On 05/15/2012 02:44 PM, Robert James wrote:
Okay, I understand why we still need VACUUM and why we can't always
CLUSTER. But my question remains: assuming I have some down time, do
I loose anything by CLUSTER. Your answer is, I believe: Not normally,
but there is one case where you do. That's an append-only table,
where you're generally interested in the most recent data, but you
cluster on something else.
Does clustering really hurt in that case? Is the planner smart enough
to realize that the data you want is towards the end only? I would
think that it doesn't know this, and will, let's say, assume it is
scattered regardless and perform a full scan. I guess the question
is: Does the natural order of data help if there's no explicit means
for the planner to prove it?
The speed benefit of clustering (aside from compacting the files and
reindexing) comes from keeping data that is likely to be used together
stored together. I.e. all the data for this week or all the records for
invoice 12345. If you are really lucky, the data a query needs might be
in one or a few contiguous disk blocks.
But say you clustered your invoice-detail data on item number. Gathering
the detail records for one invoice could require many disk seeks and a
disk-block read per record. In that case clustering would hurt so you
should choose your cluster order intelligently.
BTW, be sure to run ANALYZE after clustering a table.
See http://www.postgresql.org/docs/current/static/sql-cluster.html
Cheers,
Steve
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