Jan Wieck wrote:
> On 9/10/2007 12:49 PM, Cyril SCETBON wrote:
>>
>> Christopher Browne wrote:
>>> Cyril SCETBON wrote:
>>>>
>>>>
>>>> Jan Wieck wrote:
>>>>> On 9/10/2007 10:07 AM, Cyril SCETBON wrote:
>>>>>> Hi,
>>>>>>
>>>>>> We're hitting 400 requests per second and noticed that sl_log_? 
>>>>>> are growing a lot. This is also due to the fact that our tables 
>>>>>> have a lot of columns (more that 100). We noticed that slony 
>>>>>> stores each column and its value for each request in sl_log_x. 
>>>>>> Would it not be more optimized to store just the user request ? 
>>>>>> Actually, a user is just updating a few column, or inserting a 
>>>>>> tuple by giving some attributes but not all, however as slony 
>>>>>> stores every column with its value it sl_log_x, these tables are 
>>>>>> growing very fast :-(
>>>>>
>>>>> Slony only logs columns where the value actually has changed. 
>>>>> Please explain in detail how you propose that the log table would 
>>>>> contain which columns have changed without naming them.
>>>> when I execute this command on the master :
>>>>
>>>> insert into t1(ise) values('cyril100001');
>>>>
>>>> I can see in sl_log_1 :
>>>>
>>>> psql>select log_cmddata from sl_log_1;
>>>> (ise,id,id2) values ('cyril100001','24161',NULL)
>>>>
>>>> dbtest=# \d t1;
>>>>                                Table "public.t1"
>>>> Column |         Type          |                    
>>>> Modifiers                   
>>>> --------+-----------------------+------------------------------------------------- 
>>>>
>>>> ise    | character varying(54) | not null
>>>> id     | integer               | not null default 
>>>> nextval('t1_id_seq'::regclass)
>>>> id2    | integer               |
>>>> Indexes:
>>>>    "t1_pkey" PRIMARY KEY, btree (ise)
>>>> Triggers:
>>>>    "_CLUSTER1_logtrigger_1" AFTER INSERT OR DELETE OR UPDATE ON t1 
>>>> FOR EACH ROW EXECUTE PROCEDURE "_CLUSTER1".logtrigger('_CLUSTER1', 
>>>> '1', 'kvv')
>>>>
>>>> I agree for id which is an auto_increment but not for id2 :-(
>>>>
>>>> In another plateform my table has more than  100 columns as said 
>>>> earlier and that's really matter :-(
>>> Jan's request seems to remain perfectly good...
>>>
>>> "Please explain in detail how you propose that the log table would 
>>> contain which columns have changed without naming them."
>>>
>>> It is *not* obvious how to avoid naming all of the columns.  It is 
>>> not safe to simply assume "that column was null so we may omit it" - 
>>> different nodes may be configured differently, and it may well be 
>>> important to actually have that NULL value.
>> I was just thinking of using the same way as other statement based 
>> replication way like does mysql, that is to say just log the request 
>> that user did. However, I forgot that slony uses triggers to log 
>> requests :-( Maybe finding a way to know which current request is 
>> done on the table for which the trigger has been called would be 
>> possible ?
>
> Turning Slony into a statement based replication system would make it 
> far worse. 
You may be right. It was just a suggestion to do something to replace 
the long SQL string by a shorter one to gain space, and so performance 
when getting blocks from disks with more rows :-)
> Queries depend on the exact transaction commit order with respect to 
> the exact rows that are visible to them. That in addition to the 
> non-deterministic behavior of sequential scans and a myriad of 
> functions ... how do you expect some query like
>
>     update mytable set foo = nextval('someseq') where foo is null;
>
> to lead to exactly the same result on two different databases? Not 
> even the order in which the rows are processed is defined.
If MySQL support it , why slony would not ? 
(http://dev.mysql.com/doc/refman/5.1/en/replication-features-autoincid.html), 
but you right for deterministic functions it's hard if value is added in 
the statement. But it's not what I've suggested.
>
> Compared to Postgres, MySQL is a fairly limited system feature wise. 
> Yet with their limited functionality the MySQL user manual already 
> states that several features are not supported by their statement 
> based replication. What do you think the corresponding documentation 
> for Slony would look like?
Sorry, but you can compare it to the log shipping feature of slon with 
less lag time.
>
>
> Jan
>

-- 
Cyril SCETBON - Ingénieur bases de données
AUSY pour France Télécom - SCR/HDI/DOP/HEBEX

Tél : +33 (0)4 97 12 87 60
Jabber : cscetbon at jabber.org
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