Re: Removing unneeded self joins

Andres Freund <andres@anarazel.de>

From: Andres Freund <andres@anarazel.de>

To: David Rowley <david.rowley@2ndquadrant.com>

Cc: Simon Riggs <simon@2ndquadrant.com>, Tom Lane <tgl@sss.pgh.pa.us>, Robert Haas <robertmhaas@gmail.com>, Alexander Kuzmenkov <a.kuzmenkov@postgrespro.ru>, PostgreSQL-development <pgsql-hackers@postgresql.org>

Date: 2018-05-16T22:02:22Z

Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

Remove GUC_NOT_IN_SAMPLE from enable_self_join_elimination
- 717d0e8dd945 18.0 landed
Put enable_self_join_elimination into postgresql.conf.sample
- c2d329260cd8 18.0 landed
Get rid of ojrelid local variable in remove_rel_from_query()
- e167191dc146 18.0 landed
Implement Self-Join Elimination
- fc069a3a6319 18.0 cited
Revert: Remove useless self-joins
- d1d286d83c0e 17.0 landed
Replace lateral references to removed rels in subqueries
- 466979ef031a 17.0 landed
Replace relids in lateral subquery parse tree during SJE
- 489072ab7a9e 17.0 landed
Forbid SJE with result relation
- 8c441c082797 17.0 landed
Fix misuse of RelOptInfo.unique_for_rels cache by SJE
- 30b4955a4668 17.0 landed
Replace the relid in some missing fields during SJE
- a7928a57b9f0 17.0 landed
Revert 56-bit relfilenode change and follow-up commits.
- a448e49bcbe4 16.0 cited
Stabilize timetz test across DST transitions.
- 4a071afbd056 14.0 cited
Speed up finding EquivalenceClasses for a given set of rels
- 3373c7155350 13.0 cited
Fix mark-and-restore-skipping test case to not be a self-join.
- 24d08f3c0a1f 12.0 landed

HI,

On 2018-05-17 08:48:58 +1200, David Rowley wrote:
> On 17 May 2018 at 08:44, Simon Riggs <simon@2ndquadrant.com> 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