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Re: partitioning by highly skewed data
Posted by: Mikael Ronström
Date: June 18, 2005 02:16AM

Hi,

Hiroshi Takenaka wrote:
> Hi,
>
> Thank you for the response.
>
> Mikael Ronstrom wrote:
> >Subpartitioning is possible for RANGE and LIST
> partitions, the
> >subpartitioning is then HASH partitioning.
>
> It would be very nice if we can subpartition HASH
> partitions,
> or do somthing like:
>
> : create table t1 (high_skew_col int,
> some_low_skew_col int,...)
> : partitioned by
> range(hashfunc(high_skew_col) mod n) ...
> : subpartitioned by
> hash(hashfunc(some_low_skew_col))...;
>
> and distribute subpartitions among several disks.
> Perhaps, it will give us maximum performance, if
> future version of
> MySQL can do parallel scanning of partitions ;-)
>

Yes, exactly this will be possibly already in the first alpha version.
Yes, parallel scanning of partitions is planned for in a not too distant
future. Also many other parallel parts are planned for although we will
focus first on making partitions easy to maintain.

> In this example, I expect that we can still enjoy
> the benefit of
> (hash)partitioning on queries like 'select * from
> t1 where high_skew_col = X',
> while we can distribute data more evenly among
> disks.
>

Yes, the partitioned handler will be able to detect that only a subset of the partitions need
to be checked (in the first version not in every situation but this will improve over time as
the optimiser part is further developed)

> RANGE partitioning by high_skew_col +
> subpartitioning is not very
> suitable to our situation, because our
> high_skew_col is actually
> sparse and unevenly distributed id numbers.
>

If they are not so many you could also contemplate using LIST partitioning.

> When I wrote my previous post, my idea was to
> define
> a custom hash function like:
>
> : fn(val1, val2) mod n = hash(val2) mod k
> (if val1 is null)
> : = hash(val1)
> mod (n -k) + k (if val1 is not null)
>
> and use 'partitioned by hash(high_skew_col,
> some_low_skew_col)'.
>

PARTITION BY KEY (low_skew_col, high_skew_col)
would achieve more or less this purpose (uses a MySQL
defined hash function that takes NULL values into account.

> But, of course, it's not very useful if we rarely
> use some_low_skew_col
> in where clause. I must have been sleeping...
>


Thx for the input, I added support for NULL values in partition functions yesterday.

Rgrds Mikael

> Hiroshi

Mikael Ronstrom
Senior Software Architect, MySQL AB
My blog: http://mikaelronstrom.blogspot.com

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Re: partitioning by highly skewed data
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