Re: [HACKERS] Pluggable storage

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.