Friday, February 10, 2006

Partition Pruning uncovered

The partitioning development for 5.1 is close to its completion (apart from the
stabilisation phase). An important part of this development is the ability to
prune away partitions not used in the query. Here follows some guidelines how
the optimiser handles this at the moment.

For most functions the optimizer can only handle equality conditions thus:
where partition_field = 2
where partiiton_field IN (2,3,4)

In this case the optimiser can use this independent of what the partition
function is. It will do so by actually applying the partition function on the
equal value thus:
partition_function(2) = partition 3
partition_function(3) = partition 5
partition_function(4) = partition 3

It will set a bit in partition_used bitmap to indicate which partitions are used
to indicate to the executer which partitions to scan.

So this will work for all partition functions.

Range optimisations will work for the following partition functions using
RANGE or LIST partitioning

PARTITION BY X(f(partition_field)) where f(partition_field) is:
f(partition_field) = partition_field
f(partition_field) = year(field)
f(partition_field) = FUNC_TO_DAYS(partition_field)
(if partition_field is of date type or converted to in last two examples)

Range optimisations will also work for all partition functions and all
partitioning in the following case: When the range is either of type a list
of values (1,3,5) or where the range is a short range e.g. BETWEEN (3,5)
which is simply converted to (3,4,5).

For Range partitions the actual range of partitions is found by applying
partition_function(start_range_value) = partition 3
partition_function(end_range_value) = partition 5
=> Scan partition 3,4 and 5

For LIST partitions one will do it slightly different
partition_function(start_range_value) = x
partition_function(end_range_value) = y
(The partition functions used here must be increasing or decreasing)
Then scan all lists values from x to y (it is kept in a sorted list to enable quick
fetch of a partition given a value) and for each list value found mark the
partition as used.

For Range types that will work for all partition types the partition function will
be evaluated for each value in the interval and its corresponding partition will
be marked for use.

These optimisations apply also to subpartition so you can have conditions on
only subpartition fields and you will get pruning for subpartitions only and vice
versa for partitions and also when conditions on both at the same time.

So to use an example

CREATE TABLE t1 (a date)

SELECT * from t1 WHERE a >= '1995-01-01 AND a <= '1997-12-31';

Here we have a range optimised function =>
year(start_range_value) = year('1995-01-01') = 1995 => p0
year(end_range_value) = year('1997-12-31') = 1997 => p2

Thus p0 - > p2 will be scanned, thus p0, p1 and p2.

If we replace RANGE by LIST only and all else is kept constant then
we will scan list values from 1995 to 1997 and find that p0, p1 and p2
is to be scanned.

For HASH partitioned variants there will not be any optimisations in this
case due to that the interval is to wide since the data type of the equal
condition is date and there more than a thousand dates in the range.
However if the user makes a slight change to the data model and instead
use the following:

CREATE TABLE t1 (a_year int)
PARTITION BY HASH(any_function(a_year))

SELECT * from t1 WHERE a_year >= 1995 AND a_year <= 1997;

Then the range [1995,1997] will always be good enough for partition pruning.
And thus a maximum of 3 partitions will be scanned in the query.

1 comment:

Jayant Kumar said...


I will come directly to the problem i am facing. I have a MYISAM table which currently has 90,000,000 records. This is a very high frequency table where user information is being logged and retrieved. The table is growing by 300,000 rows every day.

I was planning to break up the table using my own logic. Which is -> if i create n tables, information of user with id x will go into table no (x%n).

However, i came across mysql partitioning and found that hash partitioning in mysql does exactly what i am planning to do.

My question is :

1. Whenever an insert/update in happening, in a partitioned table, will all partitions be locked? If yes, what task would be accomplished during the locked period or what would be duration of the lock?

2. If i do a select which needs scanning of more than 1 partition, would my queries be executed in parallel on different partitions?

Partitioning in mysql would be a very good feature if it enhances the speed of queries for large tables. I wish i could use it, since it would reduce the burden of administrative overhead on tables broken up my me.

Thanks a lot