On Thu, Aug 4, 2016 at 10:49 PM, Bruce Momjian <bruce@momjian.us> wrote:
On Thu, Aug 4, 2016 at 06:16:02PM +0100, Simon Riggs wrote: > On 4 August 2016 at 18:05, Bruce Momjian <bruce@momjian.us> wrote: > > >> Approach 2 seems more reasonable and simple. > >> > >> There are only 2 bits for lp_flags and all combinations are already used. But > >> for LP_REDIRECT root line pointer, we could use the lp_len field to store this > >> special flag, which is not used for LP_REDIRECT line pointers. So we are able > >> to mark the root line pointer. > > > > Uh, as I understand it, we only use LP_REDIRECT when we have _removed_ > > the tuple that the ctid was pointing to, but it seems you would need to > > set HEAP_RECHECK_REQUIRED earlier than that. > > Hmm. Mostly there will be one, so this is just for the first update > after any VACUUM. > > Adding a new linepointer just to hold this seems kludgy and could mean > we run out of linepointers.
Ah, so in cases where there isn't an existing LP_REDIRECT for the chain, you create one and use the lp_len to identify it as a WARM chain? Hmm.
If the root tuple still exists, we store the WARM flag (or HEAP_RECHECK_REQUIRED as used in the original post) in the tuple header itself. When the root tuple becomes dead and HOT prune decides to replace it with a LP_REDIRECT line pointer, the information is moved to lp_len (which is currently set to 0 for LP_REDIRECT items). Does that answer your question?
You can't update the indexes pointing to the existing ctid, so what you would really have to do is to write over the existing ctid with LP_REDIRECT plus WARM marker, and move the old ctid to a new ctid slot?
Not really. I hope the above answers this, but please let me know if you mean something else.
