On 16/05/16 17:09, 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
[...]
In one transaction:
1. populate the summary table
2. drop indexes on tableA
3. delete selected entries in tableA
4. recreate indexes for tableA
If deleting all entries, then simply truncate it!
N.B. I have NOT checked the fine print in the documentation, nor tested
this - so be warned! :-)
Cheers,
Gavin
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