Hello
I am not sure if this is an option, but would partial indexes be of
help? You could create your indexes in such a way that you exclude the
rows that are not needed for your current queries, i.e. those that you
would summarize in a separate table and delete using the same condition
that you use for selecting which rows must be deleted.
CREATE INDEX ... ON tablea (...) WHERE ...;
This would not make deletion quicker, but operations should have a gain.
What I also see is that you don't have any condition when selecting from
the loop, i.e. you actually go through all rows and decide on the fly
which you must delete. Maybe reducing the number of rows to be processed
by the function helps.
Finally I don't know the structure of the table, but you may select only
those columns that you need for your summary instead of *. If you have
many columns this could have an impact on the performance of the select
statement.
Regards
Charles
On 05/16/2016 07:09 AM, Haiming Zhang wrote:
Hi All,
I have a big table (200G with 728 million records), the table slows down
lots of things. It's time to clean the data up. The requirement is when
I delete I must summarise few columns to a new table for backup purpose.
So that mean I need to go through each row and add the columns' value to
the summary table (the corresponding category).
The table has indexes (multicoumn indexes) before, I removed the indexes
because I read on forum says the indexes *heavily* reduce the speed of
deleting. That's true if I only delete from the table, but my case is I
first SELECT this row, then add few values to the summary table and then
DELETE the row according to its multiple primary key. This seems to be a
bad idea now as it takes very long time to DELETE the row (as delete
needs to find the row first).
Here are the two version of the delete functions, please help to point
out how can I speed it up.
1.
CREATE OR REPLACE FUNCTION summary_delete_table()
RETURNS integer AS
$BODY$
DECLARE
rec RECORD;
subrec RECORD;
BEGIN
FOR rec
IN SELECT * FROM tableA limit 100 LOOP
BEGIN
UPDATE summaryTable set count1 = rec.count1 + rec.count1, count2 =... where category match;
delete from tableA where tableA.primaryKeyA=rec.primaryKeyA and tableA.primaryKeyB=rec.primaryKeyB;
END;
END LOOP;
return 1;
END;
$BODY$
LANGUAGE plpgsql VOLATILE
COST 100;
And then I have a .bat script to loop the above function million times.
2.
CREATE OR REPLACE FUNCTION summary_delete_table()
RETURNS integer AS
$BODY$
DECLARE
rec RECORD;
td_cursor CURSOR FOR SELECT * FROM tableA limit 100;
BEGIN
FOR rec
IN td_cursor LOOP
BEGIN
UPDATE summaryTable set count1 = rec.count1 + rec.count1, count2 =... where category match;
delete from tableA WHERE CURRENT OF td_cursor;
END;
END LOOP;
return 1;
END;
$BODY$
LANGUAGE plpgsql VOLATILE
COST 100;
Method 2 is bit faster but not much, the delete speed is 2478 rows/s for
method 2 and 2008 rows/s for method 1.
Any suggestions are welcomed.
BTW, I guess if reindex, it may take few days to finish.
Also, I tried change delete 100 rows at a time and 1000, 2000. The
result showed 1000 is faster than 100 and 2000 a time.
Thanks and Regards,
Haiming
*Haiming Zhang* | Engineer | *Redflex Group*
*T* +61 3 9674 1868 | *F* +61 3 9699 3566 | *E*
Haiming.Zhang@xxxxxxxxxxxxxx
31 Market Street, (PO Box 720), South Melbourne VIC 3205, Australia
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