pg_dump: Error message from server: ERROR: canceling statement due to conflict with recovery

I ran to this error the other day when I tried to export a dump file from a slave Postgres database.

cpg_dump: Dumping the contents of table "[table_name]" failed: PQgetResult() failed.
pg_dump: Error message from server: ERROR:  canceling statement due to conflict with recovery
DETAIL:  User query might have needed to see row versions that must be removed.
pg_dump: The command was: COPY public.[table_name] ([comma-separated column names]) TO stdout;

I turns out that we can't execute long query on Hot Standby mode server. Right now, there's no perfect solution to this problem but there are some couples of workarounds.

• Make a dump from master.
• Set max_standby_archive_delay and max_standby_streaming_delay to be bigger that amount of time it needs to export a dump. This will make slave catching up master version slower. It's not likely to work for a big database.
• Pause sync, make a dump and the resume sync. You can pause sync with function pg_xlog_replay_pause() and resume with function pg_xlog_replay_resume(). See more details on this 2 functions from 9.24. System Administration Functions and New Replication and Recovery Features in PostgreSQL 9.1. Note that we should not pause it too long (> 24 hours), otherwise I might not be able to resume.

Mailing list related to this issue: Hot Standby - ERROR: canceling statement due to conflict with recovery

[RSpec] Refer to let value that has the same name in outer scope

There is a situation in RSpec when you want to redefine let value in the inner scope base on value from the outer scope. In RSpec 2.13, you can do that by referring outer scope value with super() (parens are required)

Credit: RSpec 2.13 is released!

[Watched] Go at Google

http://www.infoq.com/presentations/Go-Google

Interesting points in Go that I learned from this presentation

• You can not have unused import statement. Makes code cleaner and faster compilation time.
• Favor small duplication over including big dependency to use only small part of it
• No implements declaration required for implementing an interface. Safety from static typing and flexibility from ad-hoc type.
• Gofix - Cool tool for changing the code to fix language version incompatibility issue.