Re: Fast way to delete big table?

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
>
>
UPDATEsummaryTable set count1 = rec.count1 + rec.count1, count2 =... where category match; 
>
>
deletefrom 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@redflex.com.au
> 31 Market Street, (PO Box 720), South Melbourne VIC 3205, Australia
>
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