Re: Multithreaded query onto 4 postgresql instances

No, this database is on a single machine, but a very powerful one.
Processors with 16 cores each and ssd disks.

I already use partitioning and tablespaces for every instance of my db
and I gain a lot with my splitted configuration.
My db is pretty huge: 600 milions of records and partitioning is not
enough...
I performed tests with a query returning more or less 100000 records and
using my C module I obtain the following results (every test performed
cleaning cache before):
- single db: 9.555 sec
- splitted in 4: 5.496 sec

So I think this can be a good approach...
I have already read this

http://www.postgresql.org/docs/9.0/interactive/xfunc-c.html

I posted it in my previous thread. But it is not clear to me how to
embed my C function into postgresql.
I mean, I know how to compile and insert it into postgresql, I've
already done it for simple function which return a boolean, for example.
But it is not clear to me how to do this if I want as return a complete
table, or set of rows...
Can you give me a minimalistic example?

By the way, my goal is to perform a SELECT query in the smallest time
possible.
For that reason an SQL function is not good beacuse slower than a C
function...

Thank you!

> On 10 Feb 2011, at 9:01, Alessandro Candini wrote:
>
>> I have installed 4 different instances of postgresql-9.0.2 on the same machine, on ports 5433, 5434, 5435, 5436.
> I do hope you intend to put those databases on different machines eventually, or some such? Otherwise you probably didn't gain anything by splitting your database up like that - you've just reduced the available resources on that single machine.
>
>> Why I have to do something tricky like this is long too explain...
> It would help to know what you're trying to achieve by splitting your database up like this. We don't need the full story, just a summary is fine; Maybe this is some experimental setup that's more related to multi-threading than to the actual database design? Maybe management smoked something outlandish and put you up with this? Maybe this is a macroscopic attempt to table partitioning?
>
>> Then I have developed a C function using libpq which creates 4 threads, each one which query a 1/4 of the db. After that I merge the results in one single response.
>>
>> My function works fine, but I need to include it inside a postgresql instance in order to launch it as a normal SQL query (SELECT myfunc(...);).
>> I have read the documentation here http://www.postgresql.org/docs/9.0/static/xfunc-sql.html#XFUNC-SQL-FUNCTIONS-RETURNING-SET, but I'm pretty confuse and I don't know if this is the right way to accomplish my goal.
> What are you confused about? That's a chapter about set-returning functions written in SQL - perhaps you're looking for information about set-returning functions from an external library like yours? Perhaps you're looking for: http://www.postgresql.org/docs/9.0/interactive/xfunc-c.html
>
> You say you don't know if this is the right way to accomplish your goal - which is...?
>
>> Have you got any ideas or suggestions?
>
> As others have suggested, you should probably have a look at table-partitioning, possibly in combination with tablespaces if you want to divide your database among multiple disks/filesystems. Putting them in different servers on the same hardware is probably not going to be a very good solution. As you've already found out, it makes querying the data silly difficult. But, we don't know the reason you're doing that of course.
>
> Alban Hertroys
>
> --
> If you can't see the forest for the trees,
> cut the trees and you'll see there is no forest.
>
>
> !DSPAM:1234,4d567a2a11731320518513!
>
>

--
Alessandro Candini

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http://www.meeo.it

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