Re: Problem with reading data from standby server ?

[Richard Huxton <dev@xxxxxxxxxxxx>]

On 20/04/12 09:39, Condor wrote:
> Hello,
>
> when I read binary replication tutorial
> (http://wiki.postgresql.org/wiki/Binary_Replication_Tutorial) I see on
> Hot Standby: Hot Standby is identical to Warm Standby, except that the
> Standby is available to run read-only queries.
> I setup hot standby server described in tutorial and it's working fine,
> no problem with that.


> I have a problem when I try to start a script that
> should read whole table, error message from php is:

> PHP Warning: pg_query(): Query failed: ERROR: canceling statement due to
> conflict with recovery

> When data is fetched it's saved into a file after some modifications.
> This script is work a 30-40 min until all data is parsed. Well, I think
> problem is started when master server send new wal file to slave, but
> how I can resolve that problem ?

Your master database is being updated all the time and your slave is 
supposed to be a perfect copy, including deleted/updated rows being no 
longer visible. So - when you run a query it might need to do one of two 
things:
  1. Pause replication
  2. Cancel the query

At some point PostgreSQL switches from doing #1 to doing #2 (otherwise 
you could get so far behind the replica could never catch up). You can 
control how long before it switches:

http://www.postgresql.org/docs/9.1/static/hot-standby.html#HOT-STANDBY-CONFLICT


--
  Richard Huxton
  Archonet Ltd

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