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

> On Jul 7, 2019, at 6:29 PM, Rob Sargent <robjsargent(at)gmail(dot)com> wrote:
>
>
>
>> O
>>>>>> 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)
>>>>>
>>>> Well not the last one of course.
>>>
>>> Better yet, “where category ~* ‘planner’"
>>>
>>
>> Hi
>>
>> Yes, I've taken a look. I'm just interested in turning off the whole
>> module and there is no parameter to do just that - an on and off switch
>> - from what I can understand. What I can do instead is to go over the
>> entire list of parameters and configure them each to generate a bad
>> plan, but I'm not sure in general how to make a configuration which
>> gives me the worst plan possible under all situations.
>>
>> I was hoping that someone has the on/off switch I'm looking for or a
>> parameter template which has been used for the same purpose or something
>> along these lines...
>>
>> This document describes the module I'm talking about:
>> https://www.postgresql.org/docs/current/planner-optimizer.html <https://www.postgresql.org/docs/current/planner-optimizer.html>
> Yeah, I guess I read that to say postgres will optimize generically (after geqo_threshold). Now maybe that’s the worst plan? Per force a lot of seq-scans in the absence of any indices. And if it isn’t the worst possible plan, so what, you’re using postgres, you won’t get a worse plan without lying to the optimizer.
> But I’m clearly in over my head. The pros will be along shortly, I’m sure.

And I didn’t notice I had pasted a url to a git project. Totally irrelevant to the issue at hand. My apologies.