Vacuuming big btree indexes without pages with deleted items

Hi all.

I have described [0] a problem with delaying replicas after vacuuming a relation with big btree index. It stucks in replaying WAL record of type XLOG_BTREE_VACUUM like that (with lastBlockVacuumed 0):

rmgr: Btree len (rec/tot): 20/ 52, tx: 0, lsn: 4115/56126DC0, prev 4115/56126D90, bkp: 0000, desc: vacuum: rel 1663/16420/16796; blk 31222118, lastBlockVacuumed 0

Master writes this record to xlog in btvacuumscan [1] function after vacuuming of all index pages. And in case of no pages with deleted items xlog record would contain lastBlockVacuumed 0.

In btree_xlog_vacuum [2] replica reads all blocks from lastBlockVacuumed to last block of the index while applying this record because there is no api in the buffer manager to understand if the page is unpinned.

So if the index is quite big (200+ GB in described case) it takes much time to do it. So the questions are:

1. Aren’t there still any api in buffer manager to understand that the page is not in shared_buffers without reading it?
2. Is it possible not to write to xlog record with lastBlockVacuumed 0 in some cases? For example, in case of not deleting any pages.

Or maybe there are some better ways of improving this situation?

[0] http://www.postgresql.org/message-id/FE82A9A7-0D52-41B5-A9ED-967F6927CB8A@simply.name
[1] http://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/backend/access/nbtree/nbtree.c;hb=refs/heads/REL9_4_STABLE#l813
[2] http://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/backend/access/nbtree/nbtxlog.c;hb=refs/heads/REL9_4_STABLE#l482

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