On 14 Feb 2011, at 9:38, Alessandro Candini wrote:
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
Is that a single query on that one DB compared to 4 queries on 4 DB's? How does
a single DB with 4 parallel queries perform? I'd expect that to win from 4
DB's, due to the overhead those extra DB instances are generating.
I do suppose you tried tuning that DB before you started making things
complicated?
Maybe my configuration and test is not clear
Single instance ---> 600 millions of records in one single db (port
5433) ---> query of 100000 records ---> 9.555 sec
Splitted instances:
600 millions of records in total splitted into 4 postgresql instances
(port 5433, 5434, 5435, 5436), let's say more or less:
5433 ---> 150 millions of records
5434 ---> 150 millions of records
5435 ---> 150 millions of records
5436 ---> 150 millions of records
I've launched the same query reported above with 4 concurrent threads
using a C program with libpq, which merges the results also,
returning the same ordered 100000 tuples retrieved in the single db case
but in 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
Did you read all the way to section 35.9.10? That explains how to create SRF's
like yours, including examples. If the step from simple functions to SRF's is
too large for you, create a few sample-functions to learn how the intermediary
steps work. We can't upload the info to your brain, after all - you'll have to
do the learning part by yourself.
Do you think is a good idea continue using libpq or should I abandon
them and focus on postgresql documentation examples?
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?
I'm not aware of any difference between adding a function returning a scalar
vs. one that returns a set. You have to create a wrapper function to your
library so that PG knows about your function, but apart from the return types
(BOOLEAN vs. SETOF something), that declaration is exactly the same.
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...
I have no idea what you're referring to, since you're top-posting, but I never
suggested to use SQL functions. I just pointed out that the document you linked
was about SQL functions instead of C functions, in the assumption that was what
was causing your confusion. You didn't explain what you were confused about
after all, I had to guess.
You're right, I copy/pasted the wrong link, sorry.
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
--
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cut the trees and you'll see there is no forest.
--
Alessandro Candini
MEEO S.r.l.
Via Saragat 9
I-44122 Ferrara, Italy
Tel: +39 0532 1861501
Fax: +39 0532 1861637
http://www.meeo.it
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Alban Hertroys
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Alessandro Candini
MEEO S.r.l.
Via Saragat 9
I-44122 Ferrara, Italy
Tel: +39 0532 1861501
Fax: +39 0532 1861637
http://www.meeo.it
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