Re: POC and rebased patch for CSN based snapshots

>When checking each tuple visibility, we always have to get the CSN
>corresponding to XMIN or XMAX from CSN SLRU. In the past discussion,
>there was the suggestion that CSN should be stored in the tuple header
>or somewhere (like hint bit) to avoid the overhead by very frequehntly
>lookup for CSN SLRU. I'm not sure the conclusion of this discussion.
>But this patch doesn't seem to adopt that idea. So did you confirm that
>such performance overhead by lookup for CSN SLRU is negligible?
This patch came from postgrespro's patch which shows a good performance,
I have simple test on current patch and result no performance decline.
And not everytime we do a tuple visibility need lookup forCSN SLRU, only xid
large than 'TransactionXmin' need that. Maybe we have not touch the case
which cause bad performance, so it shows good performance temporary.

>Of course I know that idea has big issue, i.e., there is no enough space
>to store CSN in a tuple header if CSN is 64 bits. If CSN is 32 bits, we may
>be able to replace XMIN or XMAX with CSN corresponding to them. But
>it means that we have to struggle with one more wraparound issue
>(CSN wraparound issue). So it's not easy to adopt that idea...

>Sorry if this was already discussed and concluded...
I think your point with CSN in tuple header is a exciting approach, but I have
not seen the discussion, can you show me the discussion address?

Regards,
Highgo Software (Canada/China/Pakistan)
URL : www.highgo.ca
EMAIL: mailto:movead(dot)li(at)highgo(dot)ca