Re: (select query)/relation as first class citizen

Hi

po 8. 7. 2019 v 9:33 odesílatel Roman Pekar <roma(dot)pekar(at)gmail(dot)com> napsal:

> Hi,
>
> what do you think about this idea in general? If you don't have to think
> about implementation for now? From my point of view writing Sql queries is
> very close to how functional language work if you treat "select" queries as
> functions without side-effects, and having query being first-class-citizen
> could move this even further.
>

first - please, don't send top posts.

second - my opinion is not clear. I can imagine benefits - on second hand,
the usage is relative too close to one antipattern - only one query wrapped
by functions. I see your proposal as little bit more dynamic (with little
bit different syntax) views.

With my experience I really afraid about it - it can be very effective
(from developer perspective) and very slow (from customer perspective).
This is example of tool that looks nice on paper, but can be very badly
used.

Maybe I am not the best man for this topic - I like some functional
programming concepts, but I use it locally - your proposal moves SQL to
some unexplored areas - and I think so it can be interesting as real
research topic, but not today Postgres's theme.

The basic question is why extend SQL and don't use some native functional
language. Postgres should to implement ANSI SQL - and there is not a space
for big experiments. I am sceptic about it - relational databases are
static, SQL is static language, so it is hard to implement some dynamic
system over it - SQL language is language over relation algebra - it is not
functional language, I afraid so introduction another concept to this do
more bad than good.

Regards

Pavel

> Regards,
> Roman
>
> On Sun, 7 Jul 2019 at 16:22, Roman Pekar <roma(dot)pekar(at)gmail(dot)com> wrote:
>
>> Hi,
>>
>> Yes, I'm thinking about 'query like a view', 'query like a cursor' is
>> probably possible even now in ms sql server (not sure about postgresql),
>> but it requires this paradygm shift from set-based thinking to row-by-row
>> thinking which I'd not want to do.
>>
>> I completely agree with your points of plan caching and static checks.
>> With static checks, though it might be possible to do if the query would be
>> defined as typed, so all the types of the columns is known in advance.
>> In certain cases having possibility of much better decomposition is might
>> be more important than having cached plan. Not sure how often these cases
>> appear in general, but personally for me it'd be awesome to have this
>> possibility.
>>
>> Regards,
>> Roman Pekar
>>
>> On Sun, 7 Jul 2019 at 15:39, Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>
>> wrote:
>>
>>> Hi
>>>
>>> ne 7. 7. 2019 v 14:54 odesílatel Roman Pekar <roma(dot)pekar(at)gmail(dot)com>
>>> napsal:
>>>
>>>> Hello,
>>>>
>>>> Just a bit of background - I currently work as a full-time db
>>>> developer, mostly with Ms Sql server but I like Postgres a lot, especially
>>>> because I really program in sql all the time and type system / plpgsql
>>>> language of Postgres seems to me more suitable for actual programming then
>>>> t-sql.
>>>>
>>>> Here's the problem - current structure of the language doesn't allow to
>>>> decompose the code well and split calculations and data into different
>>>> modules.
>>>>
>>>> For example. Suppose I have a table employee and I have a function like
>>>> this (I'll skip definition of return types for the sake of simplicity):
>>>>
>>>> create function departments_salary ()
>>>> returns table (...)
>>>> as
>>>> return $$
>>>> select department, sum(salary) as salary from employee group by
>>>> department;
>>>> $$;
>>>>
>>>> so that's fine, but what if I want to run this function on filtered
>>>> employee? I can adjust the function of course, but it implies I can predict
>>>> all possible filters I'm going to need in the future.
>>>> And logically, function itself doesn't have to be run on employee
>>>> table, anything with department and salary columns will fit.
>>>> So it'd be nice to be able to define the function like this:
>>>>
>>>> create function departments_salary(_employee query)
>>>> returns table (...)
>>>> as
>>>> return $$
>>>> select department, sum(salary) as salary from _employee group by
>>>> department;
>>>> $$;
>>>>
>>>> and then call it like this:
>>>>
>>>> declare _employee query;
>>>> ...
>>>> _poor_employee = (select salary, department from employee where salary
>>>> < 1000);
>>>> select * from departments_salary( _poor_employee);
>>>>
>>>> And just to be clear, the query is not really invoked until the last
>>>> line, so re-assigning _employee variable is more like building query
>>>> expression.
>>>>
>>>> As far as I understand the closest way to do this is to put the data
>>>> into temporary table and use this temporary table inside of the function.
>>>> It's not exactly the same of course, cause in case of temporary tables data
>>>> should be transferred to temporary table, while it will might be filtered
>>>> later. So it's something like array vs generator in python, or List vs
>>>> IQueryable in C#.
>>>>
>>>> Adding this functionality will allow much better decomposition of the
>>>> program's logic.
>>>> What do you think about the idea itself? If you think the idea is
>>>> worthy, is it even possible to implement it?
>>>>
>>>
>>> If we talk about plpgsql, then I afraid so this idea can disallow plan
>>> caching - or significantly increase the cost of plan cache.
>>>
>>> There are two possibilities of implementation - a) query like cursor -
>>> unfortunately it effectively disables any optimization and it carry ORM
>>> performance to procedures. This usage is known performance antipattern, b)
>>> query like view - it should not to have a performance problems with late
>>> optimization, but I am not sure about possibility to reuse execution plans.
>>>
>>> Currently PLpgSQL is compromise between performance and dynamic (PLpgSQL
>>> is really static language). Your proposal increase much more dynamic
>>> behave, but performance can be much more worse.
>>>
>>> More - with this behave, there is not possible to do static check - so
>>> you have to find bugs only at runtime. I afraid about performance of this
>>> solution.
>>>
>>> Regards
>>>
>>> Pavel
>>>
>>>
>>>
>>>> Regards,
>>>> Roman Pekar
>>>>
>>>>
>>>>