On 1 February 2018 at 08:49, Konstantin Knizhnik <k.knizh...@postgrespro.ru> wrote: > > > On 01.02.2018 03:10, Simon Riggs wrote: >> >> On 10 January 2018 at 09:54, Konstantin Knizhnik >> <k.knizh...@postgrespro.ru> wrote: >> >>> (new version attached) >> >> Why this comment? >> >> Current implementation of projection optimization has to calculate >> index expression twice >> in case of hit (value of index expression is not changed) and three >> times if values are different. >> >> Old + New for check = 2 >> plus calculate again in index = 3 >> ? >> > Sorry, I do not completely understand your question. > You do not agree with this statement or you think that this comment is > irrelevant in this place? > Or you just want to understand why index expression is calculated 2/3 times? > If so, then you have answered this question yourself: >> >> Old + New for check = 2 >> plus calculate again in index = 3 > > > Yes, we have to calculate the value of index expression for original and > updated version of the record. If them are equal, then it is all we have to > do with this index: hot update is applicable. > In this case we calculate index expression twice. > But if them are not equal, then hot update is not applicable and we have to > update index. In this case expression will be calculated one more time. So > totally three times. > This is why, if calculation of index expression is very expensive, then > effect of this optimization may be negative even if value of index > expression is not changed.
OK, thanks. Just checking my understanding and will add to the comments in the patch. -- Simon Riggs http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
