Re: Fast way to delete big table? - Mailing list pgsql-general

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