Re: Cluster table and order information

Quoting Andy Colson <andy@squeakycode.net>:

> On 2/1/2011 9:08 AM, Dario Beraldi wrote:
>> Hello,
>>
>> From the documentation of CLUSTER table
>> (http://www.postgresql.org/docs/8.4/static/sql-cluster.html) I
>> understand that clustering can be achieved by re-creating the table like
>> this:
>>
>> CREATE TABLE newtable AS
>> SELECT * FROM table ORDER BY columnlist;
>>
>> My question is: If I upload with COPY a datafile which is already
>> correctly sorted, can I inform postgres of such order, so that no
>> clustering is necessary after the import? In other words, how can I tell
>> postgres that my file is order by this and that column?
>>
>> Many thanks!
>>
>> Dario
>>
>
> The planner has no knowledge of cluster.  Meaning PG will query a
> clustered and unclustered table exactly the same way.  A table is
> not marked or anything as clustered.  And in fact, during usage of a
> table it'll become unclustered.
>
> Clustering is only useful when you are going to read multiple
> records in the same order as an index.   It turns "more random
> seeks" into "more sequential reads".
>
> If your COPY loads data in indexed order, then just dont run the cluster.
>
> -Andy
>
Thanks very much Andy, this clarifies my doubts.

I was misled by the docs saying "When a table is clustered, PostgreSQL
remembers which index it was clustered by" which made me think that
the order information is stored somewhere.

All the best
Dario

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