On Monday, Aug 18, 2003, at 07:27 US/Pacific, Vilson farias wrote:
Greetings,
I'm getting a big performance problem and I would like to ask you what
would be the reason, but first I need to explain how it happens.
Let's suppose I can't use sequences (it seams impossible but my boss doesn't like specific database features like this one).
You're just replacing the postgres implementation of sequences with your own that has a similar API. The postgres will have a few advantages over what you'll be able to write in plpgsql. :)
That said, I don't use native sequences for large projects. I do something with a similar table to yours, but I have a key cache size on each key. A container refetches keys when it runs out. I think I wrote it originally because it made work easier (it makes complex object relations easier if you can give things unique IDs before storing them), but it turns out that it performs really well because it's just an update statement to adjust the frequency of the key table access.
If you're working in java, you can use my stuff from here:
http://bleu.west.spy.net/~dustin/projects/spyjar.xtp
If you're implementing your own, you can read the particular class docs here:
http://bleu.west.spy.net/~dustin/spyjar/j2/doc/net/spy/db/GetPK.html
For sequence simulation I had created a table called cnfg_key_generation
and each tuple holds information for one of my tables (tablename,
current_sequencial_number). Lets check :
CREATE TABLE cnfg_key_generation ( department integer NOT NULL, table_name varchar(20) NOT NULL, current_key integer NOT NULL, CONSTRAINT XPKcnfg_key_generation PRIMARY KEY (department, table_name) );
Per example, for a table called 'my_test' I would have the following values
:
department = 1
table_name = 'my_test'
current_key = 1432
Everytime I want a new key to use in my_test primary-key I just increment
current_key value. For this job, I've created a simple stored procedure
called key_generation
CREATE FUNCTION key_generation(integer, varchar(20)) RETURNS integer AS'
DECLARE
the_department ALIAS FOR $1;
the_table_name ALIAS FOR $2;
new_key_value integer;
err_num integer;
BEGIN
new_value := 0;
LOCK TABLE cnfg_key_generation IN ROW EXCLUSIVE MODE;
SELECT current_value INTO new_value FROM cnfg_key_generation WHERE the_department = department AND the_table_name = table_name;
IF NOT FOUND THEN
new_key_value := 1;
INSERT INTO cnfg_key_generation VALUES (the_department,the_table_name,
new_key_value);
ELSE
new_key_value := new_key_value + 1;
UPDATE cnfg_key_generation SET current_key_value = new_key_value WHERE department = the_department AND table_name = the_table_name; END IF;
RETURN new_key_value;
END; ' LANGUAGE 'plpgsql';
Data insertion is done by the following way :
INSERT INTO my_test VALUES (key_generation(1, 'my_test'), ...other fields...);
Ok, this works, but has a huge performance problem. I read in pg docs that
everytime I make an UPDATE, a new tuple is created and the old is marked as
invalid. For a tuple that holds sequencial key generation, everytime a new
key is generated, a new tuple is created inside cfg_key_generation. It means
after 2million key generations for same table, performance will be completly
degradated because there will be 2million of old versions of same tuple. For
instance, I have a table called 'cham_chamada' that actually holds
1.5Million of tuples. The response time for key_generation execution for
this table is more than 5seconds. In this same case if I execute
key_generation for a table that has just few values (current key = 5 per
example), response time is just some miliseconds (30 to 50ms).
I tryied to fix this problem with a VACUUM and it was completly ineffective.
After execution the problem was still there. Later, after execution of every
kind of vacuum I knew (with and without ANALYZE, especific for that table,
vacuumdb from shell ...) I gave up and tryied a COPY TO/DROP/CREATE/COPY
FROM. At this time, the performance problem was fixed.
What can I do to solve this problem without table reconstruction? Is there a
performance degradation in PostgreSQL that can't be solved? If a have a huge
table with millions of data being update constantly, what can I do to keep a
good performance if vacuum isn't 100%?
Does PostgreSQL sequences deal with these kinds performance questions? Is it
clean, fast and secury to use sequences? Maybe I still can make up my boss
mind about this subject if I get good arguments here to justify the use of
sequences.
Am I doing some stupid thing?
Best regards,
----------------------------------------------------------------- José Vilson de Mello de Farias Software Engineer
Dígitro Tecnologia Ltda - www.digitro.com.br APC - Customer Oriented Applications E-mail: [EMAIL PROTECTED] Tel.: +55 48 281 7158 ICQ 11866179
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