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

> On Jul 7, 2019, at 5:49 PM, Tom Mercha <mercha_t(at)hotmail(dot)com> 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
>>> https://gitlab.com/camplab/jpsgcs 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.

Have you played with any of these settings?

postgres=# select version();
version
---------------------------------------------------------------------------------------------------------
PostgreSQL 10.7 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-36), 64-bit
(1 row)

postgres=# select name, setting, unit,short_desc from pg_settings where name ~ 'para';
name | setting | unit | short_desc
---------------------------------+---------+------+----------------------------------------------------------------------------------------------------
force_parallel_mode | off | | Forces use of parallel query facilities.
max_parallel_workers | 16 | | Sets the maximum number of parallel workers that can be active at one time.
max_parallel_workers_per_gather | 8 | | Sets the maximum number of parallel processes per executor node.
min_parallel_index_scan_size | 64 | 8kB | Sets the minimum amount of index data for a parallel scan.
min_parallel_table_scan_size | 1024 | 8kB | Sets the minimum amount of table data for a parallel scan.
parallel_setup_cost | 1000 | | Sets the planner's estimate of the cost of starting up worker processes for parallel query.
parallel_tuple_cost | 0.1 | | Sets the planner's estimate of the cost of passing each tuple (row) from worker to master backend.
ssl_dh_params_file | | | Location of the SSL DH parameters file.
(8 rows)