Robert Haas <robertmhaas@gmail.com> writes:
> On Wed, Dec 3, 2014 at 12:08 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> I would envision the planner starting out generating the first subplan
>> (without the optimization), but as it goes along, noting whether there
>> are any opportunities for join removal. At the end, if it found that
>> there were such opportunities, re-plan assuming that removal is possible.
>> Then stick a switch node on top.
>>
>> This would give optimal plans for both cases, and it would avoid the need
>> for lots of extra planner cycles when the optimization can't be applied
>> ... except for one small detail, which is that the planner has a bad habit
>> of scribbling on its own input. I'm not sure how much cleanup work would
>> be needed before that "re-plan" operation could happen as easily as is
>> suggested above. But in principle this could be made to work.
> Doesn't this double the planning overhead, in most cases for no
> benefit? The alternative plan used only when there are deferred
> triggers is rarely going to get used.
Personally, I remain of the opinion that this optimization will apply in
only a tiny fraction of real-world cases, so I'm mostly concerned about
not blowing out planning time when the optimization doesn't apply.
However, even granting that that is a concern, so what? You *have* to
do the planning twice, or you're going to be generating a crap plan for
one case or the other.
regards, tom lane
