Re: Forcing Postgres to Execute a Specific Plan

I completely understand that what I am proposing is somewhat mad and I
didn't expect it to be easy.

Basically, I'm doing some research on a new operator and would like to
start testing it by inserting it into a very specific place in very
specific plans without having to do too much work in plan generation
or optimization. I think that I could do this by writing some code to
inspect a plan and swap out the piece that I care about. I realize
this is a hack, but at the moment it's just for research purposes.
Though I have worked with the internals of other db systems, I'm still
getting familiar with postgres. Could such a piece of code be placed
in the optimizer just before it returns an optimized plan or can a
plan be modified after it is returned by the optimizer?

John Cieslewicz.


On Jun 2, 2008, at 5:24 PM, Scott Marlowe wrote:

> Not really.  It was decided long ago that in that way madness lies.
>
> OTOH, there are ways to tune the behaviour through changes to
> random_page_cose, cpu_xxx_cost and effective_cache_size settings.
>
> Then there's the mallet to the forebrain that are the set
> enable_nestloop=off type settings.  They work, but they shouldn't be
> your first line of attack so much as a troubleshooting tool to figure
> out what pgsql might be getting wrong.
>
> On Mon, Jun 2, 2008 at 1:43 PM, John Cieslewicz
> <johnc@cs.columbia.edu> wrote:
>> I'm doing some performance experiments with postgres (8.3.1) and
>> would like
>> to force postgres to execute a particular query plan. Is there a
>> straightforward way to specify a query plan to postgres either
>> interactively
>> or programatically?
>>
>> Thanks.
>>
>> John Cieslewicz.
>>
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