Re: tackling full page writes

Robert Haas wrote:
> 2. The other fairly obvious alternative is to adjust our existing WAL
> record types to be idempotent - i.e. to not rely on the existing page
> contents.  For XLOG_HEAP_INSERT, we currently store the target tid and
> the tuple contents.  I'm not sure if there's anything else, but we
> would obviously need the offset where the new tuple should be written,
> which we currently infer from reading the existing page contents.  For
> XLOG_HEAP_DELETE, we store just the TID of the target tuple; we would
> certainly need to store its offset within the block, and maybe the
> infomask.  For XLOG_HEAP_UPDATE, we'd need the old and new offsets and
> perhaps also the old and new infomasks.  Assuming that's all we need
> and I'm not missing anything (which I won't bet on), that means we'd
> be adding, say, 4 bytes per insert or delete and 8 bytes per update.
> So, if checkpoints are spread out widely enough that there will be
> more than ~2K operations per page between checkpoints, then it makes
> more sense to just do a full page write and call it good.  If not,
> this idea might have legs.

I vote for "wal_level = idempotent" because so few people will know what
idempotent means.  ;-)

Idempotent does seem like the most promising idea.

--  Bruce Momjian  <bruce@momjian.us>        http://momjian.us EnterpriseDB
http://enterprisedb.com
 + It's impossible for everything to be true. +