On 4 August 2016 at 18:17, Andres Freund <and...@anarazel.de> wrote: > On 2016-08-04 18:11:17 +0100, Simon Riggs wrote: >> On 4 August 2016 at 17:31, Andres Freund <and...@anarazel.de> wrote: >> > Hi, >> > >> > On 2016-08-04 16:29:09 +0530, Pavan Deolasee wrote: >> >> Indexes whose values do not change do not require new index pointers. Only >> >> the index whose key is being changed will need a new index entry. The new >> >> index entry will be set to the CTID of the root line pointer. >> > >> > That seems to require tracing all hot-chains in a page, to actually >> > figure out what the root line pointer of a warm-updated HOT tuple is, >> > provided it's HOT_UPDATED itself. Or have you found a smart way to >> > figure that out? >> >> Hmm, sorry, I did think of that point and I thought I had added it to the >> doc. >> >> So, yes, I agree - re-locating the root is the biggest downside to >> this idea. Perhaps Pavan has other thoughts? >> >> I think its doable, but it will be fiddly. > > I'm less afraid of the fiddlyness of finding the root tuple, than the > cost. It's not cheap to walk through, potentially, all hot chains to > find the root ctid. > > Have you considered what it'd take to allow index pointers to allow to > point to "intermediate" root tuples? I.e. have some indexes point into > the middle of an update-chain, whereas others still point to the > beginning? There's certainly some complications involved with that, but > it'd also have the advantage in reducing the amount of rechecking that > needs to be done.
"Intermediate root tuples" was my first attempt at this and it didn't work. I'll dig up the details, some problem in VACUUM, IIRC. -- Simon Riggs http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
