PostgreSql function slows down view

I am rewriting Sybase view to postgresql and encountered such problem.
Is there any way to improve performance of view with functions inside?
For example we have complex function

create function func1(pid integer)
returns char(20)
as $
begin
... do stuf
return 'aaaa';
end; $ language plpgsql;

And create view with function

create view view_client
as
select code, name, func1(pid) from client;

Such view performs terrible slow, unless "func1" is removed from it. Postgresql probalby executes function for every row even if column is not used?
count(*) from view with 2M rows takes 2+ minutes, without "func1" it takes ~10sec.

As an answer,


What exactly does the function do? Can you define it as stable or even better as immutable? – a_horse_with_no_name 12 mins ago

From the docs


IMMUTABLE / STABLE / VOLATILE These attributes inform the query optimizer
about the behavior of the function. At most one choice can be
specified. If none of these appear, VOLATILE is the default
assumption.



-
IMMUTABLE indicates that the function cannot modify the database and
always returns the same result when given the same argument values;
that is, it does not do database lookups or otherwise use information
not directly present in its argument list. If this option is given,
any call of the function with all-constant arguments can be
immediately replaced with the function value.

-
STABLE indicates that the function cannot modify the database, and
that within a single table scan it will consistently return the same
result for the same argument values, but that its result could change
across SQL statements. This is the appropriate selection for functions
whose results depend on database lookups, parameter variables (such as
the current time zone), etc. (It is inappropriate for AFTER triggers
that wish to query rows modified by the current command.) Also note
that the current_timestamp family of functions qualify as stable,
since their values do not change within a transaction.

-
VOLATILE indicates that the function value can change even within a
single table scan, so no optimizations can be made. Relatively few
database functions are volatile in this sense; some examples are
random(), currval(), timeofday(). But note that any function that has
side-effects must be classified volatile, even if its result is quite
predictable, to prevent calls from being optimized away; an example is
setval().



For additional details see Section 36.6

The real question is essentially, can you optimize away f(x)

• Over everything (mathematical purity)

• Over the same scan

• Never (uses IO or system calls)

A somewhat important note is that even in production applications you can sometimes lie and say IMMUTABLE and it's perfectly fine. Just be aware of the consequences.

StackExchange Database Administrators Q#165028, answer score: 2