Re: Proposal: implement Postgres CLUSTER USING by default for MtM tables

[Russell Keith-Magee]

Hi Bryce,

An interesting idea. How are you proposing that this feature be exposed as an API? 

Is there any reason that this couldn't be handled as a DB-admin operation? i.e., Django just does what it normally does, and if a DB admin notices that table clustering is a problem, they apply the cluster command at the database level. Given that this is a very low level optimisation, is there any reason that this needs to be exposed as a Django-level API optimisation option?

Yours,

Russ Magee %-)

On Wed, May 22, 2013 at 9:28 AM, <bry...@obviously.com> wrote:

The tables created by django for MtM fields are unordered.  Loading all the relationships may result in a number of I/O operations approaching the number of relationships:

# select * from basetable;

 id |basetable_id| mtm_id 

----+------------+----------

  6 |          3 |        1

  7 |          2 |        2

 15 |          1 |        1

 16 |          1 |        2

 18 |          3 |        5

Postgres has a feature to 'defragment' such tables packing data that gets accessed at the same time into the same disk blocks.  The number of I/O operations can be as small as one:

# \pset pager off

# cluster basetable using table_mem_id;

# select * from basetable;

 id |basetable_id| mtm_id 

----+------------+----------

 15 |          1 |        1

 16 |          1 |        2

  7 |          2 |        2

  6 |          3 |        1

 18 |          3 |        5

On a large table this can put hundreds of MtM relationships sequentially on disk.  This makes for less I/O operations and less cache space used.

----

This feature is postgres specific.

And it would involve django making a guess about the use pattern of the relationship.  Either the forward or reverse is optimized.

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[Donald Stufft]

What do you mean by "clustering by default". CLUSTER is a one time operation. You use to to arrange the orders of the row in a table but after they have been arranged it has no long lasting affects past that.

-----------------

Donald Stufft

PGP: 0x6E3CBCE93372DCFA // 7C6B 7C5D 5E2B 6356 A926 F04F 6E3C BCE9 3372 DCFA