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the stats are inaccurate or outdated and our expected 3 loops turns into

 Craig Ringer                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services

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