Re: One process per session lack of sharing

Robert Haas <robertmhaas@gmail.com> writes:
> On Fri, Jul 15, 2016 at 4:28 AM, Craig Ringer <craig@2ndquadrant.com> wrote:
>> I don't think anyone's considering moving from multi-processing to
>> multi-threading in PostgreSQL. I really, really like the protection that the
>> shared-nothing-by-default process model gives us, among other things.

> We get some very important protection by having the postmaster in a
> separate address space from the user processes, but separating the
> other backends from each other has no value.

I do not accept that proposition in the least.  For one thing, debugging
becomes an order of magnitude harder when you've got multiple threads
in the same address space: you have essentially zero guarantees about
what one thread might have done to the supposedly-private state of
another one.

> ... enough other people have
> written complex, long-running multithreaded programs that I think it
> is probably possible to do so without unduly compromising reliability.

I would bet that every single successful project of that sort has been
written with threading in mind from the get-go.  Trying to retro-fit
threading onto thirty years' worth of single-threaded coding is a recipe
for breaking your project; even if you had control of all the code running
in the address space, which we assuredly do not.
        regards, tom lane