On 21 December 2015 at 23:57, Viktor Leis <l...@in.tum.de> wrote:
> > Please have a look at Figure 6 (page 6) in > http://www.vldb.org/pvldb/vol9/p204-leis.pdf Disabling nested loop > joins without index scan (going from (a) to (b)) results in great > improvements across the board. And even more importantly, it avoids > most of the cases where queries took unreasonably long and timed > out. Basically this amounts to the being able to run the query on > PostgreSQL or not. > For that data, yes. But you're ignoring other important cases. Small or even 1-element lookup tables can be one where a nestloop over a seqscan turns out to be by far the fastest way to do the job. This can really add up if it's deep in a subplan that's excuted repeatedly, or if it's part of queries that get run very frequently on a busy OLTP system. That said, these cases are also the ones that land up hurting very badly if the stats are inaccurate or outdated and our expected 3 loops turns into 3000. -- Craig Ringer http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services
