Thread: Contrib -- PostgreSQL shared variables

This is a first pass on a simple shared memory variable system for
PostgreSQL. I would appriciate anyone interested in this functionality to
rip it apart.

It basically adds this functionality:

SetShared('name', value);
GetSharedInt('name');
SetSharedText('name);
RemovedShared('name');

I also added two extra functions that are sort of a kludge, but could be
very helpful.

AddSharedInt('name', value);
SubSharedInt('name', value);

These add or subtect the 'value' from the variable and return the result.

LockShared('name');

pgsql@mohawksoft.com wrote:
> This is a first pass on a simple shared memory variable system for
> PostgreSQL. I would appriciate anyone interested in this functionality to
> rip it apart.
>
> It basically adds this functionality:
>
> SetShared('name', value);
> GetSharedInt('name');
> SetSharedText('name);
> RemovedShared('name');
>
> I also added two extra functions that are sort of a kludge, but could be
> very helpful.
>
> AddSharedInt('name', value);
> SubSharedInt('name', value);
>
> These add or subtect the 'value' from the variable and return the result.
>
>
> ------------------------------------------------------------------------
>
>
> ---------------------------(end of broadcast)---------------------------
> TIP 9: the planner will ignore your desire to choose an index scan if your
>       joining column's datatypes do not match

> LockShared('name');

Hmmm, I thought about that, but it is *WAY* more complicated than it
looks. What if after a "Lock" the process crashes before it can call
"Unlock?" It is this problem that inspired the "add" and "sub" calls.



>
> pgsql@mohawksoft.com wrote:
>> This is a first pass on a simple shared memory variable system for
>> PostgreSQL. I would appriciate anyone interested in this functionality
>> to
>> rip it apart.
>>
>> It basically adds this functionality:
>>
>> SetShared('name', value);
>> GetSharedInt('name');
>> SetSharedText('name);
>> RemovedShared('name');
>>
>> I also added two extra functions that are sort of a kludge, but could be
>> very helpful.
>>
>> AddSharedInt('name', value);
>> SubSharedInt('name', value);
>>
>> These add or subtect the 'value' from the variable and return the
>> result.
>>
>>
>> ------------------------------------------------------------------------
>>
>>
>> ---------------------------(end of broadcast)---------------------------
>> TIP 9: the planner will ignore your desire to choose an index scan if
>> your
>>       joining column's datatypes do not match
>

pgsql@mohawksoft.com wrote:
> This is a first pass on a simple shared memory variable system for
> PostgreSQL. I would appriciate anyone interested in this functionality to
> rip it apart.
>
> It basically adds this functionality:
>
> SetShared('name', value);
> GetSharedInt('name');
> SetSharedText('name);
> RemovedShared('name');
>
> I also added two extra functions that are sort of a kludge, but could be
> very helpful.
>
> AddSharedInt('name', value);
> SubSharedInt('name', value);
>
> These add or subtect the 'value' from the variable and return the result.
>
Something that I've found very useful when dealing with shared memory is
the ability to do atomic exchange of values. With that in place, its
possible to perform atomic operations involving several variables.

Perhaps it could be as simple as splitting your SetShared into
SetSharedInt and SetSharedString and then have them return the old value?

Here's an example of what I mean (I use C syntax for clarity, I know the
intended use is from SQL).

     /* Loop until something other than LOCKED is returned. When
      * that happens, we have the lock.
      */
     while(SetSharedInt('lock', LOCKED) == LOCKED)
         usleep(WAIT_PERIOD);

     Set a group of variables here.

     /* Unlock
      */
     SetSharedInt('lock', UNLOCKED);


Regards,

Thomas Hallgren

This new version contains, in addition to the previous version,
SwapShared(..) which allows you to swap a value in a variable.