Re: (select query)/relation as first class citizen - Mailing list pgsql-hackers

Hi

ne 7. 7. 2019 v 14:54 odesílatel Roman Pekar <roma.pekar@gmail.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