On Sat, Aug 20, 2011 at 4:57 AM, Gokulakannan Somasundaram

> <gokul...@gmail.com> wrote:
> > by your argument, if WALInserLock is held for 't' seconds, you should
> > definitely be holding visibility map lock for more than time frame 't'.
> Nope, that's not how it works.  Perhaps you should read the code.
> See, e.g., heap_update().
> --
OK. I took a look at the patch you have supplied in
There is a code like this.

         all_visible_cleared = true;
+        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??


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