Re: Breaking up a PostgreSQL COPY command into chunks? - Mailing list pgsql-general

Hi,

Hmm, ok, I'll pass that onto our DBA/operations guys, and see if that helps.

Do these settings still work if you only have a single Postgres instance? (I'll need to check out setup).

So my understanding is that the default is 30 seconds (http://www.postgresql.org/docs/9.3/static/runtime-config-replication.html) - but we're increasing it to 600 seconds, and that should give the COPY command enough time to pull down the data?

As a rough guide, the dumped CSV file is around 600 Mb.

Is there any other background you might be able to give on what you think might be happening, or how this might fix it?

And you'd recommend tweaking these values over trying to chunk up the COPY/SELECT, is that right?

I've just realised the LIMIT/ORDER thing may not work well to paginate, since there may be new records, or deleted records between each time I call it?

Cheers,

Victor

On Fri, Nov 8, 2013 at 1:15 PM, wd <wd@wdicc.com> wrote:

Try this,

max_standby_archive_delay = 600s        # max delay before canceling queries                                       # when reading WAL from archive;                                       # -1 allows indefinite delay
max_standby_streaming_delay = 600s      # max delay before canceling queries                                       # when reading streaming WAL;                                       # -1 allows indefinite delay

or try 
pg_xlog_replay_pause()
pg_xlog_replay_resume()

On Fri, Nov 8, 2013 at 10:06 AM, Victor Hooi <victorhooi@yahoo.com> wrote:

Hi,

We're using psycopg2 with COPY to dump CSV output from a large query.

The actual SELECT query itself is large (both in number of records/columns, and also in width of values in columns), but still completes in around under a minute on the server.

However, if you then use a COPY with it, it will often time out.

We're using psycopg2 to run the command, the trace we get is something like:

Traceback (most recent call last):

 File "foo.py", line 259, in <module>

   jobs[job].run_all()

 File "foo.py", line 127, in run_all

   self.export_to_csv()

 File "foo.py", line 168, in export_to_csv

   cur.copy_expert(self.export_sql_statement, f)

psycopg2.extensions.TransactionRollbackError: canceling statement due to conflict with recovery

DETAIL:  User was holding shared buffer pin for too long.

My question is, what are some simple ways we can use to chunk up the query?

Could we pull down a list of all the ids (auto-incrementing int), break this list up, then use a WHERE clause to break it up, running multiple COPY commands?

Or would it be better to use LIMIT/OFFSET to break it up? I'm not sure how we'd figure out when we reached the end of the results set though (apart from just counting the results?).

Or are there other approaches you guys could recommend?

Cheers,

Victor