Re: Measuring the Query Optimizer Effect: Turning off the QO?

On 7/7/19 6:49 PM, Tom Mercha wrote:
> On 08/07/2019 01:46, Rob Sargent wrote:
>>> On Jul 7, 2019, at 5:22 PM, Tom Mercha <mercha_t(at)hotmail(dot)com> wrote:
>>>
>>> Hi All
>>>
>>> As we know, a query goes through number of stages before it is executed.
>>> One of these stages is query optimization (QO).
>>>
>>> There are various parameters to try and influence optimizer decisions
>>> and costs. But I wanted to measure the effect of such a stage by turning
>>> it off completely and I can't find such a parameter which explicitly
>>> does that. Then I could execute a query to get the effect of "QO active
>>> and "QO inactive" and compare.
>>>
>>> Obviously, I know well what the results would generally look like but I
>>> am just interested in measuring the differences for various types of
>>> queries. I am also aware that this is a simple comparison - there are
>>> more interesting comparisons to perform with QO tweaks, but right now I
>>> am interested in something basic.
>>>
>>> So how would one shut down QO? Or at least, obtaining the guarantee of
>>> generating the worst plan possible, ideally without touching many
>>> parameters?
>>>
>>> Best,
>>> Tom
>> Drop all indices?
>>
> Sorry, maybe my question wasn't clear enough.
>
> A query can be rewritten in various ways by applying rules and costs of
> relational algebra operators, as well as their parallelisation. I am
> talking about turning off this query optimization, so I am already
> assuming that indexes aren't present.

It seems as though you're asking what the "First Approximation" plan is,
before it tries to get Too Clever.  However, I don't think there's a
separation between Query *Planner* and Query *Optimizer*.
https://www.postgresql.org/docs/9.6/planner-optimizer.html

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