| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Simon Riggs <simon(at)2ndquadrant(dot)com> |
| Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, Alexander Kuzmenkov <a(dot)kuzmenkov(at)postgrespro(dot)ru>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Removing unneeded self joins |
| Date: | 2018-05-16 20:10:45 |
| Message-ID: | 10951.1526501445@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
Simon Riggs <simon(at)2ndquadrant(dot)com> writes:
> What I would add is that I've seen cases where the extra joins do NOT
> hurt performance, so the extra CPU used to remove the join hurts more
> than the benefit of removing it. Yes, we tried it.
Interesting. The concern I had was more about the cost imposed on every
query to detect self-joins and try to prove them useless, even in queries
where no benefit ensues. It's possible that we can get that down to the
point where it's negligible; but this says that even the successful-proof
case has to be very cheap.
regards, tom lane
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