Re: large table

> Date: Mon, 22 Sep 2014 14:38:52 -0400
> From: wmoran@potentialtech.com
> To: lukecoldiron@hotmail.com
> CC: pgsql-general@postgresql.org
> Subject: Re: [GENERAL] large table
>
> On Mon, 22 Sep 2014 11:17:05 -0700
> Luke Coldiron <lukecoldiron@hotmail.com> wrote:
>
> > I am trying to figure out what would have caused a table in a PostgreSQL 8.4.16 to get into a state where there is only 1 live tuple and has only ever had one 1 tuple but the size of the table is huge.
> >
> > CREATE TABLE public.myTable( myColumn timestamp with time zone NOT NULL);
> >
> > Note: there is no primary key or index on this table.
> > CREATE OR REPLACE FUNCTION public.myFunc()RETURNS VOID AS $$BEGIN UPDATE public.myTable SET myColumn = CLOCK_TIMESTAMP();
> > IF NOT FOUND THEN INSERT INTO public.myTable(myColumn) VALUES (CLOCK_TIMESTAMP()); END IF;END;$$ LANGUAGE plpgsql VOLATILE STRICT SECURITY DEFINER;
> > CREATE OR REPLACE FUNCTION public.wasUpdated(OUT was_updated boolean)RETURNS BOOLEAN AS $$BEGIN was_updated := COALESCE((SELECT myColumn FROM public.myTable) > (CLOCK_TIMESTAMP() - INTERVAL '5 SECOND'), FALSE);END;$$ LANGUAGE plpgsql VOLATILE STRICT SECURITY DEFINER;
> > SELECT *FROM pg_stat_all_tablesWHERE relname = 'myTable';
>
> [snip]
>
> > The actual size of the table is around 33 MB.
> > The myFunc function is called every 2.5 seconds and the wasUpdated function every 2 seconds by separate processes.
> > I realize that running a FULL VACUUM or CLUSTER command on the table will resolve the issue but I am more interested in a root cause that explains why this table would end up in this state. I have tried to reproduce this issue by running the exact setup and have not been able to get the table to grow like this example. Any plausible cause'es or explanations would be much appreciated.
>
> The UPDATE in myFunc() creates a new row every 2.5 seconds when it updates the row.
> The data from those rows is only reclaimed when a vacuum is run. So (for example)
> if autovacuum only triggers a vacuum every 250 seconds, there will be 249 rows worth
> of space in the table, on average. With the other process querying the table, it's
> possible that the row that it's looking at will be a something that _should_ be
> reclaimable, so vacuum may not clear up all the free space.
>
> As far as running the exact setup: if you're not getting the same results, then
> your setup isn't exactly the same. It's likely that there are things going on in the
> setup you're curious about that you're not aware of, such as additional queries on
> the table, additional load that causes operations to take a little longer, thus
> resulting in different overlap of competing operations, etc.

It is possible and that is part of what I am trying to discover however I am very familiar with the system / code base and in this case there is a single process updating the timestamp and a single process reading the timestamp. There are no other user processes programmed to interact with this table outside of potentially what Postgres is doing.

>
> Keep in mind that a short-lived incident might have resulted in table bloat that
> won't be reclaimed by autovacuum. I.e., if autovacuum wasn't running for a while,
> this table would just keep bloating; then when you start autovacuum, it will
> maintain the table size, but it won't get any smaller.

I thought this as well and have run tests with autovacuum turned off and I don't see this issue occur over my 1000s of updates. The updates become hot updates and reuse dead tuples.

>
> I can't make any sense of the data you provided, it's all on seperate rows and I've
> given up on trying to figure out what number goes with which value, so I don't know
> exactly what the situation is. It's likely that you can improve on the situation
> by tweaking the autovacuum settings for this table to vacuum it more aggressively.

Sorry about that the email client that I am using messed up the formatting. Here is another attempt.

So far having autovacuum on or off has not caused the problem to occur. Originally I was thinking that having autovacuum off would make this happen for sure but since the table doesn't have an index it appears to be able to do a hot update.

>
> Although, you don't seem to have a _problem_ that you've stated. Are you seeing
> performance issues? Is 33M too much data and filling up the drive (not being
> sarcastic here, as there are various mobile applications where 33M could be
> important, even now). Because, if this isn't actually causing any problems, I
> wouldn't really worry about it.

Yes, this is a performance issue. The query takes around 10 longer to perform that would normally be expected. Not a big surprise after looking at how large the table is and having to scan it for all the live tuples (1 tuple). The space is not so much of an issue but the query is being done on a embedded appliance that is sensitive to this dramatic of a change in performance.  For the future I will be redesigning this mechanism and it will be done outside of the database but before I do so I wanted to see if anyone could explain why this might have occurred possible bug that was fixed in PostgreSQL, etc.

>
> --
> Bill Moran
> I need your help to succeed:
> http://gamesbybill.com