Re: Removing unneeded self joins

HI,

On 2018-05-17 08:48:58 +1200, David Rowley wrote:
> On 17 May 2018 at 08:44, Simon Riggs  wrote:
> > What I was advocating was an approach that varies according to the
> > query cost, so we don't waste time trying to tune the heck out of OLTP
> > queries, but for larger queries we might take a more considered
> > approach.
> 
> That's tricky. If we do this, it should be done before Path
> generation, so not much is known about the costs in those case.
> 
> Perhaps something can be done by looking at the number of relpages,
> but I've no idea what that would be. Perhaps we need to see how costly
> this operation is first before we try to think of ways to only apply
> it conditionally?

I'm also not buying that this isn't a benefit in OLTP in general. Sure,
for a single query RTT costs are going to dominate, but if you use
prepared statements the costs are going to pay of over multiple
executions.  Even just avoiding initializing unnecessary executor nodes
shows up in profiles.

Greetings,

Andres Freund