Re: A little COPY speedup

Tom Lane wrote:
> As you say, pd_tli is not really pulling its weight, but I'm also loath
> to remove it, as in a multi-timeline situation the page LSN is really
> not well defined if you don't know which timeline it refers to.
>
> Now we'd only need 16 bits to store the last-used offset, or a flags
> field if you'd prefer that, so one possible compromise is to store only
> the 16 least significant bits of TLI (which ought to be enough to
> disambiguate in any real-world situation), and insert the new field
> where the MSBs had been.

Sounds good to me. It's nice to keep the TLI for debugging/forensics
purposes even if it's not used at the moment, but 16 bits is enough for
that.

Another possibility would be to use the unused bits in
pd_upper/lower/special, but that requires more bit-trickery.

> I'm not sure whether I like your flag approach better than the
> last-used-offset one. The previous patch probably buys some teeny
> amount more performance, but the flag seems more robust (noting in
> passing that neither patch attempts to WAL-log its changes, so we really
> need to treat the values as hints). And a change as sketched here would
> leave us 15 free bits for future expansion, which might be nice.

I'll post a patch along those lines.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com