Re: the big picture for index-only scans

On Sat, Aug 20, 2011 at 4:57 AM, Gokulakannan Somasundaram
OK. I took a look at the patch you have supplied in http://postgresql.1045698.n5.nabble.com/crash-safe-visibility-map-take-five-td4377235.html
There is a code like this.

     {
         all_visible_cleared = true;
         PageClearAllVisible(BufferGetPage(buffer));
+        visibilitymap_clear(relation,
+                            ItemPointerGetBlockNumber(&(heaptup->t_self)),
+                            &vmbuffer);
     }

Here, you are not making an entry into the WAL. then there is an assumption that the two bits will be in sync without any WAL entry. There is a chance that the visibility map might be affected by partial page writes, where clearing of a particular bit might not have been changed. And i am thinking a lot of such issues. Can you just explain the background logic behind ignoring the principle of WAL logging? What are the implemented principles, that protect the Visibility map pages??

Thanks,
Gokul.