postgres-xc-general Mailing List for Postgres-XC

On Tue, Jan 22, 2013 at 7:11 AM, Filip Rembiałkowski <
fil...@gm...> wrote:

> Hi PG-XC authors.
>
> I am reading the PGCon2012 Tutorial presentation
> [
> http://postgresxc.wikia.com/wiki/File:20120515_PGXC_Tutorial_global_1.pdf]
>
> Thank you for this document - it really covers many things.
>
> On page 44 you write about data distribution, and using distributed
> tables versus replicated tables.
> You use the term "Statement based replication" - and this alerted me
> instantly.
>
> What _exactly_ did you mean by this?
>
A query string is generated on Coordinator and used to communicate with the
remote nodes.
For replicated tables, the same query is used on all the nodes if SQL is a
DML, on one node if it is a SELECT.

As you probably know, pgpool project uses exactly same term, and for
> them it means sending same statement to many parallel nodes.
> This approach is proven to be flawed by design - at least when you use
> any volatile functions when writing, it makes replicas inconsistent.
>
The term might be the same, but XC is integrated directly with the Postgres
core, while pgpool is a different instance running on top of Postgres
servers. In consequence, the query planner in XC can use the concept of
query clause shippability. This means that some part of the query can be
shipped to remote nodes (meaning that the function or clause is managed on
remote Datanodes) or non-shippable (in this case clause is compiled on
Coordinator). A volatile function is always considered as non-shippable as
it plays by definition with the database data, so in the case of XC its
value is computed on Coordinator once and what is sent to Datanodes is:
1) A query of the type "SELECT * FROM tab WHERE val = $1" (It might have
been something similar to "SELECT * from tab WHERE val = nextval('seq')" on
Coordinator, $1 is replaced by the value computed on Coordinator)
2) The value computed.

Make a try with an XC with a typical example of this type:
CREATE TABLE tab (a serial) DISTRIBUTE BY REPLICATION;
INSERT INTO tab VALUES (DEFAULT);
Then have a look at the logs of the Datanodes, you will notice that what
has been sent down is not the query received by Coordinator, but something
like:
Received query from Coordinator 'INSERT INTO tab VALUES ($1)'
Received values: $1 = '1'

Or similar.
So no need to worry about data inconsistencies in those cases.
-- 
Michael Paquier
http://michael.otacoo.com

Hi PG-XC authors.

I am reading the PGCon2012 Tutorial presentation
[ http://postgresxc.wikia.com/wiki/File:20120515_PGXC_Tutorial_global_1.pdf ]

Thank you for this document - it really covers many things.

On page 44 you write about data distribution, and using distributed
tables versus replicated tables.
You use the term "Statement based replication" - and this alerted me instantly.

What _exactly_ did you mean by this?

As you probably know, pgpool project uses exactly same term, and for
them it means sending same statement to many parallel nodes.
This approach is proven to be flawed by design - at least when you use
any volatile functions when writing, it makes replicas inconsistent.

Thanks,
Filip

Hello,

Please visit http://postgresxc.wikia.com/wiki/Postgres-XC_Wiki.   I
added a couple of link and files uploaded in this page.

I will find other useful materials and will upload them.

Good luck.
----------
Koichi Suzuki


2013/1/21 Ashutosh Bapat <ash...@en...>:
> Hi Raju,
> There is a quite substantial (considering the age of this product)
> material for Postgres-XC available at various places.
>
> Take a look at http://postgres-xc.sourceforge.net/, that's project homepage.
>
> The product documentation can be found at
>
> http://postgres-xc.sourceforge.net/docs/
>
> You will find more material at
> http://sourceforge.net/projects/postgres-xc/files/?source=navbar
>
> Hope that's helpful.
>
> On Sat, Jan 19, 2013 at 4:13 AM, Raju Angani <an...@gm...> wrote:
>> Hi Ashutosh,
>>
>> This morning I came accross postgres-xc.
>> I'm very much interested in learning about postgres-xc, could you please
>> send me some pointers(pdf, details notes)
>> or even better if you could point me to tutorial(implementation,
>> maintenance, troubleshooting).
>>
>> I wanted to know the internals, I'm familiar with oracel RAC, and MSSQL
>> HADR
>>
>> Appreciate your help!
>>
>> Thank you
>> Raju
>
>
>
> --
> Best Wishes,
> Ashutosh Bapat
> EntepriseDB Corporation
> The Enterprise Postgres Company
>
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Hi Raju,
There is a quite substantial (considering the age of this product)
material for Postgres-XC available at various places.

Take a look at http://postgres-xc.sourceforge.net/, that's project homepage.

The product documentation can be found at

http://postgres-xc.sourceforge.net/docs/

You will find more material at
http://sourceforge.net/projects/postgres-xc/files/?source=navbar

Hope that's helpful.

On Sat, Jan 19, 2013 at 4:13 AM, Raju Angani <an...@gm...> wrote:
> Hi Ashutosh,
>
> This morning I came accross postgres-xc.
> I'm very much interested in learning about postgres-xc, could you please
> send me some pointers(pdf, details notes)
> or even better if you could point me to tutorial(implementation,
> maintenance, troubleshooting).
>
> I wanted to know the internals, I'm familiar with oracel RAC, and MSSQL
> HADR
>
> Appreciate your help!
>
> Thank you
> Raju



-- 
Best Wishes,
Ashutosh Bapat
EntepriseDB Corporation
The Enterprise Postgres Company