-organized table
wouldn't ever support BitmapHeapScan, because it refers tuples by PK values
not TIDs. However, in this case, storage AM might have some alternative to
our BitmapHeapScan. So, index-organized table might have some compressed
representation of ordered PK values set and use it for bulk fetch of PK
index.

Therefore, I think it would be nice to make BitmapHeapScan an
heap-storage-AM-specific scan method while other storage AMs could provide
other storage-AM-specific scan methods. Probably it would be too much for
this patchset and should be done during one of next work cycles on storage
AM (I'm sure that such huge project as pluggable storage AMs would have
multiple iterations).

Similarly, SampleScans contain storage-AM-specific logic. For instance,
our SYSTEM sampling method fetches random blocks from heap providing high
performance way to sample heap. Coming back to the example of
index-organized table, it could provide it's own storage-AM-specific table
sampling methods including sophisticated PK tree traversal with fetching
random small ranges of PK. Given that tablesample methods are already
pluggable, making them storage-AM-specific would lead to user-visible
changes. I.e. tablesample method should be created for particular storage
AM or set of storage AMs. However, I didn't yet figure out what should API
exactly look like...

------
Alexander Korotkov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company