Re: WAL logging volume and CREATE TABLE

On 02.08.2011 16:34, Bruce Momjian wrote:
> Our docs suggest an optimization to reduce WAL logging when you are
> creating and populating a table:
>
>     http://www.postgresql.org/docs/9.0/static/runtime-config-wal.html#RUNTIME-CONFIG-WAL-SETTINGS
>     
>     In minimal level, WAL-logging of some bulk operations, like CREATE
>     INDEX, CLUSTER and COPY on a table that was created or truncated in the
>     same transaction can be safely skipped, which can make those operations
>     much faster (see Section 14.4.7). But minimal WAL does not contain
>     enough information to reconstruct the data from a base backup and the
>     WAL logs, so either archive or hot_standby level must be used to enable
>     WAL archiving (archive_mode) and streaming replication.
>
> I am confused why we issue significant WAL traffic for CREATE INDEX?
> Isn't the index either created or removed if the transaction fails?
> What crash recovery activity state do we need WAL logging for?  I
> realize we have to do WAL logging for streaming replication, but CREATE
> TABLE isn't going to affect that.   I also realize the index has to be
> on disk on commit, but the same is true for doing the CREATE TABLE in
> the same transaction block.

I'm confused about what you're confused about. Crash recovery doesn't 
need the WAL for CREATE INDEX, but WAL archiving does.

--   Heikki Linnakangas  EnterpriseDB   http://www.enterprisedb.com