Re: Troubleshooting a long running delete statement

On 10/6/21 14:00, Dirschel, Steve wrote:

> Question:
> How would one troubleshoot this issue in Postgres as to why the delete
> was running so long?  My background is Oracle and there are various
> statistics I may look at:
>
> * One could estimate the number of logical reads the delete should
> do based on expected number of rows to delete, expected logical
> reads against the table per row, expected logical reads against
> each index per row.
> * One could look in V$SQL and see how many logical reads the query
> was actually doing.
> * One could look at V$SESS_IO and see how many logical reads the
> session was doing.
>
> In this case you would see the query was doing way more logical reads
> that expected and then try and think of scenarios that would cause that.
> Here is what I could see in Postgres:
>
> * When I did an explain on the delete I could see it was full
> scanning the table. I did a full scan of the table interactively
> in less than 1 second so the long runtime was not due to the full
> tablescan.
> * I could not find the query in pg_stat_statements to see how many
> shared block reads/hits the query was doing to see if the numbers
> were extremely high.  Based on documentation queries do not show
> up in pg_stat_statements until after they complete.
> * pg_stat_activity showed wait_event_type and wait_event were null
> for the session every time I looked.  So the session was
> continually using CPU.
>
> I started looking at table definitions (indexes, FK's, etc.) and
> comparing to Oracle and noticed some indexes missing.  I then could
> see the table being deleted from was a child table with a FK pointing
> to a parent table.  Finally I was able to see that the parent table
> was missing an index on the FK column so for every row being deleted
> from the child it was full scanning the parent.  All makes sense after
> the fact but I'm looking for a more methodical way to come to that
> conclusion by looking at database statistics.
> Are there other statistics in Postgres I may have looked at to
> methodically come to the conclusion that the problem was the missing
> index on the parent FK column?
> Thanks
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Hi Steve,

First, check whether you have any triggers on the table. The best way of
doing it is to use information_schema.triggers. I have seen triggers
introduce some "mysterious" functionality in Oracle as well. Second,
check constraints. Is the table you're deleting from the parent table of
a foreign key constraint(s)? If the constraints are defined with "ON
DELETE CASCADE", you maybe deleting more than you think. If it is not
defined with "ON DELETE CASCADE" or "ON DELETE SET NULL", you would get
an error. If that passes the muster, then check the processes doing the
most of IO using "iotop" or "atop". I like the latter. You can then
check what the busy processes are doing using strace -e trace=file and,
for good measure, 'perf top".

Regards

--
Mladen Gogala
Database Consultant
Tel: (347) 321-1217
https://dbwhisperer.wordpress.com